Indirect solar gain through specially designed walls is a little more complicated and costly, but is an effective way of using solar heat after dark.

A Trombe wall is the most popular indirect passive solar item. It typically consists of an insulated layer of glass outside a dark, thick masonry wall. The glass helps the 8- to 16-inch wall absorb and trap the heat.

The wall slowly radiates the absorbed heat into the room over several hours it radiates into the living space. An 8-inch-thick wall will absorb and radiate noontime heat at 8 p.m. because masonry transfers heat at an average rate of 1 inch per hour.

New Trombe walls also have top and bottom vents that allow hot air to escape from the top during the summer. If a door or window is open across the room, it can create a cooling current by drawing in cooler air from the bottom of the Trombe wall.