If insulation is the ability of a material to hold in temperature, thermal mass is the ability of a material to absorb and store temperature. Sometimes referred to as a heat sink, we can use this to keep a building cool in the summer and warm in the winter in one of the simplest methods to reduce heating and cooling costs.

If you’ve ever gone swimming at night, you’ve experienced thermal mass. The sun heats up the pool water all day, and the water stores that heat. At night, when the surrounding temperature drops and the water releases this heat, the pool feels like bathwater. You’re feeling the stored heat being released in the water. This is thermal mass in action (water has a very high thermal mass).

Heavy, massive materials such as concrete, brick, and stone have a high thermal mass. This is why your basement is always so cool in the summer, and why your dog sleeps on the tiles. The mass of the concrete basement and tile floor store in the cold and release it when the air around it is warm.

Tip: An exposed concrete slab is a simple way to utilize thermal mass. In the winter, let the sun come through the windows and warm up the concrete. In the summer, use curtains or overhangs to block the sun from coming in, keeping the concrete cool. Just this simple method will ensure you maintain a consistent temperature year-round.

Thermal mass is ideal in a climate where it’s warm during the day and cool at night. Build the south-facing walls of your home out of adobe, rammed earth, or concrete. The sun will warm the walls all day, and keep the house warm at night. (Find more information on natural building methods )

You can use thermal mass to maintain a consistent temperature in your home. Materials with a high thermal mass are not affected by sudden temperature swings; they take a long time to heat up and cool down. Using thermal mass in the winter requires positioning a mass wall such that the sun warms it all day (see the figure below). At night, after the temperature drops, the wall will release heat and keep you warm.

Because thermal mass requires changes in temperature in order to work, it won’t work in areas that are always hot or always cold. In fact, using thermal mass in these areas will work against you. For example, a concrete building in a hot desert, where it’s hot all day and night, will never have the chance to release the heat. This building will be even hotter inside from the saturated thermal mass.