Jonathan Hammond, a Davis, Calif., architect, recently designed a heat-absorbing sofa for a client who wanted more thermal mass in his home. It is made of three water-filled culvert pipes (one to sit on, two for the back) covered with appropriate upholstery. It works very well, apparently, as do the 55-gallon drums, also water-filled, that are stacked up in front of a south-facing window in Mr. Hammond's own home.

Richard Karg of Brunswick, Maine, does similar things with water-filled plastic jars in the basement of his home.In this instance the water absorbs the heat thrown out by a wood- burning stove.

Heat-storing materials, often referred to as thermal mass, are an essential part of any solar house. Without them, the house would become unbearably hot on a sunny day and quickly become cold the moment the sun went down. Thermal mass also enables the homeowner to use the heat from a wood or coal stove much more efficiently.

Stone, brick, concrete, and any masonry, along with water, are all good heat-storing materials. They work this way: The sun's heat, pouring in through the windows during the day, is absorbed by these materials and then returned to the house when air temperatures start to fall after dark. Similarly, they soak up the excess heat of a wood- or coal- burning stove (which is most efficient when the fuel is burning rapidly), then feed it back into the room when the fire dies down.

Including enough thermal mass in any new construction is relatively simple (using brick or concrete walls with the insulation on the outside is one easy way). But what about the existing house? The use of water- or stone- filled drums, jars, or even culvert pipes is one relatively inexpensive way for a house built on a concrete slab. But the pipes are very heavy and can readily add more weight than wooden floor joists can withstand. If you intend to go this route, be very careful where you place such additional weight or be prepared to add supporting columns under a wooden floor.

On the other hand, there are some more- moderate steps you can take. Didier Thomas of the Northeast Solar Energy Center suggests adding another row of sheetrock to the inside walls. This will add appreciable thermal mass to the house right where the building is designed to support the weight. Another option, he says, is to add slate or ceramic tile over the floors. In this instance, the additional weight is so evenly spread out over floor area that no additional support should be necessary. Check to see if the floor joists are sagging at all and add the necessary prop.

Thermal mass also helps cool a house in summer. By leaving windows open at night, most of the heat is exhausted from the thermal mass. Then during the day , with draperies excluding the sun, the masonry, water barrels, or what have you cool down the home by absorbing the heat from the surrounding air.

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