Since ancient times, people have used thick walls to trap the sun's heat during the day and release it slowly and evenly at night. Welcome to the trombe wall.

Trombe Wall

Today's passive solar buildings often improve on the ancient technique described above by incorporating a thermal storage and delivery system called a Trombe wall. Named after French inventor Felix Trombe in the late 1950s, the Trombe wall continues to serve as an effective feature of passive solar design.

A typical Trombe wall consists of an 8 to 16-inch thick masonry wall coated with a dark, heat-absorbing material and faced with a single or double layer of glass. The glass is placed from ¾ inch to 2 inches from the masonry wall to create a small airspace. Heat from sunlight passing through the glass is absorbed by the dark surface, stored in the wall, and conducted slowly inward through the masonry.

Applying a selective surface to a Trombe wall improves its performance by reducing the amount of infrared energy radiated back through the glass. The selective surface consists of a sheet of metal foil glued to the outside surface of the wall. It absorbs almost all the radiation in the visible portion of the solar spectrum and emits very little in the infrared range. High absorbency turns the light into heat at the wall's surface, and low emittance prevents the heat from radiating back towards the glass. Although not as effective as a selective surface, painting the wall with black, absorptive paint will also help the wall to absorb the sun's heat.

For an 8-inch-thick Trombe wall, heat will take about 8 to 10 hours to reach the interior of the building (heat travels through a concrete wall at rate of about one inch per hour). This means that rooms remain comfortable through the day and receive slow, even heating for many hours after the sun sets, greatly reducing the need for conventional heating. Rooms heated by a Trombe wall often feel more comfortable than those heated by forced-air furnaces because of the radiantly warm surface of the wall, even at lower air temperatures.

Architects can use Trombe walls in conjunction with windows, eaves, and other building design elements to evenly balance solar heat delivery. Strategically placed windows allow the sun's heat and light to enter a building during the day to help heat the building with direct solar gains. At the same time, the Trombe wall absorbs and stores heat for evening use. Properly sized overhangs shade the Trombe wall during the summer when the sun is high in the sky. Shading the Trombe wall prevents the wall from getting hot during the time of the year when heating is not needed.