CLIMATE RESPONSIVE ARCHITECTURE - Thermal mass: walls and floors


Thermal mass surfaces should be large enough to store adequate heat & cold.
There are three basic approaches to sizing the area & thickness of thermal mass. The first is appropriate for storage system like trombe walls, roof ponds & sun spaces, where the collection area equals the surface area of the mass storage and the thickness is the primary consideration. The second is more appropriate for direct gain, where both mass surface area and thickness is the primary consideration. The second is more appropriate for direct gain, where both mass surface area and thickness are variable. Because the storage mass is in the inhabited space in direct gain schemes, air and mass temperatures can not be allowed to get too high or too low, so this system depends on the larger transfer areas afforded by relatively thin masonry walls. The third approach is when water is used for storage, where sizing is based on volume and the surface areas can be smaller, because heat is transferred, by convection more readily between the surface and the bulk of mass in water than it is in masonry.

To size the mass for passive solar heating, enter the graph, sizing Thermal Mass for Direct Gain Rooms & Sun spaces, on the horizontal axis with the estimated solar savings fraction for the design. Move vertically to intersect the diagonal line for the mass type and thickness, then horizontally to read the recommended ratio of mass area to solar glazing area.

Sun spaces are assumed to have a masonry wall between the sun space and the room; this should be 230-305mm thick. The floor and side walls of the sun space should also be massive, with a minimum of 3m2 of exposed mass per m2 of south facing sun space glazing. Sun spaces with an insulated wall between the sun space and the room they heat by convection should have 170-365L of water per m2 of south collector glass.

Evaluation of the dynamic thermal performance of massive wall systems combines experimental and theoretical analysis. For complex three-dimensional building envelope components, it is based on dynamic three-dimensional finite difference simulations, whole building energy computer modeling, dynamic guarded hot box tests, and sometimes, comparative field performance investigations. Dynamic hot box tests serve to calibrate detailed computer models. It is important to know, that all these costly and time-consuming steps are not necessary for all wall assemblies. For simple one-dimensional walls, only theoretical analysis can be performed without compromising accuracy.
Masonry or concrete walls having a mass greater than or equal to 146 kg/m2 (30 lb/ft2) and solid wood walls having a mass greater than or equal to 98 kg/m2 (20 lb/ft2) as massive walls. They have heat capacities equal to or exceeding 266 J/m2K (6 Btu/ft2 0F). The same classification is used in this work.
Since 95 percent of U.S. residential buildings is constructed using light-weight building envelope technologies, energy performance of wood-framed walls is utilized as a base for performance comparisons in this work. A wide range of traditional wood-framed wall assemblies is considered, R-values from 0.4 to 6.9 Km2/ W (2.3 to 39.0 hft2 F/Btu). Energy performance data, generated by whole building energy simulations for residential buildings containing wood-framed walls, is compared against similar data generated for four basic types of massive walls. Each wall type consists of the same materials, concrete and insulating foam. Within the same type of walls, all sequences of materials are the same, however, individual material thicknesses change to match necessary R-values. Massive wall R-values range in this work from R - 0.88 m2K/W (5.0 hft2F/Btu) to R - 3.03 m2K/W (17.2 hft2F/Btu). Four basic material configurations are considered for massive walls:
- Exterior thermal insulation, interior mass (Intmass)
- Exterior mass, interior thermal insulation (Extmass)
- Exterior mass, core thermal insulation, interior mass, and (CIC)
- Exterior thermal insulation, core mass, interior thermal insulation (ICI).
The four types of massive walls above approximate most of the currently used multilayer massive wall configurations. For example, the first two wall configurations may represent any masonry block wall insulated with rigid foam sheathing. The last wall configuration may represent Insulated Concrete Forms (ICF) walls. Therefore, results presented in this work can be used for approximate energy calculations of most massive wall systems.

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