Increasing thermal mass

If you’ve read about increasing the thermal mass to improve energy but don’t know what that means, it’s fairly simple. It means increasing the ability of materials to retain heat energy.

This can be done anytime, but it is ideal when remodeling. Increasing the thermal mass does not require a “Fred Flintstonesque” decor with a pile of rocks in the center of every room. Actually, most improvements are quite attractive.

Increasing thermal mass means having the house structure and interior objects absorb and hold as much heat energy as possible. The heavier the items in your home, the more mass it has to absorb heat. As outdoor temperatures change, thermal mass helps moderate indoor temperature swings to improve comfort and efficiency.

It is comparable to the way a heavy, high “physical” mass automobile provides a smoother ride by absorbing the energy from bumps.

Old-fashioned solid log and heavy timber-framed houses used this thermal mass concept. Even though a log house has a small amount of actual insulation R-value when compared to a typical framed house, its utility bills are often not significantly higher, particularly during summer.

Winter months

By reducing the indoor temperature swings during winter months, less heat is lost through walls and windows. This is particularly true in rooms where heat is generated, such as the kitchen or bathroom, or rooms with large south-facing windows.

During the day, heat produced 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.

Summer months

In summer, high thermal mass delays the need for the air conditioner to kick on 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 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. The thermal mass of air is very low, so the cold outdoor air quickly absorbs heat from the indoor thermal mass.

Selecting materials

Increase the thermal mass in your house by selecting proper high mass, or heavier, materials. They should be located throughout, but concentrate on rooms that generate heat or tend to overheat during summer. When 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 materials is rated by heat capacity properties. 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 or tile ranges from 18 to 36 depending upon type. Their natural thermal properties, in addition to their densities, determine the relative heat capacity numbers.

A dark, ceramic-tile floor near a door or window is an excellent source of thermal mass. Decorative walls featuring solid bricks work well with a fireplace or large windows. Heavy granite or slate countertops in a kitchen are effective, as are wooden floors.

Damp soil in large potted plants can store heat. For emergencies, storing old milk jugs full of water under cabinets and near your heating system increase thermal mass.