©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
Last week I wrote about sunspaces  and how they can be used to deliver passive solar heat to our homes. Another option for passive solar heating is the Trombe wall, or thermal storage wall.
While the sunspace is an "isolated-gain" solar system, a Trombe wall is an "indirect-gain" system. Here's how it works: On the south side of a house you have a high-mass concrete or masonry wall whose exterior surface is painted a dark color. A layer of glass (or some other type of clear or translucent glazing) is held away from the wall surface by a few inches or more. Sunlight shines through the glazing and is absorbed by the dark wall, which heats up. The solar heat conducts into the wall where it is stored, and it gradually moves through the wall to the inner surface, where it radiates its warmth to the room.
A short history of Trombe walls
The Trombe wall is named after a French engineer Félix Trombe, who popularized this heating system in the early 1960s. The idea actually goes back a lot further. A thermal-mass wall was patented in 1881 by Edward Morse. In the U.S., interest in Trombe walls emerged in the 1970s, aided by researchers at Los Alamos National Laboratory in New Mexico.
I was fortunate enough to be working in Santa Fe in the late 70s, for the New Mexico Solar Energy Association, and I became particularly interested in Trombe walls. In fact, my first article in a national magazine was on Trombe wall retrofits — in Solar Age magazine in 1979. I also wrote the obscure Thermal Storage Wall Design Manual in 1979 (see photo), which gained some prominence among the small cadre of passive solar designers around that time.
Trombe walls are particularly well-suited to sunny climates that have high diurnal (day-night) temperature swings, such as the mountain-west. They don't work as well in cloudy climates or where there isn't a large diurnal temperature swing. In New Mexico, where homes have been built out of adobe (dried mud) bricks for hundreds of years, even an unglazed south wall will deliver some heat into the house — if you add a frame and layer of glazing on the outside of the wall the performance improves dramatically.
We are used to insulating walls, but with Trombe walls there is no insulation. The system works with a material that is both very heavy (high heat capacity) and fairly conductive (low R-value per inch). The trick is to choose the right material and size the wall thickness so that the solar heat makes it through to the inner surface by nighttime. If it's too thick, it won't be as effective at delivering solar heat, and if it's too thin it will result in too much heat loss at night.
Tweaking a Trombe wall
An overhang is typically built that extends out over the Trombe wall above it. This will shade the wall from direct sun during the summer (when the sun is high overhead), but allow full solar exposure in the winter (when the sun is lower in the sky).
Top and bottom vents can be installed through the masonry wall to deliver more heat into the house during the daytime hours. Warm air in the space between the glazing and wall surface rises and enters the room, being replaced by air from the house entering through the lower vents in a convective loop. These vents should be closed at night so that the air circulation doesn't reverse, with air next to the glazing cooling off and pulling in warm air from the room through the upper vents and delivering chilled air to the room through lower vents.
Vents through the glazing can also be installed and seasonally opened and closed. In the summer months — when you don't want the Trombe wall delivering heat into the house — these vents are left open. Screens on the vents keep out insects and other unwanted visitors.
Like other passive solar heating systems, Trombe walls don't require fans or pumps. Part of the house itself is turned into the solar heating system.
In addition to this Energy Solutions blog, Alex contributes to the weekly blog BuildingGreen's Product of the Week , which profiles an interesting new green building product each week.
Alex is founder of BuildingGreen, Inc.  and executive editor of Environmental Building News , which is now in its 20th year. To keep up with his latest articles and musings, you can sign up for his Twitter feed .