Thermal Mass Improves Efficiency

Dear Jim: I am adding a room and doing some general remodeling in my home. I have heard adding thermal mass to my home can lower my utility bills and improve comfort. How can I do this in the old and new rooms?

-- Dean S.

Dear Dean: What you heard about adding thermal mass to a home is correct. The primary energy savings come from the increased comfort which allows you to set the furnace thermostat lower during winter and higher during summer. It also can actually reduce the heat loss, particularly in the kitchen and bathrooms, where heat is generated from cooking or bathing.

This is the concept that keeps log homes fairly comfortable in all weather conditions. Even though walls constructed of solid logs have a low insulation R-value relative to an insulated framed wall, log homes stay comfortable. This is the result of the tons of thermal mass from the heavy solid logs.

Any object in your home absorbs heat from the warm air or hot water radiators when the furnace or boiler comes on. Once the furnace stops, these warm objects radiate heat back to the air and to your body. An object with higher thermal mass can hold more heat energy.

The keys to lowering your utility bills with thermal mass are to select the proper materials and locate them so they absorb excess heat and then slowly release it to the room air.

Thermal mass of various materials is rated by their heat capacities.

Water has a high 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.

For your new construction, install a thick concrete floor even if it is over a basement or crawl space. Precast concrete panels are a good choice. Using radiant floor heating is effective with this design. Use decorative solid brick or stone for the interior wall where the new room attaches to the existing house. Tile flooring adds additional thermal mass to the room.

In order to increase the thermal mass in your existing rooms, consider installing a ceramic tile floor in the foyer. This is particularly effective if the sun shines in through windows in the door. The thickness and weight of the tile is more important than its color, although darker colors are slightly better.

Brick panels can be added to interior walls. Although they are not as thick as a new solid brick wall, they add some mass and look realistic.

If you use a fireplace, build a thick raised brick or stone hearth. In the kitchen, install thick granite, marble or slate countertops and tile backsplashes to absorb the excess heat when cooking and baking.

Houseplants with large pots of moist soil create thermal mass. If you store water in jugs for emergencies, store them in a closet or under the sink instead of in the garage or basement. After taking a hot bath, let the water cool down first before draining the tub.

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