Energy
The introduction has stressed the need for a reduction in the use of
energy in buildings; where the part played by a strategic role for
daylighting can provide considerable savings in energy, and therefore of
carbon dioxide emissions, leading to a reduction in greenhouse gases and
ultimately a reduction in global warming.
This is now recognized by most governments, though there is still a
reluctance to take sufficient measures to overcome the problems
involved. The ‘fossil fuels’ which provide the bulk of the energy we
use at present, are still thought of as cheap alternatives to action, ignoring
the fact that coal, gas, and oil are a finite resource with limited life for the
future, leading to a potential energy crisis.
Even where this is acknowledged, most governments have not put the
necessary investment into alternative forms of energy, by developments
in the fields of wave, wind or solar power. In the past there have been
exceptions; one being in the development of hydroelectric power, where
conditions have permitted and lucky the countries which have benefitted;
another is in the use of solar power in certain countries which have
exploited their natural environment; this is an area where a developing
technology can play an important part in the future.
Nuclear power in the UK has not proved to be the answer, unlike early
projections from journalists that energy would become almost free. The
generation of energy by means of nuclear power stations, has become too
expensive, added to the unsolved problems of the disposal of nuclear
waste, to a point where it is unlikely using present technology for nuclear
to provide the alternative to fossil fuels; the development of nuclear
energy is more an issue for the environmentalist. There are countries,
such as France, where a large part of their energy is derived from nuclear
plants, but in the UK there does not at present seem to be either the will
or the means.
The future therefore appears to lie in the development of alternative
sources of energy, but the problem facing us today is in taking action to
ameliorate the energy crisis as it exists.
The reduction in the use of energy in buildings has been identified as a
major objective, of which electrical energy for lighting is a significant factor.
Lighting accounts for between a third and a half of the energy use in
commercial buildings and significant savings in energy can be obtained
where the positive use of daylight has been planned; associated with
control systems, by means of ‘daylight linking’, natural light provides the
major light source during the day with variable artificial light as back-up.
It will be found that in many of the Case Studies mentioned later in the
book, daylight has provided the necessary amount of light for large parts
of the building during the day, whilst providing the interior space with
an overall impression of daylight, even in areas where the actual daylight
factors may be relatively low, allowing light from artificial sources to be
reduced, with consequent savings in energy.
ARTIFICIAL LIGHT
All forms of energy use in buildings should be analysed, related to the
different needs of individual architectural programmes, to see where
savings can be made; for example in homes, the use of the natural source
has always been paramount during the day, so few savings can be made.
At night however, developments in lamp technology have produced
significantly more efficient artificial light sources and this is an area
where, due to the large quantity of residential property, significant
savings have yet to be made; moreover major energy savings in the home
are to be found in the means of heating and insulation. Table 4.1
illustrates the different aspects of the main types of lamp, providing
comparisons to assist the architect in making his choice. The different
factors identified are those of efficacy, lamp life and colour, but other
factors that must also be considered are those of cost and control.
It can be seen from the column under ‘Lamp efficiency’ that the
favourite domestic lamp – Incandescent Tungsten – has an efficiency of
only 7–14 Lm/watt, whilst the compact fluorescent (CFC) has an
efficiency of between 40–87 Lm/watt. At present the CFC lamps cannot
be dimmed economically, but there are many areas in homes, where
dimming is not a requirement, and with satisfactory colour (2700 K) there
is no reason not to take advantage of their long life and lower wattage.
The newer generation of lower-energy lamps such as the compact and
T-5 linear fluorescent lamps can in many cases replace less efficient
incandescent sources, which can be four to eight times more efficient;
they can also have more than eight times longer lamp life. Used in
conjunction with high frequency electronic control gear further reductions
of 20 per cent in power consumption or energy savings can be
made.
To realize these gains they must relate not only to the lamp, but also to
matching this with the the correct luminaire or light fitting. It is no use
simply fitting energy-efficient lamps into inappropriate luminaires,
resulting in unsatisfactory installations; furthermore an energy efficient
scheme demands regular, consistent and informed maintenance. It may
also be cost effective in large installations to operate a system of ‘bulk
replacement’ of lamps after a specific period irrespective of how many
lamps may have failed.
In buildings for industrial use, no doubt savings may be possible in a
rigorous investigation of the plant required to run industrial processes;
but the area most likely to result in the greatest savings is in building
services, and the greatest of these will be in the lighting, where daylight is
the key.
One of the problems has been in the ‘cheap energy policy’ of
Government; there may be other good reasons for this, but it has led in
the past to a prodigal use of cheap energy, and it is only recently, with a
looming energy crisis, that government has woken up to the vital need
for savings to be made.
The first line of defence must be in avoidance of waste; for how many
times do we pass a building with every light burning in the middle of the
day when daylight is quite adequate, or after dark when the building is
largely unoccupied. The total amount of energy wasted on a daily basis
may not have been calculated, but it is considerable and arguably equals
the amount of savings which can be made in other ways.
A particular example of this might be in transport buildings where
artificial light is used all day irrespective of the level of daylight. There is
no doubt a need for the level of daylight never to drop below the
statutory design level, but this can be solved by adopting a system of
control which links artificial light to the available daylight to ensure that
the design level is maintained, whilst allowing significant reductions in
the use of artificial light, which can be off for most of the day.
DAYLIGHT
The most obvious vehicle for energy saving in buildings is in exploiting
the most abundant source of light available to us – daylight. Environmentally
conscious assessments of building design are recognizing that
daylight (and natural fresh air) is an important commodity and should be
exploited to the full. Generally, people when asked, always prefer to
work in a daylit environment. There is a growing acknowledgement that
daylight produces positive effects, both physiological and psychological.
Forms of control are necessary to limit the potentially excessive levels
of daylight, if it is not to become a nuisance, particularly on bright sunny
days. A wide range of devices are available, from relatively inexpensive
and simple internal blinds (roller, venetian etc.) through to high tech,
computer-controlled heliodens, which track the sun.
Whilst a daylighting strategy will be needed in those buildings where a
decision to provide air-conditioning has been adopted, it is in those
buildings described as ‘passive’ where the greatest savings can be made.
A passive building is one in which the greatest use is made of natural
resources . . . natural light, solar power and ventilation derived from
making use of the natural environment. Nature cannot provide all that is
necessary, and even during the day there may well be a need for some
additional energy use, in terms of lighting from artificial sources, or
ventilation from some form of fan assistance, whilst in terms of solar
power, this can be used to advantage.
CONTROLS
The careful introduction of lighting controls can ensure that the
maximum use is made of the available daylight; so that the amount of
artificial light is reduced automatically when all, or most of it, is no longer
required to meet the design level.
The term ‘daylight linking’ has been used already, and this perhaps
needs some explanation. It is used in the sense that the artificial lighting
in a building is planned and controlled to support the natural light
40 Daylighting: Natural Light in Architecture
available during the day, to ensure that the combined lighting level meets
the desired design level.
This can be done by planning the artificial lighting circuits so as to
allow control by simple switching, so that those sources close to the
window may be switched on only when required. Such unsophisticated
means of control suffer from the human factor, in that once the artificial
light close to the window is switched on it tends to be left on all day.
A more sophisticated method known as Permanent Supplementary
Artificial Lighting (PSALI) was proposed by Prof. Hopkinson in the late
1950s; the first practical application of the technique being developed for
the Esso Building (see Lighting Modern Buildings, p. 89) where there was
dual switching for day and night, with the same lamp energy used
throughout, but using the daylight available close to the windows to
achieve the required design level when available. This still relied on the
human factor to turn on the switch.
One of the greatest advances in the technology of lighting is in the
development of modern control systems. These will be associated with
light fittings which can react by photocell to the level of daylight available
outside, enabling the design level to be maintained throughout the day,
but offering considerable savings in energy.
The control system should be appropriate to the occupation of a space,
and in a leaflet published by the British Research Establishment, Watford,
UK, the following are identified.
1. Variable occupation. Occupants spend part of their time in the space,
and part elsewhere, e.g. an office
2. Intermittent scheduled occupation, a meeting room
3. Full occupation, reception area
4. Intermittent occupation, storeroom areas.
Before deciding on the appropriate type of control it is useful to
analyse the type of ‘occupation’ as above, as this may help to determine
the nature of the control system.
It is unnecessary to dwell on the many types of control system, from
‘intelligent’ light fittings which react automatically to the ambient light
level, adjusting the total light to meet the design level; to systems where
each fitting may be controlled individually by an occupant to meet his or
her needs by means of a manual controller, or groups of fittings which
can be controlled by means of proximity switches, reacting to an
occupant’s presence.
It should be emphasized that the control system for a particular
building or part of a building should be appropriate for its use, for
example the control system for a church will be very different to that of
an office or a factory. Each programme should be analysed and those
areas of buildings where there is intermittent use, such as storage or
warehouse, need to be provided with an appropriate control regime; if
daylight is available, artificial light may not be required during the day at
all, or by some means of occupancy or proximity switching.
Control systems are at the heart of energy savings, and daylight linking
is an essential part of the solution, and may be linked into the BEMS
(Building Energy Management System).
Energy 41
SOLAR
There are two distinct aspects to the question of the relationship of
energy to the power of the sun. First there is the heat gain from the sun to
those surfaces of the building which are insolated, for the most part on
the south elevation but with some additions to east and west. This can be
a useful addition to the heating of the building in the winter, but on the
obverse side can produce overheating in the summer, which must be
dealt with.
This however is not a matter to be dealt with under the heading of
daylighting, it being more concerned with the heating and ventilation
equation.
The second aspect, is very much one of daylighting; that of the use of
the sun to generate power by means of solar panels or photovoltaics, this
is an aspect of the relationship of the sun to energy, and a growing
technology.
Despite the fact that we lack the climate to provide large quantities of
solar power (as for example in Israel, where solar panels generating
power are the rule on properties rather than the exception) the
conversion of the sun’s energy into useful power has been shown to be
effective.
The building industry has a long way to go before the technology
already available makes a substantial impact, but as the energy crisis
becomes closer the means will be found (see Lighting Modern Buildings,
Case Study No. 11. The Solar Office at Doxford International Business
Park). It has been shown that some 30 per cent of the energy required for
an office building can be provided by means of photovoltaic panels,
provided that the orientation and construction of the building has been
planned for it.
LEGISLATION
Up to the twenty-first century little effort had been made to limit the
amount of energy used for the lighting of buildings by legislation; but a
start was made by Part L of the Building Regulations of 1995, dealing with
the conservation of fuel and power; this was a start to limiting the
amount of energy used for lighting in buildings, and this coupled with
the increased efficacy of the lamps and light fittings available from the
lighting industry, had a material effect upon the energy demand.
In 2002, revisions to Part L made it a requirement to consider the need
for ‘energy efficient lighting’ more seriously, and architects should be
aware of the current regulations, which in themselves will no doubt be
further updated and modified, to increase the need for further energy
savings for the future.
The new Part L requires that ‘Reasonable provision shall be made for
the conservation of fuel and power in buildings other than dwellings, by
. . . installing in buildings artificial lighting systems which are designed
and constructed to use no more fuel and power than is reasonable in the
circumstances and making reasonable provision for controlling such
systems’. There is some flexibility for lighting designers to comply with
the regulations, and there is every reason for the spirit of the regulations
to be wholeheartedly adopted.
The regulations are divided into two parts, the first (Part L1) dealing
with dwellings, and the second (Part L2) with non-domestic buildings.
42 Daylighting: Natural Light in Architecture
The latter takes in offices, industrial buildings and those of multiresidential
use, such as hotels, hostels, old people’s homes, hospitals and
boarding schools. This is a very broad sweep of the majority of buildings,
and architects should be aware of the implications . . . it will not be
sufficient to say that your client has demanded illumination levels of 1000
lux in a hotel foyer when to provide this level the amount of energy used
is far in excess of the amount allowed for this type of space.
To give an example of the legislation the following is a quotation from
Part L2: This refers to general lighting efficiency in office, industrial and
storage buildings:
1.43 Electric lighting systems serving these buildings should be provided
with ‘reasonably efficient lamp/luminaire combinations.’ A
way of meeting the requirements would be to provide lighting
with an initial efficacy averaged over the whole building of not
less than 40 luminaire-lumens/circuit watt. This allows considerable
design flexibility to vary the light output ratio of the luminaire, the
luminous efficacy of the lamp, or the efficiency of the control gear.
Whilst this clearly precludes the use of tungsten lamps for general use,
they can still be used in some areas which may demand their use; where
the average over the whole building does not exceed the predetermined
40 luminaire-lumens/circuit watts . . . there is flexibility.
A major difference in the new regulations is that they apply to display
lighting, defined as ‘lighting designed to highlight displays of exhibits or
merchandise.’ (Examples of display lighting are included in the Case
Studies shown later, a good example being the Sainsbury Store in
Greenwich (Case Study pp. 164–167) where high levels of environmental
lighting are available during the day by the use of natural light from roof
lights, but where in terms of Part L the overall energy use is below the
limits of the requirements.)
Part L of the building regulations encourages the use of daylight
linking, stressing the relationship between the available daylight, and
controlled artificial light sources. Daylighting can be at the heart of
energy savings in buildings, and whilst in the early twentieth century this
was largely forgotten, at the beginning of the twenty-first it has been
shown to be a key to the future.
Daylighting Natural Light in Architecture 3
3 Windows
The window is an opening in a wall or side of a building admitting light
and often air to the interior. Early windows were developed before the
introduction of glass, so initially windows were left open to the external
atmosphere, or filled by some form of closure to minimize the heat loss at
night. The more sophisticated buildings would have had thin slabs of
marble, mica or oiled paper for this purpose.
In mediaeval times wooden shutters were installed on the interior, and
these were left open or closed to regulate the light and air. With the
introduction of glass, used first in small panes in Roman architecture, the
window as we know it today had its beginnings. The concept of small
panes of glass, divided by bronze or later lead divisions, as used in early
buildings dies hard and window manufacturers still offer these as alternatives to fully glazed windows in new domestic work, however
inappropriate they may appear.
Windows can broadly be divided into two main types, first the window
set in the side walls of a building, and second the opening light set into
the roof, generally known as rooflights.
The daylight penetration from side windows will depend upon the
ceiling height, and in early buildings where the ceiling heights were low,
the penetration of daylight into the building was severely limited . . . with
the design of the important houses of the seventeenth and eighteenth
centuries the ceiling heights were raised and daylight was able to reach
further into the interiors. However as buildings became grander, even this was not enough, and the concept of the rooflight was developed to
introduce daylight into interiors far from the side windows.
Illustrations of some window types indicate the variety of window
shapes that have evolved over the centuries, set into the vertical sides of
buildings.
The horizontal window is perhaps the most well known of all, starting
as it did in mediaeval times, limited by the construction methods of the
day. It is still much used in today’s domestic architecture. Provided the
horizontal window is placed high in the wall the daylighting will
penetrate well into the space, but other features of the window need to be
considered, such as the view out which will be prejudiced where the cill
is too high.
A logical development of this type is where the horizontal window
extends the entire length of the external wall, a device used in nineteenth
century industrial buildings to provide even and sufficient light for
machine operators. This type of window required new structural
techniques to overcome the need for vertical support to the structure
above.
Yet a further example is the clerestorey; found mainly in tall buildings
such as churches, generally associated with other forms of window at
lower level to provide the main daylight. Clerestoreys are placed at high
level to assist in getting daylight further into the interior and to light the
roof structure.
A logical development of the extended horizontal window, is the floor
to ceiling window; as structural techniques were perfected, this type of
window has become almost universal in some types of architectural
programme such as the office. The 1930s saw the innovation of the wrap
around corner window as further structural techniques were made
possible.
Finally and in no chronological order comes the vertical window.
Vertical windows were popular from the fourteenth century, having
perhaps their most glorious period in the eighteenth century, when the
Georgian window with its sophisticated detailing was almost universal.
Tall windows, set apart by masonry at intervals, provided a simple
structural solution and this formed the pattern of development in
residential and other building types for several centuries.
The windowcan be said to be the most important architectural feature of
a building; this is the first experience that a visitor will have when seeing
the building for the first time, and architects have naturally considered the
form of the window and its relationship to the exterior to be vital.
The illlustrations of these buildings along the Embankment illustrate
three different approaches to fenestration. All buildings are of the
twentieth century. The first, on the left, shows the more traditional
separate windows, whilst that in the middle is an example of the
continuous horizontal window, where the individual floors are expressed
as important horizontal bands. The building on the right is the further
development where the window becomes a subsidiary part of the
external cladding, for a total glass fac¸ade. The appearance of the buildings
says little about the success of the daylighting, it says more about
architectural fashion.
ROOFLIGHTS
Whilst rooflights could properly have been said to have started with the
central courtyards or atria of the Roman house, these were open to the
sky and rain; and despite providing daylight to the surrounding dwelling
space, would not have modified the exterior climate in the manner of a
roof light.
The rooflight by definition permits daylight to enter from above
through a glazed opening in the roof protecting the interior from wind
22 Daylighting: Natural Light in Architecture
Three buildings on London’s Embankment
DP Archive
and weather. The early rooflights were perceived either as domes such as
that at Chiswick House with ordinary windows in the sides allowing in
the daylight, but by the nineteenth century structural techniques had
developed sufficiently to allow fully glazed barrel vaults or glazed domes
to be placed above areas of building remote from the side walls and the
proximity of windows. Examples of nineteenth century shopping malls
still exist today where these overhead lights permit daylight to reach
deep into the interior of buildings.
Much innovation was used in the nature of these roofights, and it is of
interest to study the section of the Soane Museum, to see the many
different shapes and sizes of overhead light Soane devised to introduce
daylight to the different spaces, in what was at the time his private house.
By the twentieth century the use of rooflights had been reduced almost
entirely to industrial buildings, and the CIBSE Lighting Guide LG10,
‘Daylighting and Window Design’ (published October 1999) illustrates a
number of different types, the most common of which were the shed
roof, the sawtooth, and the monitor.
The advantages and disadvantages are set out in CIBSE LG10
indicating that the original shed roof, the cheapest solution, has serious
defects and is unlikely to be used today; whilst the many different forms
of monitor roof can be adapted to fit most roof situations to solve the
daylighting problems below.
New roof forms are still being developed for the admission of daylight
to large open areas not restricted to industrial buildings . . . from
supermarkets to universities and swimming pools. An excellent example
of an early solution to a factory in which the services are rationalized and
placed inside ducts which are a part of the overall roof structure and do
not obstruct the daylight is shown in Lighting Modern Buildings pp. 138/9,
the York Shipley Factory; whilst the roof design for the Sainsbury
Supermarket in Greenwich (Case Study pp. 164/167) shows an elegant
solution to the roof form, providing a high level of daylight to the store.
ATRIA
Whilst the word atrium started as the central court of a Roman house,
admitting light and air to the surrounding dwelling space, the word has
taken on a wider meaning as described in the CIBSE LG10 daylight and
window design.
‘An interior light space enclosed on two or more sides by the walls
of a building, and daylit from a roof of transparent or translucent
material and, sometimes, from glazed ends or sides. It permits the
entry of light to the other interior spaces, linked to it by glazed or
unglazed openings.’
The atrium is therefore a further development of the dome or vault
allowing daylight into the central areas of the great houses. The modern
atrium will be covered by a glazed skylight, which, whilst slightly
reducing the amount of daylight, monitors the external atmosphere
keeping out the rain, whilst contributing to ventilation, and reducing the
necessity for air-conditioning.
The proportions of the atrium and the reflective capacity of the
enclosing wall surfaces are critical, and those atria which are wide in
relation to their height, will perform better than taller, narrower spaces in
ensuring that daylight reaches the lower levels. Having said this, the
elongated atrium, which can act as an internal street at the low level, has
proved successful in providing the impression of a daylit interior, even if
due to its height the measured level of daylight at the lowest level will be
much reduced.
In order to optimize the daylight at the lower levels one method is to
set back the floor plans at the higher levels to maximize the direct view of
the sky at the lower; but this has planning limitations and economic
implications for the building owner.
Summarizing the advantages of atrium design
First, the human advantages: by getting daylight into the centre of deep
plan buildings, this provides the occupants with a sense of orientation,
information on the time, weather and the world outside the building;
together with a sense of space and expansive views which may
compensate for the lack of external views from the building.
Second, the environmental advantages: there is a potential for savings
of energy, assistance with the problems of ventilation, and a reduction in
the need for air-conditioning. Depending upon the orientation and detail
of the rooflight there may be a need for some solar shading.
On balance the advantages outweigh the disadvantages, as over the life
of the building the cost of the people who work there greatly exceeds
those of its construction, and the work environment is crucial.
Glazing
There is now a large amount of alternative glazing for windows, and it
is necessary for the architect, in conjunction with his services
consultant, to write a detailed performance specification; this must
include the orientation of the window, its thermal and acoustic
characteristics, together with its capacity for solar shading. This is of
course in addition to the main function of the window which is the
admission of daylight and the introduction of the view to outside.
Further factors which may need to be taken into account, are whether it
is thought desirable to have windows which open or are fixed, and its
relationship to ventilation.
But here our concern is with the types of glazing which are available.
As already stated the main purpose of a window is for the admission of
daylight, and associated with this the view to the exterior.
Glazing types which reduce the impression of daylight significantly,
darken both the interior, and the view, whilst the view from the outside
towards the building makes the fac¸ade look black. It is only when
comparisons are made between the view through a clear glass window
and one with a modifying glass that reduces the daylight, that the results
create disappointment . . . it is true to say that it is human nature to
appreciate the natural environment, with all its variations of colour, light
and shade.
This is particularly true of residential properties where some form of
dark glass has been applied to the fac¸ade, giving the impression of a dull
Windows 25
day seen from the interior, as compared to the view through a clear
window.
There are basically three main types of glazing as follows.
1. Clear glazing
This can be single sheet, double or triple glazed or alternatively a ‘thick’
glass, but the more sheets or the greater the thickness of glass the more
the daylight will be diminished, although the impression of the colour of
the exterior will still be perceived as natural.
Clear glass whilst allowing a high transmission of daylight, will at the
same time and on certain building fac¸ades allow a high transmission of
solar radiation. It is this fact that has led to the development of the more
high-tech glasses designed to reduce solar gain, with their consequent
loss of daylight transmission. Other means such as interplane blinds,
located between the panes of glass, may present a solution. These would
only need to to be installed on fac¸ades subject to solar gain and then only
activated when required.
2. Tinted glass
This is of two types: the first where the clear glass is itself modified in
such a way as to produce different radiant heat transmission characteristics,
therefore the thicker the glass the lower the transmission of
daylight, and the greater the control of radiant heat from sunlight.
The second type of glasses are those coated with microscopically thin
layers of metallic oxides which reflect the heat away and out of the
building. These coatings are applied to the inside layer of glass generally
in association with other panes in a sealed double glazed unit as a
protection, since on their own they would be vulnerable to damage.
These coated glasses can be designed to have high daylight transmission,
due to the very thin layer of reflective material; so that they almost
give the appearance of clear glass, and do not suffer from the objections
raised by tinted glasses which reduce the daylight significantly.
Additionally they do not obstruct the view; however they do have cost
implications, and should only be used where the specification demands
it. Highly reflective glasses are available, but need to be used with care to
avoid the danger of glare to other buildings or motorists.
3. Miscellaneous glazing
A number of different types of glazing are placed in this category, largely
because they cannot be lumped together into a single category; they
consist of the following:
Patterned glass, wired glass, laminated glasses and glass blocks.
Patterned glass
Any number of patterns can be rolled into semi-molten glass, to provide
decorative or diffusing sheets for various purposes, though rarely for
windows, since their capacity for light transmission will be modified.
Wired glass
A similar process is used for the manufacture of wired glass, where a wire
mesh is sandwiched within the thickness of the glass. This used generally
26 Daylighting: Natural Light in Architecture
in security situations, and sometimes as a protection to vulnerable
skylights.
Laminated glasses
Similar methods of manufacture are used for laminating sheets of plastic
between sheets of glass, again used for security reasons as resistance to
impact. These reduce the transmission of daylight.
In museums where exhibits are exposed to daylight, it will be necessary
to control the entry of UV light. This may be done by the use of laminated
glasses, where UV absorbing filters can be laminated between the sheets
of clear glass.
Glass blocks
These were a popular form of glass wall in the 1930s, having thermal
characteristics due to the hollow nature of the blocks, which, because of
their structural nature are still in use today for the introduction of
daylight into new buildings, but special openings will be required to
provide a view.
High tech glazing
There are a number of glazing types which fall into this category, the
most advanced of which are the photovoltaics, where the glass itself is
designed to generate electricity from solar radiation on south facing
exposures, which can then be used within the building to reduce the
energy required for the artificial lighting. Some buildings already use this
method, and the UK Government is now putting research money into its
further development. (See Doxford International Building Park. Lighting
Modern Buildings. Case No 11, pp. 124/5).
Two other types of high tech glass deserve mention, but are not at
present economically viable for general use in buildings.
The first are the photochromic glasses, which respond directly to an
environmental stimulus (temperature or light) rather like the special
sunglasses which are already available which alter their transmission
factor depending upon the brightness of the ambient light; alternatively
there are the electrochromic glasses designed to respond indirectly by the
application of an electrical current which alters their visual and thermal
characteristics. These glasses are still at the experimental stage, but are
likely to be developed further to a point where they may become viable.
The choice of glazing in a large complex is one of the greatest
importance, having implications both on first cost, and the cost in use of
the project.
WINDOW DETAILS
The Georgian window developed in the eighteenth century satisfied all
the known criteria at the time. It admitted unadulterated daylight, it
provided ventilation when required, and it could be controlled by
internal shutters, providing additional security. The splay at the sides
(and sometimes at the cill as well) together with the careful detailing of
the glazing bars, assisted in balancing the brightness between the inside
of the room and the outside. However it did little for thermal insulation,
and on sunny elevations problems of solar gain and the possibility of
glare, were considered less important at the time.
Windows have developed a long way from this point, from the
standard horizontal or vertical windows set into the side walls of the
majority of residential properties, to the window walls commonly found
in modern office blocks. The Georgian window, however, provides some
lessons which have apparently not been learnt today, mostly to do with
the subtlety of the detailing.
1. To assist with modern methods of production, the timber sections
used for dividing the opening parts from the fixed glazed areas, tend to
be heavy, interrupting the view out, particularly where they cross the
sight lines of those inside.
2. Where glazing bars are required, between the different glazed areas
they are often very heavy, and where in the Georgian window the
detailing would have allowed the light to flow around the bar, reducing
its apparent size, the modern glazing bar tends to create unsatisfactory
shadows within and a barrier to the view, further reducing the amount of
daylight available.
3. The use of splayed sides between the window and the wall, to
balance the brightness of the window seen against the brightness of the
interior of the room has almost been forgotten, a lesson learnt in our
mediaeval churches, and which is equally relevant today; the use of the
splay to conceal security shutters may however not be required. This is of
course not to deny the advantages of the modern domestic window, in
terms of both thermal and acoustic capacity, with the introduction of
double glazing.
Whilst the majority of windows are of the type discussed, set into walls at
intervals, either horizontal or vertical, each having their own characteristics
in determining the quality of daylight entering the room. It is more
likely that wall-to-wall windows will be used in modern office blocks, and
these will have their own structural detailing; for example there may be
no need to have horizontal divisions since the glass sizes will generally be
able to stretch between the cill and the ceiling level or spandrel, whilst the
divisions between the wide panes of glass horizontally can be minimized
to avoid the break-up of the view.
It will however be important to consider the junction at the point
where the window meets a wall at right angles at a major subdivision of
the space, or the end of the building; here the reflection factor of the wall
needs to be kept high, to avoid a conflict of brightness. Alternatively the
architect may wish to break up the elevation of his building, by the
introduction of structural elements which articulate the perimeter of the
fac¸ade. In such cases the wide horizontal windows located between the
vertical structures might be treated in the same manner as the splays of
the more traditional building. The window elevations of buildings need
to be carefully considered when related to the orientation of the fac¸ades,
with care taken to provide solutions to any exposure where there is a
need for solar shading and protection from glare.
Windows can provide a degree of symbolism; this was apparent on the
type of window used for Anglican churches, which from frequent use
become symbolic of this type of church. Many examples exist of
symbolism in the windows of churches, not least in the stained glass
infilling. A modern example of symbolism in a new extension to an
Anglican church in Boxmoor, where it is clear that the Christian cross is
visible; this may be compared with the original windows.
SOLAR SHADING
This is a subject where expert advice should be sought. There are many
different forms of solar shading; each has its own characteristics,
advantages and disadvantages, and the architect must be sure of the
criteria that should be taken into account when determining the nature of
the shading required and whether some form of adjustability is desirable.
The BRE pamphlet ‘Solar Shading of Buildings’ states that the principal
reasons for needing shading are as follows:
1. To reduce the effect of heat gain from the sun
2. To cut down sun glare experienced through the windows
3. The provision of privacy. This will not normally be a requirement,
but it may be important in certain circumstances.
1. Reduction of heat gain from the sun
At some times of the year this may be of the greatest importance, but its
need will not be permanent; for certain times of the year the heat gain
may be welcome.
The problem is most acute on South facing exposures, but there may be
special conditions in the building such as abnormally high internal heat
gains, or where the building has to be kept at a low temperature.
Since once the heat gain is within the building envelope, it is difficult to
control, the shading system which stops the heat from getting in in the
first place . . . the external system . . . will be better. When contemplating
external shading, it is important to bear in mind the question of structural
stability and the need for periodic cleaning.
2. Reduction of sun glare
Glare may result from a direct view of the sun, by reflection from some
outside source such as the building opposite a North facing exposure, or
by reflection from items inside the building; most noticeably from items
which are the object of attention, such as a business machine or
computer. Glare, unlike heat, can be controlled easily from within the
building.
3. Provision of privacy
This is really the ‘net curtain’ solution, if it is needed at all. Some form of
translucent material which lets through the maximum amount of
daylight, but breaks up the internal image seen from the outside. This
is less important during the day when the light ouside exceeds that
within and there is no disadvantage in placing the diffusing material on
the inside of the window. In certain security situations special materials
have been developed which in addition to the provision of privacy,
capture the shards of glass which occur when a window is broken
Solar shading solutions can broadly be divided into the following three
types:
1 External shading
2 Internal shading
3 Alternative glazing
(Note. The BRE pamphlet further lists as a solution the reduction in the
area of glazing, comparing this with the loss of daylight associated with
the use of some form of tinted ‘sun control’ glazing. On the basis that the
window areas have been calculated correctly in the first place, this must
reduce the daylight available, and for this reason has not been included.)
1. External shading
The following methods are available: Overhangs and canopies, light
shelves, fixed and movable louvres, shutters, vertical fins, deep window
reveals, egg-crate baffles, and roller blinds.
When choosing a method of external shading, the most crucial decision
that must be taken is the long-term viability of the hardware involved,
associated with the climatic conditions which will be experienced on site;
there is also the architect’s preoccupation with the exterior appearance of
the building with which the former is associated.
Whilst it is best to control the heat gain before it enters the building by
external means, any method of external shading can be vulnerable, and
the cost and long-term viability of the method employed must be
established. Comparisons should be made with internal shading methods,
to establish whether the gains in heat control are sufficient to
warrant what will initially be expensive, and possibly a long-term
maintenance problem.
The following list of shading types are some of the options available,
each having their own advantages and disadvantages; although the
visual appearance of each type may have more influence with tbe
architect as to how he sees the impact on the elevation of the building:
Overhangs and canopies
Continental shutters, and awnings
Light shelves
Fixed and movable louvres
Egg-crate louvres
External roller blinds
Brise soleil
2. Internal shading
It must be recognized that any form of shading within the building
envelope is bound to be less efficient as a control of heat gain than an
Windows 31
external device, since the heat which is generated has already entered the
building, and is more difficult to extract; however the type of shade will
be less vulnerable than that outside, will be easier to maintain and to
clean, so that an overall view must take into account all the factors in
coming to a decision. If of course it is not possible to control the solar gain
sufficiently inside, then other means will have to be adopted.
The most common form of control and one used almost universally in
residential building, is the curtain, and provided these are carefully
designed, perhaps with a reflective lining to reduce the solar gain when
pulled across the opening (whilst at night they can keep out the cold) can
be perfectly satisfactory in our temperate climate . . . indeed we welcome
the sun on all but the exceptional day.
A more flexible form of control is the venetian blind, which has the
advantage of adjustability in that it can be raised when not required for
sun control, to permit maximum daylight entry.
The demise of this excellent tool has been predicted for many years, but
it survives, offering excellent glare control, can be motorized when used
in large office projects, can be incorporated within panes of glass to
protect it from damage, and specialist versions are available where the tilt
of the blades can be varied to enable the top of the blind to reflect light up
to the ceiling of a room, whilst the lower blades control the sunlight by
reflecting it away from the building. A further advantage of the venetian
blind is that the surface design of the horizontal slats can be varied to
meet the individual requirements of the building.
The obvious advantage of the venetian blind is that it can and should
be raised when not needed for sun control; the problem is that once
lowered it tends to be left in the closed position. A procedure should be
adopted to ensure that their use is optimized and a simple solution might
be for the office cleaners to open the blinds to ensure that each day starts
with them open to admit the maximum daylight. Venetian blinds have a
lot of life in them yet .
Other types of blind are also available, the vertical hung louvre blind
where the louvre slats can be rotated, or moved to one side offer
flexibility, provide privacy, and together with roller blinds and those of
other materials can provide low-cost solutions in the domestic situation.
The heat gain from the sun can be controlled by the type of glass used,
various options being available, First there are the low emissivity
glazings, developments in this field continue, and the thermal properties
of the glass can now be tailored to give good solar control. Their big
advantage is that they admit higher levels of daylight than the original
tinted versions, and can control heat loss.
Prismatic glazing panels have also proved useful; these are limited to
small panels of prismatic glazing which, when attached to high level
rooflights, can allow daylight to enter, but redirects the sunlight on to the
ceiling of the space, or excludes it altogether.
Finally there are the high tech glazings already referred to under the
glazings available for windows. These include the following:
Electrochromic and liquid crystal glazings, which can be made to
darken on application of an electric current
Photochromic glass, which darkens when sunlight falls on it
Thermochromic glass which alters its transmission value on the
introduction of heat.
None of the latter is in the mainstream of development, and it is
unlikely that these will have a major impact for some years.
32 Daylighting: Natural Light in Architecture
INNOVATIVE DAYLIGHTING SYSTEMS
In 1998 Paul Littlefair of the BRE wrote a seminal paper on this subject,
listing as its aims: to improve the distribution of daylight in a space and to
control direct sunlight.
Of the various methods none can be said to have achieved a universal
application, but each has a specific use and is worthy of mention.
Mirrors. There are many ways in which the interaction of light or
sunlight with a mirrored surface can be used for reflection. From the use
of a large hand-held mirror to throw light into the dark recesses of a
renaissance church for the delight of visitors, to the fixed mirrored
louvres which may be related to vertical windows, installed to direct light
upwards to a ceiling; alternatively there are those which, when related to
glazed openings in a roof, can project light downwards to the interior
(see Case Study of the Central United Methodist Church in Wisconsin pp.
142–143). These tend to be specialist solutions requiring the mirror to be
controlled by a motorized tracking system or heliostat . . . not for general
application.
Prismatic glazing. The principle is to use methods of refraction of light,
rather than reflection. Whilst this method can be applied to vertical
windows, they are perhaps more successful when associated with
systems of rooflight, a good example being Richard Rogers’ redevelopment
of Billingsgate fish market to a modern computer centre (see
Lighting Historic Buildings, p. 64) where sunlight is refracted away from
the occupants to eliminate glare, whilst allowing daylight to the space
below. As these have only a limited application they are expensive.
Light shelves. It is possible by means of comparatively inexpensive
building construction, to provide light shelves. These have already been
mentioned in terms of solar shading, but they are useful also to provide a
view window below the light shelf, with the light above reflected to the
ceiling to redistribute daylight further into the room. It must be
recognized that light shelves do not increase the daylight factors in a
room, but they alter the distribution, assisting in getting light further
towards the back of the room so that uniformity is improved. Light
shelves are relatively cheap to install, and are less subject to damage than
those used externally, but do require cleaning on a regular basis.
Light pipes. Of all the methods of innovative daylighting, the light pipe
has had the most universal application. It is basically a method of
rooflighting, which by means of association with reflective tubes, directs
the light to a lower level. Whilst it can be employed to direct light
through several floors, this has the disadvantage of locating the pipes
through the upper floors, taking up useful floor space.
Light-pipe installations can be associated with a means of ventilation,
and also with sources of artificial light which take over after dark or when
the daylight outside is insufficient, using a light control system. A
particularly useful application has been in domestic buildings, where a
light pipe can be directed to an area in the property, such as an upstairs
landing, which otherwise might receive no daylight.
THE FUTURE
The design of ‘the window’ for a new building is of the first importance,
not only because it will determine the appearance of the building, which
it does, but because it is being asked to take a major role in the control of
the building environment. It will be seen in the Case Studies to follow
later in the book, that with the large increase in ‘passive’ buildings, it is
the window which is at the leading edge of new development,
development of which is as yet far from exhausted.
To quote but one example of leading edge technology; a window
designed by the architects Studio E. and developed to a practical stage by
a manufacturer (Colt) as the ‘interactive’ window, shows an integrated
approach to the environmental control of a building. It is of particular
interest in that it does not demand the need for high tech glass solutions,
using low-cost clear window glass.
The window is designed with the following criteria in mind:
1. The provision of daylight
2. To solve the problem of mechanically controlled building ventilation
without creating draughts
3. To cater for adequate thermal insulation
4. To provide adequate sound insulation for normal circumstances.
5. To control solar gain and diminish sun and sky glare.
The features of the window allow individual control by occupants,
accepted as an important characteristic in user satisfaction as is also the
provision of a view, and can be tailored to suit individual environmental
requirements. This is one example of the way in which industry is being
led by architects to satisfy the needs of the environment.
Finally to quote from conclusions made at a conference at the RIBA in
1996:1
1. Windows are an essential element in building design, for the following
reasons: change, colour, sunlight, modelling, orientation and
view.
2. Window design, associated with the need to reduce energy in buildings,
is leading towards high tech window design, where associated
problems of ventilation, solar gain, glare and noise pollution suggest
an integrated solution.
3. Air-conditioning, at least in this country, will become the exception
rather than the rule.
4. There is a convergence between the provision of optimum visual and
environmental conditions in building, and the world needs to come
to terms with global warming, and the reduction of carbon dioxide
emissions.
The window is an opening in a wall or side of a building admitting light
and often air to the interior. Early windows were developed before the
introduction of glass, so initially windows were left open to the external
atmosphere, or filled by some form of closure to minimize the heat loss at
night. The more sophisticated buildings would have had thin slabs of
marble, mica or oiled paper for this purpose.
In mediaeval times wooden shutters were installed on the interior, and
these were left open or closed to regulate the light and air. With the
introduction of glass, used first in small panes in Roman architecture, the
window as we know it today had its beginnings. The concept of small
panes of glass, divided by bronze or later lead divisions, as used in early
buildings dies hard and window manufacturers still offer these as alternatives to fully glazed windows in new domestic work, however
inappropriate they may appear.
Windows can broadly be divided into two main types, first the window
set in the side walls of a building, and second the opening light set into
the roof, generally known as rooflights.
The daylight penetration from side windows will depend upon the
ceiling height, and in early buildings where the ceiling heights were low,
the penetration of daylight into the building was severely limited . . . with
the design of the important houses of the seventeenth and eighteenth
centuries the ceiling heights were raised and daylight was able to reach
further into the interiors. However as buildings became grander, even this was not enough, and the concept of the rooflight was developed to
introduce daylight into interiors far from the side windows.
Illustrations of some window types indicate the variety of window
shapes that have evolved over the centuries, set into the vertical sides of
buildings.
The horizontal window is perhaps the most well known of all, starting
as it did in mediaeval times, limited by the construction methods of the
day. It is still much used in today’s domestic architecture. Provided the
horizontal window is placed high in the wall the daylighting will
penetrate well into the space, but other features of the window need to be
considered, such as the view out which will be prejudiced where the cill
is too high.
A logical development of this type is where the horizontal window
extends the entire length of the external wall, a device used in nineteenth
century industrial buildings to provide even and sufficient light for
machine operators. This type of window required new structural
techniques to overcome the need for vertical support to the structure
above.
Yet a further example is the clerestorey; found mainly in tall buildings
such as churches, generally associated with other forms of window at
lower level to provide the main daylight. Clerestoreys are placed at high
level to assist in getting daylight further into the interior and to light the
roof structure.
A logical development of the extended horizontal window, is the floor
to ceiling window; as structural techniques were perfected, this type of
window has become almost universal in some types of architectural
programme such as the office. The 1930s saw the innovation of the wrap
around corner window as further structural techniques were made
possible.
Finally and in no chronological order comes the vertical window.
Vertical windows were popular from the fourteenth century, having
perhaps their most glorious period in the eighteenth century, when the
Georgian window with its sophisticated detailing was almost universal.
Tall windows, set apart by masonry at intervals, provided a simple
structural solution and this formed the pattern of development in
residential and other building types for several centuries.
The windowcan be said to be the most important architectural feature of
a building; this is the first experience that a visitor will have when seeing
the building for the first time, and architects have naturally considered the
form of the window and its relationship to the exterior to be vital.
The illlustrations of these buildings along the Embankment illustrate
three different approaches to fenestration. All buildings are of the
twentieth century. The first, on the left, shows the more traditional
separate windows, whilst that in the middle is an example of the
continuous horizontal window, where the individual floors are expressed
as important horizontal bands. The building on the right is the further
development where the window becomes a subsidiary part of the
external cladding, for a total glass fac¸ade. The appearance of the buildings
says little about the success of the daylighting, it says more about
architectural fashion.
ROOFLIGHTS
Whilst rooflights could properly have been said to have started with the
central courtyards or atria of the Roman house, these were open to the
sky and rain; and despite providing daylight to the surrounding dwelling
space, would not have modified the exterior climate in the manner of a
roof light.
The rooflight by definition permits daylight to enter from above
through a glazed opening in the roof protecting the interior from wind
22 Daylighting: Natural Light in Architecture
Three buildings on London’s Embankment
DP Archive
and weather. The early rooflights were perceived either as domes such as
that at Chiswick House with ordinary windows in the sides allowing in
the daylight, but by the nineteenth century structural techniques had
developed sufficiently to allow fully glazed barrel vaults or glazed domes
to be placed above areas of building remote from the side walls and the
proximity of windows. Examples of nineteenth century shopping malls
still exist today where these overhead lights permit daylight to reach
deep into the interior of buildings.
Much innovation was used in the nature of these roofights, and it is of
interest to study the section of the Soane Museum, to see the many
different shapes and sizes of overhead light Soane devised to introduce
daylight to the different spaces, in what was at the time his private house.
By the twentieth century the use of rooflights had been reduced almost
entirely to industrial buildings, and the CIBSE Lighting Guide LG10,
‘Daylighting and Window Design’ (published October 1999) illustrates a
number of different types, the most common of which were the shed
roof, the sawtooth, and the monitor.
The advantages and disadvantages are set out in CIBSE LG10
indicating that the original shed roof, the cheapest solution, has serious
defects and is unlikely to be used today; whilst the many different forms
of monitor roof can be adapted to fit most roof situations to solve the
daylighting problems below.
New roof forms are still being developed for the admission of daylight
to large open areas not restricted to industrial buildings . . . from
supermarkets to universities and swimming pools. An excellent example
of an early solution to a factory in which the services are rationalized and
placed inside ducts which are a part of the overall roof structure and do
not obstruct the daylight is shown in Lighting Modern Buildings pp. 138/9,
the York Shipley Factory; whilst the roof design for the Sainsbury
Supermarket in Greenwich (Case Study pp. 164/167) shows an elegant
solution to the roof form, providing a high level of daylight to the store.
ATRIA
Whilst the word atrium started as the central court of a Roman house,
admitting light and air to the surrounding dwelling space, the word has
taken on a wider meaning as described in the CIBSE LG10 daylight and
window design.
‘An interior light space enclosed on two or more sides by the walls
of a building, and daylit from a roof of transparent or translucent
material and, sometimes, from glazed ends or sides. It permits the
entry of light to the other interior spaces, linked to it by glazed or
unglazed openings.’
The atrium is therefore a further development of the dome or vault
allowing daylight into the central areas of the great houses. The modern
atrium will be covered by a glazed skylight, which, whilst slightly
reducing the amount of daylight, monitors the external atmosphere
keeping out the rain, whilst contributing to ventilation, and reducing the
necessity for air-conditioning.
The proportions of the atrium and the reflective capacity of the
enclosing wall surfaces are critical, and those atria which are wide in
relation to their height, will perform better than taller, narrower spaces in
ensuring that daylight reaches the lower levels. Having said this, the
elongated atrium, which can act as an internal street at the low level, has
proved successful in providing the impression of a daylit interior, even if
due to its height the measured level of daylight at the lowest level will be
much reduced.
In order to optimize the daylight at the lower levels one method is to
set back the floor plans at the higher levels to maximize the direct view of
the sky at the lower; but this has planning limitations and economic
implications for the building owner.
Summarizing the advantages of atrium design
First, the human advantages: by getting daylight into the centre of deep
plan buildings, this provides the occupants with a sense of orientation,
information on the time, weather and the world outside the building;
together with a sense of space and expansive views which may
compensate for the lack of external views from the building.
Second, the environmental advantages: there is a potential for savings
of energy, assistance with the problems of ventilation, and a reduction in
the need for air-conditioning. Depending upon the orientation and detail
of the rooflight there may be a need for some solar shading.
On balance the advantages outweigh the disadvantages, as over the life
of the building the cost of the people who work there greatly exceeds
those of its construction, and the work environment is crucial.
Glazing
There is now a large amount of alternative glazing for windows, and it
is necessary for the architect, in conjunction with his services
consultant, to write a detailed performance specification; this must
include the orientation of the window, its thermal and acoustic
characteristics, together with its capacity for solar shading. This is of
course in addition to the main function of the window which is the
admission of daylight and the introduction of the view to outside.
Further factors which may need to be taken into account, are whether it
is thought desirable to have windows which open or are fixed, and its
relationship to ventilation.
But here our concern is with the types of glazing which are available.
As already stated the main purpose of a window is for the admission of
daylight, and associated with this the view to the exterior.
Glazing types which reduce the impression of daylight significantly,
darken both the interior, and the view, whilst the view from the outside
towards the building makes the fac¸ade look black. It is only when
comparisons are made between the view through a clear glass window
and one with a modifying glass that reduces the daylight, that the results
create disappointment . . . it is true to say that it is human nature to
appreciate the natural environment, with all its variations of colour, light
and shade.
This is particularly true of residential properties where some form of
dark glass has been applied to the fac¸ade, giving the impression of a dull
Windows 25
day seen from the interior, as compared to the view through a clear
window.
There are basically three main types of glazing as follows.
1. Clear glazing
This can be single sheet, double or triple glazed or alternatively a ‘thick’
glass, but the more sheets or the greater the thickness of glass the more
the daylight will be diminished, although the impression of the colour of
the exterior will still be perceived as natural.
Clear glass whilst allowing a high transmission of daylight, will at the
same time and on certain building fac¸ades allow a high transmission of
solar radiation. It is this fact that has led to the development of the more
high-tech glasses designed to reduce solar gain, with their consequent
loss of daylight transmission. Other means such as interplane blinds,
located between the panes of glass, may present a solution. These would
only need to to be installed on fac¸ades subject to solar gain and then only
activated when required.
2. Tinted glass
This is of two types: the first where the clear glass is itself modified in
such a way as to produce different radiant heat transmission characteristics,
therefore the thicker the glass the lower the transmission of
daylight, and the greater the control of radiant heat from sunlight.
The second type of glasses are those coated with microscopically thin
layers of metallic oxides which reflect the heat away and out of the
building. These coatings are applied to the inside layer of glass generally
in association with other panes in a sealed double glazed unit as a
protection, since on their own they would be vulnerable to damage.
These coated glasses can be designed to have high daylight transmission,
due to the very thin layer of reflective material; so that they almost
give the appearance of clear glass, and do not suffer from the objections
raised by tinted glasses which reduce the daylight significantly.
Additionally they do not obstruct the view; however they do have cost
implications, and should only be used where the specification demands
it. Highly reflective glasses are available, but need to be used with care to
avoid the danger of glare to other buildings or motorists.
3. Miscellaneous glazing
A number of different types of glazing are placed in this category, largely
because they cannot be lumped together into a single category; they
consist of the following:
Patterned glass, wired glass, laminated glasses and glass blocks.
Patterned glass
Any number of patterns can be rolled into semi-molten glass, to provide
decorative or diffusing sheets for various purposes, though rarely for
windows, since their capacity for light transmission will be modified.
Wired glass
A similar process is used for the manufacture of wired glass, where a wire
mesh is sandwiched within the thickness of the glass. This used generally
26 Daylighting: Natural Light in Architecture
in security situations, and sometimes as a protection to vulnerable
skylights.
Laminated glasses
Similar methods of manufacture are used for laminating sheets of plastic
between sheets of glass, again used for security reasons as resistance to
impact. These reduce the transmission of daylight.
In museums where exhibits are exposed to daylight, it will be necessary
to control the entry of UV light. This may be done by the use of laminated
glasses, where UV absorbing filters can be laminated between the sheets
of clear glass.
Glass blocks
These were a popular form of glass wall in the 1930s, having thermal
characteristics due to the hollow nature of the blocks, which, because of
their structural nature are still in use today for the introduction of
daylight into new buildings, but special openings will be required to
provide a view.
High tech glazing
There are a number of glazing types which fall into this category, the
most advanced of which are the photovoltaics, where the glass itself is
designed to generate electricity from solar radiation on south facing
exposures, which can then be used within the building to reduce the
energy required for the artificial lighting. Some buildings already use this
method, and the UK Government is now putting research money into its
further development. (See Doxford International Building Park. Lighting
Modern Buildings. Case No 11, pp. 124/5).
Two other types of high tech glass deserve mention, but are not at
present economically viable for general use in buildings.
The first are the photochromic glasses, which respond directly to an
environmental stimulus (temperature or light) rather like the special
sunglasses which are already available which alter their transmission
factor depending upon the brightness of the ambient light; alternatively
there are the electrochromic glasses designed to respond indirectly by the
application of an electrical current which alters their visual and thermal
characteristics. These glasses are still at the experimental stage, but are
likely to be developed further to a point where they may become viable.
The choice of glazing in a large complex is one of the greatest
importance, having implications both on first cost, and the cost in use of
the project.
WINDOW DETAILS
The Georgian window developed in the eighteenth century satisfied all
the known criteria at the time. It admitted unadulterated daylight, it
provided ventilation when required, and it could be controlled by
internal shutters, providing additional security. The splay at the sides
(and sometimes at the cill as well) together with the careful detailing of
the glazing bars, assisted in balancing the brightness between the inside
of the room and the outside. However it did little for thermal insulation,
and on sunny elevations problems of solar gain and the possibility of
glare, were considered less important at the time.
Windows have developed a long way from this point, from the
standard horizontal or vertical windows set into the side walls of the
majority of residential properties, to the window walls commonly found
in modern office blocks. The Georgian window, however, provides some
lessons which have apparently not been learnt today, mostly to do with
the subtlety of the detailing.
1. To assist with modern methods of production, the timber sections
used for dividing the opening parts from the fixed glazed areas, tend to
be heavy, interrupting the view out, particularly where they cross the
sight lines of those inside.
2. Where glazing bars are required, between the different glazed areas
they are often very heavy, and where in the Georgian window the
detailing would have allowed the light to flow around the bar, reducing
its apparent size, the modern glazing bar tends to create unsatisfactory
shadows within and a barrier to the view, further reducing the amount of
daylight available.
3. The use of splayed sides between the window and the wall, to
balance the brightness of the window seen against the brightness of the
interior of the room has almost been forgotten, a lesson learnt in our
mediaeval churches, and which is equally relevant today; the use of the
splay to conceal security shutters may however not be required. This is of
course not to deny the advantages of the modern domestic window, in
terms of both thermal and acoustic capacity, with the introduction of
double glazing.
Whilst the majority of windows are of the type discussed, set into walls at
intervals, either horizontal or vertical, each having their own characteristics
in determining the quality of daylight entering the room. It is more
likely that wall-to-wall windows will be used in modern office blocks, and
these will have their own structural detailing; for example there may be
no need to have horizontal divisions since the glass sizes will generally be
able to stretch between the cill and the ceiling level or spandrel, whilst the
divisions between the wide panes of glass horizontally can be minimized
to avoid the break-up of the view.
It will however be important to consider the junction at the point
where the window meets a wall at right angles at a major subdivision of
the space, or the end of the building; here the reflection factor of the wall
needs to be kept high, to avoid a conflict of brightness. Alternatively the
architect may wish to break up the elevation of his building, by the
introduction of structural elements which articulate the perimeter of the
fac¸ade. In such cases the wide horizontal windows located between the
vertical structures might be treated in the same manner as the splays of
the more traditional building. The window elevations of buildings need
to be carefully considered when related to the orientation of the fac¸ades,
with care taken to provide solutions to any exposure where there is a
need for solar shading and protection from glare.
Windows can provide a degree of symbolism; this was apparent on the
type of window used for Anglican churches, which from frequent use
become symbolic of this type of church. Many examples exist of
symbolism in the windows of churches, not least in the stained glass
infilling. A modern example of symbolism in a new extension to an
Anglican church in Boxmoor, where it is clear that the Christian cross is
visible; this may be compared with the original windows.
SOLAR SHADING
This is a subject where expert advice should be sought. There are many
different forms of solar shading; each has its own characteristics,
advantages and disadvantages, and the architect must be sure of the
criteria that should be taken into account when determining the nature of
the shading required and whether some form of adjustability is desirable.
The BRE pamphlet ‘Solar Shading of Buildings’ states that the principal
reasons for needing shading are as follows:
1. To reduce the effect of heat gain from the sun
2. To cut down sun glare experienced through the windows
3. The provision of privacy. This will not normally be a requirement,
but it may be important in certain circumstances.
1. Reduction of heat gain from the sun
At some times of the year this may be of the greatest importance, but its
need will not be permanent; for certain times of the year the heat gain
may be welcome.
The problem is most acute on South facing exposures, but there may be
special conditions in the building such as abnormally high internal heat
gains, or where the building has to be kept at a low temperature.
Since once the heat gain is within the building envelope, it is difficult to
control, the shading system which stops the heat from getting in in the
first place . . . the external system . . . will be better. When contemplating
external shading, it is important to bear in mind the question of structural
stability and the need for periodic cleaning.
2. Reduction of sun glare
Glare may result from a direct view of the sun, by reflection from some
outside source such as the building opposite a North facing exposure, or
by reflection from items inside the building; most noticeably from items
which are the object of attention, such as a business machine or
computer. Glare, unlike heat, can be controlled easily from within the
building.
3. Provision of privacy
This is really the ‘net curtain’ solution, if it is needed at all. Some form of
translucent material which lets through the maximum amount of
daylight, but breaks up the internal image seen from the outside. This
is less important during the day when the light ouside exceeds that
within and there is no disadvantage in placing the diffusing material on
the inside of the window. In certain security situations special materials
have been developed which in addition to the provision of privacy,
capture the shards of glass which occur when a window is broken
Solar shading solutions can broadly be divided into the following three
types:
1 External shading
2 Internal shading
3 Alternative glazing
(Note. The BRE pamphlet further lists as a solution the reduction in the
area of glazing, comparing this with the loss of daylight associated with
the use of some form of tinted ‘sun control’ glazing. On the basis that the
window areas have been calculated correctly in the first place, this must
reduce the daylight available, and for this reason has not been included.)
1. External shading
The following methods are available: Overhangs and canopies, light
shelves, fixed and movable louvres, shutters, vertical fins, deep window
reveals, egg-crate baffles, and roller blinds.
When choosing a method of external shading, the most crucial decision
that must be taken is the long-term viability of the hardware involved,
associated with the climatic conditions which will be experienced on site;
there is also the architect’s preoccupation with the exterior appearance of
the building with which the former is associated.
Whilst it is best to control the heat gain before it enters the building by
external means, any method of external shading can be vulnerable, and
the cost and long-term viability of the method employed must be
established. Comparisons should be made with internal shading methods,
to establish whether the gains in heat control are sufficient to
warrant what will initially be expensive, and possibly a long-term
maintenance problem.
The following list of shading types are some of the options available,
each having their own advantages and disadvantages; although the
visual appearance of each type may have more influence with tbe
architect as to how he sees the impact on the elevation of the building:
Overhangs and canopies
Continental shutters, and awnings
Light shelves
Fixed and movable louvres
Egg-crate louvres
External roller blinds
Brise soleil
2. Internal shading
It must be recognized that any form of shading within the building
envelope is bound to be less efficient as a control of heat gain than an
Windows 31
external device, since the heat which is generated has already entered the
building, and is more difficult to extract; however the type of shade will
be less vulnerable than that outside, will be easier to maintain and to
clean, so that an overall view must take into account all the factors in
coming to a decision. If of course it is not possible to control the solar gain
sufficiently inside, then other means will have to be adopted.
The most common form of control and one used almost universally in
residential building, is the curtain, and provided these are carefully
designed, perhaps with a reflective lining to reduce the solar gain when
pulled across the opening (whilst at night they can keep out the cold) can
be perfectly satisfactory in our temperate climate . . . indeed we welcome
the sun on all but the exceptional day.
A more flexible form of control is the venetian blind, which has the
advantage of adjustability in that it can be raised when not required for
sun control, to permit maximum daylight entry.
The demise of this excellent tool has been predicted for many years, but
it survives, offering excellent glare control, can be motorized when used
in large office projects, can be incorporated within panes of glass to
protect it from damage, and specialist versions are available where the tilt
of the blades can be varied to enable the top of the blind to reflect light up
to the ceiling of a room, whilst the lower blades control the sunlight by
reflecting it away from the building. A further advantage of the venetian
blind is that the surface design of the horizontal slats can be varied to
meet the individual requirements of the building.
The obvious advantage of the venetian blind is that it can and should
be raised when not needed for sun control; the problem is that once
lowered it tends to be left in the closed position. A procedure should be
adopted to ensure that their use is optimized and a simple solution might
be for the office cleaners to open the blinds to ensure that each day starts
with them open to admit the maximum daylight. Venetian blinds have a
lot of life in them yet .
Other types of blind are also available, the vertical hung louvre blind
where the louvre slats can be rotated, or moved to one side offer
flexibility, provide privacy, and together with roller blinds and those of
other materials can provide low-cost solutions in the domestic situation.
The heat gain from the sun can be controlled by the type of glass used,
various options being available, First there are the low emissivity
glazings, developments in this field continue, and the thermal properties
of the glass can now be tailored to give good solar control. Their big
advantage is that they admit higher levels of daylight than the original
tinted versions, and can control heat loss.
Prismatic glazing panels have also proved useful; these are limited to
small panels of prismatic glazing which, when attached to high level
rooflights, can allow daylight to enter, but redirects the sunlight on to the
ceiling of the space, or excludes it altogether.
Finally there are the high tech glazings already referred to under the
glazings available for windows. These include the following:
Electrochromic and liquid crystal glazings, which can be made to
darken on application of an electric current
Photochromic glass, which darkens when sunlight falls on it
Thermochromic glass which alters its transmission value on the
introduction of heat.
None of the latter is in the mainstream of development, and it is
unlikely that these will have a major impact for some years.
32 Daylighting: Natural Light in Architecture
INNOVATIVE DAYLIGHTING SYSTEMS
In 1998 Paul Littlefair of the BRE wrote a seminal paper on this subject,
listing as its aims: to improve the distribution of daylight in a space and to
control direct sunlight.
Of the various methods none can be said to have achieved a universal
application, but each has a specific use and is worthy of mention.
Mirrors. There are many ways in which the interaction of light or
sunlight with a mirrored surface can be used for reflection. From the use
of a large hand-held mirror to throw light into the dark recesses of a
renaissance church for the delight of visitors, to the fixed mirrored
louvres which may be related to vertical windows, installed to direct light
upwards to a ceiling; alternatively there are those which, when related to
glazed openings in a roof, can project light downwards to the interior
(see Case Study of the Central United Methodist Church in Wisconsin pp.
142–143). These tend to be specialist solutions requiring the mirror to be
controlled by a motorized tracking system or heliostat . . . not for general
application.
Prismatic glazing. The principle is to use methods of refraction of light,
rather than reflection. Whilst this method can be applied to vertical
windows, they are perhaps more successful when associated with
systems of rooflight, a good example being Richard Rogers’ redevelopment
of Billingsgate fish market to a modern computer centre (see
Lighting Historic Buildings, p. 64) where sunlight is refracted away from
the occupants to eliminate glare, whilst allowing daylight to the space
below. As these have only a limited application they are expensive.
Light shelves. It is possible by means of comparatively inexpensive
building construction, to provide light shelves. These have already been
mentioned in terms of solar shading, but they are useful also to provide a
view window below the light shelf, with the light above reflected to the
ceiling to redistribute daylight further into the room. It must be
recognized that light shelves do not increase the daylight factors in a
room, but they alter the distribution, assisting in getting light further
towards the back of the room so that uniformity is improved. Light
shelves are relatively cheap to install, and are less subject to damage than
those used externally, but do require cleaning on a regular basis.
Light pipes. Of all the methods of innovative daylighting, the light pipe
has had the most universal application. It is basically a method of
rooflighting, which by means of association with reflective tubes, directs
the light to a lower level. Whilst it can be employed to direct light
through several floors, this has the disadvantage of locating the pipes
through the upper floors, taking up useful floor space.
Light-pipe installations can be associated with a means of ventilation,
and also with sources of artificial light which take over after dark or when
the daylight outside is insufficient, using a light control system. A
particularly useful application has been in domestic buildings, where a
light pipe can be directed to an area in the property, such as an upstairs
landing, which otherwise might receive no daylight.
THE FUTURE
The design of ‘the window’ for a new building is of the first importance,
not only because it will determine the appearance of the building, which
it does, but because it is being asked to take a major role in the control of
the building environment. It will be seen in the Case Studies to follow
later in the book, that with the large increase in ‘passive’ buildings, it is
the window which is at the leading edge of new development,
development of which is as yet far from exhausted.
To quote but one example of leading edge technology; a window
designed by the architects Studio E. and developed to a practical stage by
a manufacturer (Colt) as the ‘interactive’ window, shows an integrated
approach to the environmental control of a building. It is of particular
interest in that it does not demand the need for high tech glass solutions,
using low-cost clear window glass.
The window is designed with the following criteria in mind:
1. The provision of daylight
2. To solve the problem of mechanically controlled building ventilation
without creating draughts
3. To cater for adequate thermal insulation
4. To provide adequate sound insulation for normal circumstances.
5. To control solar gain and diminish sun and sky glare.
The features of the window allow individual control by occupants,
accepted as an important characteristic in user satisfaction as is also the
provision of a view, and can be tailored to suit individual environmental
requirements. This is one example of the way in which industry is being
led by architects to satisfy the needs of the environment.
Finally to quote from conclusions made at a conference at the RIBA in
1996:1
1. Windows are an essential element in building design, for the following
reasons: change, colour, sunlight, modelling, orientation and
view.
2. Window design, associated with the need to reduce energy in buildings,
is leading towards high tech window design, where associated
problems of ventilation, solar gain, glare and noise pollution suggest
an integrated solution.
3. Air-conditioning, at least in this country, will become the exception
rather than the rule.
4. There is a convergence between the provision of optimum visual and
environmental conditions in building, and the world needs to come
to terms with global warming, and the reduction of carbon dioxide
emissions.
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