Before choosing your building system, you should analyze your ability to benefit from the use of high thermal mass building systems in your climate. Thermal mass can be any material that can capture heat and hold it for hours after exposure, then release it slowly. The most common materials used for exterior thermal mass are types of masonry, e.g., stone, rammed earth, adobe or concrete.
During the hot days, as these materials absorb heat, it slowly moves inward through the walls. About the time that the sun sets, the heat has reached the interior space. Throughout the evening, as temperatures drop, the exterior walls cool down because the outdoor temperature is well below the inside temperature, directing the flow of heat toward the outside. The materials continue to release their heat as they become increasingly cooler, until they reach the ambient nighttime low temperature. Then, as day breaks, the interior is able to maintain these cooler temperatures well into the hot afternoon hours as the direction of flow reverses again. This can be one of the most efficient natural heating and cooling strategies available, and effectively replace mechanical heating and cooling systems-for big cost savings!
This is an application of the second law of thermodynamics: heat moves from warm toward cold, and only works if nighttime temperatures where you live drop rapidly to well below the desired indoor temperature and you use very thick thermal mass exterior walls. Building sites with big diurnal temperature swings (high temperatures during the day with starkly contrasting low night temperatures), such as those located in mountainous or arid regions, such as the desert southwest, will benefit the most from using exterior thermal mass.
In areas with long, hot humid summers, where nighttime temperatures don’t really cool off that much, exterior thermal mass does not work very well, as the walls never get the chance to cool sufficiently. In this case, the outdoor heat is constantly moving inward, making it impossible for your air conditioner to keep up. These walls do not act like insulation.
If you live in an area with long, cold winters, where it never gets warm during the day, these types of mass walls will constantly move the heat outward. This is one of the reasons why native peoples build lightweight homes in warm, humid parts of the world and heavy-mass earthen-walled homes in arid deserts with hot days and cold nights. They may not have known physics as such, but they figured out what worked through trial and error.
There are other ways to use thermal mass if you don’t live in that high-diurnal type of climate. Trombe walls and tile floors abutting south- facing windows that collect solar heat all day release it into the living space at night. Venting the wall assembly correctly to create convection loops intensifies the effect, which can be controlled by closing the vents at night and sizing overhangs for protection in the summer.
In cold climates (and some people even use this strategy in warm climates), we can add thermal mass inside the home as a component of our passive heating design strategy. Concrete foundations that serve as the finished floor (i.e., stained or polished concrete), ceramic tile floors and interior stone walls are the most common thermal mass features used in these climates. To serve this function, the materials must have measurable direct solar exposure for several hours during the day or be used in conjunction with an indoor heating appliance, like a masonry fireplace. Passive solar design strategies incorporate windows and shading devices to control solar exposure to these mass features. It also means that radiant heat is not blocked by any materials specifically designed to do that, such as low solar heat gain coefficient (SHGC)- rated windows. The types of materials specified for each location and application must work together for passive systems to optimize benefits. Again, these strategies can be used to lower your mechanical heating and cooling costs, saving energy and money.
It is important that you understand and build what is right and will work well in your climate. Don’t believe that everything you see on TV is right for your climate and will work well where you live. If the building advice or system comes from a cool, rainy and mild climate and you live in a hot and humid climate, beware. Using the perfect applied building science for Minneapolis when building in Atlanta will prove disastrous, and vice versa!
