1.4 Optimal Use of Mass and Insulation
Note: Italicized text is reprinted from the California Energy Commission Passive Solar Handbook.
A heavy wall must have two qualities in order to dampen diurnal changes in the exterior environment and thus keep the internal temperature of a room relatively constant:heat capacity — the ability to store heat, and low heat conductivity — the ability to resist, or to insulate against heat flow.
If one intermittently exposes an adobe brick first to a blow torch and then to cold water (and if each exposure time is relatively short) the temperature of the brick never reaches either extreme, but oscillates somewhere in between. The heat capacity of the brick keeps its temperature from rising rapidly with the small heat addition, or dropping rapidly with the small heat extraction. The brick’s insulating quality prevents heat from entering or leaving very rapidly.
Adobe, however, does not happen to have the optimum combination of heat capacity and insulation. This problem can be resolved by the way the material is used which is as important as what material is used. The most effective way of maximizing the two qualities — heat capacity and insulation — in a building wall is to use two separate materials. Ideally, one would choose a material with … little resistance to heat flow …. By placing the insulating material next to the external environment, little heat is allowed into or out of the building. And with the high heat capacity material next to the inside environment, what heat does enter or leave (primarily through windows and interior heat generation) can’t change the temperature of the heat capacity material rapidly.
Thus, little heat is let in or out, and the high capacity material slowly stores heat. The building’s thermal mass damps out temperature fluctuations.
Monolithic, in agreement with the Passive Solar Handbook, concludes that a more ideal wall than adobe alone would be one made of externally insulated adobe or externally insulated concrete. This concept of externally insulated, high mass construction is common to all of the passive concepts in this handbook except those using isolated heat storage arrangements, such as the rock bed thermo-siphon system.
If you think an externally insulated, high mass wall is effective, consider the Monolithic Dome roof/ceiling. It stores and releases huge amounts of energy, and it is easy to tap into this massive simple storage.