The different construction materials that make up our homes have different thermal mass, that is, they have a different ability to absorb and store heat. Wood and other lightweight materials have a low ability: they don’t store heat for long; they release the heat quickly.
Heavy, massive materials such as stone, brick and concrete, on the contrary, have a high thermal mass. They can store the solar heat gains by up to 10-12 hours, before beginning to release it. And that can be used in some heating and cooling strategies.
High mass strategies require some prerequisites to be met. Issues like design, insulation and exposure or protection from the sun are critical for good results. Otherwise, thermal mass strategies can fail or be counterproductive.
High thermal mass strategies are part of the passive solar design principles, formulated in the 1970s and 1980s, but are now being subject to some criticism.
thermal mass strategies
High thermal mass strategies requires changes in ambient temperatures, in order to work properly.
Do not implement high mass strategies in places where the diurnal temperature range is small; low-thermal materials can be a better choice.
And the same is true in cold climates, in regions where solar gains are small; the more important, in this case, is massive insulation; wood and lightweight materials can be better choices for thermal comfort.
Heating with high thermal materials
The basic idea when using high-mass strategies is simple: to allow materials to store solar heat during the day (or from the heating system) in order to benefit from that same heat throughout the night.
The image at right shows how the idea works, with a high mass floor capturing the sunlight during the day and releasing it during the night (it could also be an interior wall - or the inside surface of an exterior wall, properly insulated in its outer surface – designed to capture the sun from clerestory windows (second image, below).
Cooling with high thermal materials
The idea behind passive cooling is also simple: to allow night breezes and convection air movement to pass over the floor (or walls), drawing out the heat.
As long as the floor and walls are properly protected from the summer sun (by using overhangs, trees, shutters, awnings…) and there is night breezes, this strategy can be effective.
The image at right shows the mechanism. Notice the sun’s angle and how the overhang protects the house from it.
The climate issue
High thermal mass strategies vary a lot with the climate. As mentioned earlier they may work against us in climates where the diurnal range of temperature is small (lower than 18ºF/10°C); and they rarely make sense in cold climates.
Thermal mass rules
Do not install too much thermal mass in your walls and floors. The ideal mass varies according to the climate and the area of the windows, and it is typically estimated at between three and six times the area of that glazing.
The thickness of thermal mass walls is often estimated to 4-5 inches (10cm-13 cm); thermal mass floors can be slightly thicker, but never more than 8 inches (20 cm). See, for more details: Thermal Mass Rules for Floors and Walls
High mass construction is typically not recommended in humid climates with a small diurnal range. High mass walls and floors will not allow a quick release of the heat; low mass buildings are easier to cool, in these climates.
The risk of overheating, and large periods where the strategy doesn't work, is significant.
High thermal mass construction relies heavily on design, to avoid unwanted heat loss and overheating. Design means, in this case, the size and location of the windows, and issues like overhangs, shade and natural ventilation, but also the spots where thermal mass is located and its amount (see box).
The location of thermal mass within the building (concrete floors and walls) is critical. A special attention should also be paid to the windows or other glazing elements facing the winter sun; they have to be protected during the summer, and details like dark-colored floors may also be important (do not use light-colored floors, or carpets!).
More complex strategies may involve clerestory windows, natural ventilation issues (especially in hot climates), trombe-walls or sunspaces...