Dense building materials such synthetic stone increase your home’s indoor thermal mass due to their ability to retain heat energy.
Photo by El Dorado Stone
Question: I’ve read that increasing thermal mass can improve energy efficiency. What exactly does this mean? Is it something we can do when we remodel our older home?
Answer: Increasing the thermal mass means increasing the ability of materials in your home to retain heat energy. This can be done anytime, but it is ideal when remodeling. Plus, many of these improvements are quite attractive.
The concept of increasing thermal mass means having your house’s structure and interior objects absorb and hold as much heat energy as possible. The heavier the items in your home—for example, slate flooring or brick interior walls—the more mass there is to better absorb heat. As outdoor temperatures change, thermal mass helps moderate indoor temperature swings to improve comfort and efficiency.
Thermal mass can save energy in your home by reducing indoor temperature swings during winter months—less heat is lost through the walls and windows. This is particularly true in rooms where heat is generated, such as the kitchen, bathroom, or rooms with large, south-facing windows.
On cold winter days, heat generated by the HVAC system is absorbed by the home’s thermal mass. At night, heat is released to warm the home. The greater the mass, the more it will keep the home comfortable.
During summer months, high thermal mass can help keep rooms cooler and delay the need for the air conditioner to kick on in the morning and throughout the daytime. Unless high humidity is an issue, opening windows at night and running a whole-house or window fan allows the thermal mass to lose the heat stored from the daytime. Close the windows in the morning, and the cooled thermal mass absorbs heat coming in without causing the room temperature to rise as fast.
You can increase the thermal mass in your house by selecting high-mass, or heavier, materials and locating them throughout the home. Concentrate on rooms that generate heat or tend to overheat during the summer. If you are building a house or adding rooms, use heavy foam insulating sheathing on the outside of the wall framing. This allows the framing lumber to become part of the interior thermal mass.
The thermal mass of various materials is rated by heat-capacity properties. For example, water has a high relative-heat capacity of 62.4 per cubic foot, compared to drywall at only 1.3 per cubic foot. Wet soil rates about 55, concrete is about 31, brick is about 27, and stone/tile ranges from 18 to 36, depending on type. Their natural thermal properties, in addition to their densities, determine the above relative numbers.
As you remodel, opt for heavier materials over lighter ones where you can. Installing dark, ceramic-tile flooring near a door or window provides an excellent source of thermal mass. A decorative brick wall works well in a room with a fireplace or large windows. Heavy granite or slate countertops in a kitchen or wooden floors can also be a good source for thermal mass.
Even the damp soil in large potted plants can store heat, while the plants naturally purify indoor air. Old milk jugs full of water, stored under cabinets and near your heating system, will also increase thermal mass.
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