Thermal Mass: What It Is and When It Improves Comfort
Heavy or massive objects like masonry, earth, and water can hold a lot of heat. Because of this capacity to act as a heat source (warming their surroundings) or a heat sink (drawing heat from and cooling their surroundings), materials with thermal mass affect comfort both indoors and out. (Oceans and lakes, for example, moderate air temperature changes because their thermal mass acts as a buffer.)
Buildings in climates with large diurnal (day-night) temperature swings, like the high-elevation Southwest, offer a classic example of the time-lag effect of thermal mass. Adobe and other types of masonry walls absorb intense daytime heat, keeping temperatures comfortable inside. During the cold night, the walls pour out their accumulated heat, keeping the inside warm. By morning the walls, if they are designed correctly, can again absorb the daytime heat.
In most of North America, under most conditions, temperatures vary over the course of 24 hours but stay either above or below the comfort level. Heating or cooling is then necessary for most buildings, so building a tight envelope with materials that insulate well, or have a high R-value, should be the top priority. Do materials with high thermal mass also insulate well? Some manufacturers would like us to think so, wielding a metric called “effective R-value” as evidence.
And indeed, the time-lag provided by thermal mass saves energy in some climate conditions, but the effect is very circumstantial (see EBN Vol. 7, No. 4). As a more general rule, the most effective thermal storage materials are fairly good conductors and thus poor insulators. A thermal mass like poured concrete insulates poorly with R-0.08 per inch, compared with R-3.70 for cellulose. But even in climates where insulation is the priority, buildings can use thermal mass.
For example, night-flush cooling and passive solar heating can be viable strategies in the same location during different seasons. Using thermal mass inside a well-insulated building envelope aids both strategies, because the mass can absorb solar heat during the day and release it at night. In climates where significant cooling is needed, thermal mass outside the insulation can also save costs by delaying the peak cooling period until nightfall, when office buildings are unoccupied and need less cooling; electricity may be less expensive; and cooling equipment operates more efficiently. Many uses of thermal mass can reduce energy consumption and improve comfort. In buildings that are only occupied sporadically, however, it is often more efficient to minimize the interior mass so they can warm up (or cool down) quickly when needed. Also, thermal mass can be expensive and space-intensive, so architects and builders tend to use it where it can also serve other functions: as structure, as a durable interior surface like flooring, or in a heating system like a masonry stove.