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Indirect Gain (Trombe Walls)

An indirect-gain passive solar home has its thermal storage between the south-facing windows and the living spaces.

An illustration of a cross-section of a passive solar home with a Trombe wall constructed on the south side. It shows how the angle of the winter sun hits the wall, which absorbs the heat and distributes it within the home's interior. It also shows how the angle of the summer sun hits a roof overhang, constructed above the Trombe wall, blocking the sun's heat from entering the home.

Using a Trombe wall is the most common indirect-gain approach. The wall consists of an 8–16 inch-thick masonry wall on the south side of a house. A single or double layer of glass is mounted about 1 inch or less in front of the wall's surface. Solar heat is absorbed by the wall's dark-colored outside surface and stored in the wall's mass, where it radiates into the living space.

The Trombe wall distributes or releases heat into the home over a period of several hours. Solar heat migrates through the wall, reaching its rear surface in the late afternoon or early evening. When the indoor temperature falls below that of the wall's surface, heat begins to radiate and transfer into the room. For example, heat travels through a masonry wall at an average rate of 1 hour per inch. Therefore, the heat absorbed on the outside of an 8-inch-thick concrete wall at noon will enter the interior living space around 8 p.m.