Thermal mass in a house keeps the sun on the job after sundown

Great-grandma never heard the term thermal mass, but she employed all the principles involved every time she sent her little boy off to school with a hot potato in his pocket.

Whenever solar energy is discussed, references are made to thermal mass, or heat storage, and its importance to a sun-heated building. By storing the excess heat that pours into a house on a cold but sunny day, thermal mass enables the home to remain relatively warm long after the sun goes down. Excess heat given off by a wood- or coal-burning stove can also be stored in this manner for later release when the stove goes out.

What is thermal mass? Anything that stores heat. That baked potato, for instance, stored heat and, come lunchtime, was part of a hearty meal. But earlier, its stored heat helped warm grandpa's hands as he trudged to school.

When you lift the frying pan or pot from the stove, it doesn't go cold the moment it is taken from the heat source. That's because both the pot and the food it contains have enough thermal mass to store heat. Indeed, it is a good thing that food itself can store heat, or we would never know a hot meal. Conversely, a meal can quickly go cool if placed on a cold plate, because the thermal mass of the plate draws the heat from the food.

A most common application of the principles of thermal mass a generation ago (and slowly regaining popularity in this new era of lower nighttime temperatures) involved the hot-water bottle. The bottle (at first a glass bottle with a screw-in stopper, but later of rubber) was filled with hot water from the kettle and taken to bed at night. Water stores a lot of heat for its volume, so the hot-water bottle stayed warm for hours.

Heated bricks or heated soapstone were frequently used in the same way.

The adobe ovens of the American Southwest work so effectively because of their thermal mass. An intense fire is burned in the oven for a short while, during which the heat from the flames is soaked up by the surrounding wall of thick adobe clay. When the fire is out, the ashes are raked away and the food placed in the oven, where the stored heat radiates back to bake the bread or whatever.

Today, stove manufacturers sometimes increase the thermal mass in their products by lining them with firebricks. Soapstone stoves, because of the heat-storing material from which they are made, continue to radiate warmth long after the fire goes out. Austrian tile stoves likewise hold heat well.

So much for what thermal mass is. Why is it so important to a solar-heated or stove-heated home? Quite simply, it moderates the extremes of temperature that would otherwise occur within the home. It keeps it from overheating when the sun shines and from getting cold the moment it doesn't.

A friend of mine, now an acknowledged expert in the solar field but still a neophyte when he built his first home, loaded the house with large, south-facing windows but didn't compensate with additional thermal mass. The results were predictable. Even when the temperature was 10 degrees F., the home would quickly approach saunalike temperatures indoors if it remained closed up on a sunny day. By 10 a.m. the windows had to be opened, and all the heat that would have been so valuable at night had to be spilled outdoors.

The effectiveness of thermal mass was recently brought strongly home to me. Identical wood stoves heat the living room of my Massachusetts home and the central room of the cottage I am building in Maine. Because of the liberal use of mortar in its construction, the cottage has considerably more thermal mass than the all-wood Massachusetts house.

In Massachusetts, the stove will quickly back us up against the walls if we don't damp it down once the fire is going well. In contrast, the same type and size of stove never overheats the cottage, because the masonry work soaks up the heat the way a sponge draws up water.

True enough, the cottage may take somewhat longer to warm up when we first arrive. But thereafter, it is by far the more comfortable place to be. Long after the stove has burned itself out at night, the temperature remains comfortable. In theory, that same thermal mass will keep the cottage comfortably cool on hot summer days.

A rule of thumb suggests the exposed area of thermal mass should be three times the area of the south-facing glass.

Including adequate amounts of thermal mass is a relatively simple matter in a new house, but can heat-storing materials be added to the existing home? Obviously, there are ways that this can be done, but all involve care in seeing that the materials involved do not add more weight to a floor than the existing joists can support.

An acquaintance of mine is adding afieldstone facing to part of his living-room wall to get needed thermal mass into his otherwise all-wood home. Before doing this, however, he plans to build a wall of concrete blocks from the ground to the floor so that there will be no sagging of the floor and consequent cracking of the fieldstone facing. Bricks can be used in a similar fashion.

Water can be turned into useful thermal mass by storing it in containers (anything from glass or plastic jars to steel drums or commerially made fiber-glass tubes) and standing them where the sun will strike them directly. While thermal mass will absorb heat from the surrounding air, it absorbs vastly greater quantities if it is bathed in sunlight or is directly in the path of heat radiating from a stove.

Before adding themal mass to your home, it would be well to know how well the various materials hold heat.

One cubic foot of concrete and stone will store 22.46 BTUs (a measurement of heat) for every 1 degree F. that its temperature is raised. Brick is marginally better at 24.6 BTUs, and water almost three times as effective at 62.40 BTUs per degree. Vastly more effective - but also vastly more expensive - are phase-change salts now being marketed. A cubic foot of the salts will store 134. 76 BTUs for every degree F. increase in temperature.

By contrast, a cubic foot of air requires a mere 0.018 BTUs to warm up one degree. This means that a relatively small amount of thermal mass can warm up a large volume of air.