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Energy Mass Management

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

Thermal mass can improve comfort because it moderates temperature swings in your house, particularly in the summer.

During winter, moderating indoor temperature swings can result in less heat being lost to the outdoors. This helps in rooms where heat is generated indirectly, such as cooking in the kitchen or bathing in the bathroom. It also helps in rooms with south-facing windows, which receive passive solar heating. A low-thermal-mass room can overheat and lose heat. A high-thermal-mass room overheats less and stores the heat.

During the summer, thermal mass can delay the need for air conditioning until later in the day. Once the outdoor temperature drops, windows can be opened and the thermal mass cools overnight.

If your house is heated by a heat pump with electric resistance backup heat, additional thermal mass is a definite plus. Heat pump efficiency and its heat output are greater when the outdoor air is warmer, such as during a sunny afternoon. With adequate thermal mass, enough heat may be stored indoors so the backup resistance heating does not have to come on or run as long when the outdoor temperature drops at night.

The keys to increasing the thermal mass in your home are to select the proper materials and incorporate as much of them as possible. Thermal mass should be located evenly throughout your house, with more in areas where it can absorb excess heat and then slowly release it.

The thermal mass of various materials is rated by their heat capacity properties. 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 about 31, brick about 27, and stone/tile ranges from 18 to 36. Their natural thermal properties, in addition to densities, determine the numbers.

For your new construction, install a thick concrete floor. Precast concrete panels are a good choice. 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.

In order to increase thermal mass in 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 color, although darker colors are slightly better. Ceramic tile also has a unique thermal property that allows it to easily reradiate the heat.

Decorative 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. The hearth absorbs heat from the fire and stays warm for hours after the fire is out.

In the kitchen, install thick granite, marble, or slate countertops and tile backsplashes to absorb excess heat when cooking and baking. If you hand-wash dishes, leave the hot water in the sink until its thermal mass gives off its heat to the room air as the water cools.

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 before draining the tub.

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