Solar windows let sunlight directly into the house. The heat is usually stored in a heavy floor or in interior walls. Thermal storage walls, as solar walls are often called, are exactly what their name implies- walls built primarily to store heat. The most effective place to build them is directly inside the windows, so that the sunlight strikes the wall instead of directly heating the house. The directly sun-heated wall gets much hotter, and thereby stores more energy, than thermal mass placed elsewhere.

These "solar walls" conduct heat from their solar hot side to their interior cooler side, where the heat then radiates to the house. But this process takes a while. In a well-insulated house, a normal number of windows in the south wall will admit enough sun to heat the house during the day. Thermal storage walls will then pick up where the windows leave off and provide heat until morning.

South-facing windows with an area of less than 10 percent of the floor area of the house are probably not large enough to provide enough heat during the day. If this is the case, vents could be added at both the base and the top of a solar wall. The wall can then provide heat to the house during the day just as solar chimneys do. Although the vents need be only 10 to 12 square inches for each

lineal foot of wall, they can add cost and complication. Therefore, it is best not to use them unless heat is needed during daylight hours. Thermal storage walls with vents are normally called Trombe Walls, after Dr. Felix Trombe who, in the early 1960s, built several homes with this design in the French Pyrenees. 1

One type of thermal storage wall uses poured concrete, brick, adobe, stone, or solid (or filled) concrete blocks. Walls are usually one foot thick, but slightly thinner walls will do, and walls up to 18 inches thick will supply the most heat. Further thicknesses save no additional energy. Containers of water are often used instead of concrete. They tend to be slightly more efficient than solid walls because they absorb the heat faster, due to convective currents of water inside the container as it is heated. This causes immediate mixing and quicker transfer of heat into the house than solid walls can provide. One-half cubic foot of water (about 4 gallons) per square foot of wall area is adequate, but unlike solid walls, the more water in the wall, the more energy it saves.

The main drawback of solar walls is their heat loss to the outside. Double glazing (glass or any of the plastics) is

adequate for cutting this down in most climates where winter is not too severe (less than 5000 degree days:

Boston, New York, Kansas City, San Francisco). Triple glazing or movable insulation is required in colder climates.

This glazed thermal storage wall is comprised of glazing frame members milled from cedar 4x4's bolted to an eight-inch thick structural brick wall. The bricks are dense paving bricks-a dark umber color on the outside, standard terra cotta color on the inside-and are laid up with all cavities filled with mortar. The triple glazed panels, designed for use in the northeast, reduce heat losses to the outside from the warm wall. Standard operable triple-glazed casement windows are incorporated into the wall to provide direct gain heating, light, views and ventilation. Double glazing is suitable for use in milder climates. (Construction details, the Brookhaven House.)