The trombe idea

Passive solar techniques have entered the mainstream of home remodeling. Here and on page 154 we show how one passive approach a mass storage wall has been adapted in three existing houses.

Also called Trombe walls after one of their French developers, these are simply walls of dense materials like brick or concrete behind glazing (the tinted areas in these photographs). Solar radiation penetrates the glass and strikes the dark-painted wall, which turns it to heat that won't readily pass back out through the glass. (Water or eutectic salts can also be used as the mass for heat storage.)

Solar collection depends on wall area the bigger the wall, the more you get. Distribution speed depends on thickness 8 to 12 inches is optimal.

Mass walls provide thermal lag, a delayed response to outdoor temperature changes, which helps provide steady comfort. The accumulating heat is conducted slowly through the wall, reaching the living space later in the day when it's more likely to be needed. The wall heats by radiation: your body feels heat even if the room air is cooler than normal. Walls can also absorb heat from indoor sources such as wood-burning stoves.

Many mass walls have vent openings to boost heat more quickly in daytime: heat rises into rooms through upper vents, causing heavier, cooler room air to be pulled outward through lower vents for reheating. This approach leaves somewhat less heat stored in the wall for night use. To prevent a heat-stealing reverse air flow as the wall cools, at least the upper vents should be closed at night.

Basic considerations

Where's a good place for a mass wall? You can replace a south-facing stud wall or window, or simply glaze over an uninsulated masonry wall. You're dealing with hundreds of pounds maybe tons so consult an engineer about foundation and support, especially in earthquake areas.

Another consideration is nighttime heat loss: the protective glazing should be double unless you're willing to operate shutters or curtains between wall and glass. A popular glazing choice is patio door "seconds.' Also look into the new heat-retaining glass substitutes (see page 59 of the January 1983 Sunset).

The glass should be held 2 to 4 inches out from the wall preferably with screw-on mullions that remove to allow cleaning but are sealed to keep out moisture and prevent heat loss. Because the higher summer sun will partly glance off the glass, you'll likely need either a shading overhang or vents to outdoors at the top of the glass framing. (A wall cooled by nighttime venting in turn helps cool your house, absorbing daytime heat buildup.)

Four books for further reading

Sunset's new book Solar Remodeling (Lane Publishing Co., Menlo Park, Calif. 94025; $4.95) illustrates ways you can use the sun to heat your present house.

These three publications give more detailed help. The Passive Solar Energy Book, by Edward Mazria (Rodale Press, Emmaus, Pa., 1979; $14.95 in paperback), has a wealth of sizing information for different climates. Thermal Storage Wall Design Manual, by Alex Wilson (New Mexico Solar Energy Association, Box 2004. Santa Fe 87504, 1979; $4.75, including postage), discusses materials and construction. Retrotrombe (Total Environmental Action Foundation, Harrisville, N.H. 03450, 1981; $9.95 including postage) contains a set of plans for adapting existing masonry walls to solar.

Photo: Salt Lake City: existing brick wall becomes a heater

Uninsulated brick or masonry walls are notorious heat losers, conducting precious warmth outward at night. But painted dark brown and faced with a 39-foot span of double glazing, this south wall absorbs enough heat to cut energy costs by 36 percent. Fans feed heat indoors; cool air flows back through under-house ducts to holes in bottom of framing. Bays enclose existing windows, while old wall radiates warmth. Design: John Whitaker. Owners: John Whitaker, Jody Williams