Five Ways Thermal Mass Can Improve Your Home
Thermal mass is any material that holds significant heat — for buildings, it is most often masonry, concrete, or earthen walls. Many builders and professionals ignore it, but this is a costly and unfortunate mistake. Here’s why:
Thermal Mass Lowers Heating Bills
Thermal mass can absorb heat from the sun in homes designed for passive solar heating. In the Midwest, it is reasonable to expect 20 to 50 percent of such a home’s heat requirements to be freely supplied by the sun. In the Southeast, this figure is more like 50 to 70 percent, and in the Southwest, up to 80 or 90 percent .
Thermal Mass Lowers Cooling Bills
It may seem ironic that something that holds heat can also decrease cooling needs, but it is true. Consider the large temperature swings in many regions where it is too hot for comfort during the day and too cold for comfort at night. A building structure with a high thermal mass will tend to resist this cycling effect, keeping the interior cooler during the day and warmer at night.
Consider how much more stable the temperatures are in regions surrounded by water — a substance with high heat capacity. The daily temperature swing near Las Vegas is almost double that in San Francisco, even though the average temperatures are pretty similar.
Thermal Mass Makes Structures Stronger
Materials with high thermal mass are often stronger than the alternatives. Large multifamily and commercial buildings rely on concrete (which has high thermal massl) to add structural strength. Even simpler, more “rustic” construction types such as rammed earth and earthbag homes can withstand gale force winds and tornadoes. The Borough House Plantation of South Carolina has many homes with walls built simply out of compacted dirt, or “rammed earth” that have been standing for almost 200 years .
Thermal Mass Insulates Against Noise
Noise creates stress and is second only to air pollution as an environmental cause of health problems . Some noises can be blocked by traditional insulation, but others, especially low frequencies like a booming subwoofer, require a large mass of material between the source and the living quarters. Materials with high thermal mass tend to be dense and great at dampening sound waves.
Thermal Mass is More Comfortable
Even if the air temperature is the same, you may often feel more comfortable if the interior surfaces of your home are at a lower temperature. Your body loses heat not only through contact with the air, but through radiation to other surfaces. This is why you may feel colder when beside a window on a cold day, even if the air temperature is roughly the same as anywhere else indoors. If the thermal mass buffers the ambient temperature swings correctly, you may find that you don’t need to turn up the heat or AC quite to what you would think.
Sources Passive Solar Design Handbook, Volume 3, Page 18.  Wikipedia: Borough House Plantation  New Scientist Magazine: Noise kills, and blights lives in Europe