The Importance of Thermal Mass
Thermal mass is akin to a battery for heat. It takes time to charge (i.e. heat up) and also time to discharge (i.e. cool down). This has the effect of buffering short, intense periods of heat gain or heat loss. Consider how a bowl of soup stays hot for a long time while a less “thermally massive” material, like popcorn, almost immediately returns to room temperature. Building materials can act in the same way.
Understanding Thermal Mass
Thermal mass is a specifically quantifiable metric that depends on the density of a material and what is known as its “specific heat capacity”. For example, the specific heat of concrete is .880 (J/g*K) and it has a density around 2,400 kg/m^3. Multiplying these two values yields a thermal capacity of 2.112 megaJoules/m^3.
Thermal mass can work with both ambient temperature and solar irradiance to decrease heating (and sometimes cooling) needs. When thermal mass is designed to mainly interface with ambient temperatures, it is most useful in climates with a large diurnal temperature swing. For example, adobe buildings are popular in Santa Fe, New Mexico where the temperature range between night and day often exceeds 30 °F. However, thermal mass can also work as a passive solar strategy such that by placing it in the path of direct solar radiation, it becomes warmer than ambient temperature and therefore provides benefit even in perpetually colder climates.
Thermal mass strategies can vary with the type of material used and how it is incorporated into the building. Some of the most thermally massive materials include water, concrete, and adobe. As you might expect, some materials are easier to incorporate into buildings than others.
The placement of thermal mass can be divided into categories of direct gain (in which the thermal mass is placed within the living space) and indirect gain (in which the thermal mass is between the sun and the living space). Examples of direct gain are a concrete floor or an internal brick wall. Indirect gain systems may be a trombe wall, roof pond, or sun space.
Sizing Thermal Mass
Using the Load Collector Ratio method (developed by the Los Alamos National Laboratory), one can refer to tables that provide the expected heating savings based on the amount of thermal mass in the design, the area and properties of windows, and the climate.
Thermal mass can also be simulated using various energy modeling programs, although they vary in how much of an effect they attribute to thermal mass. RemRate, for example, seems to under-predict savings. The Load Collector Ratio is unique in that it is based on experimental results rather than theoretical calculations.
Thermal mass is an oft-overlooked component affecting building energy consumption, even though it can have a profound effect. Even for a building that does not intend to utilize “passive solar strategies”, the floors, walls, and windows inevitably create a configuration of thermal mass and solar gain that should be considered by the building owner as part of the lifetime building performance. At the very least, parameters such as window properties and slab thickness should be varied within an energy model to see if they positively or negatively affect overall energy performance.