Look Ma, No Trombe Wall

Reader Contribution by John Kosmer
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Black, brown, dark grey or any another dark color ceramic floor tile will absorb heat/light faster than light color tiles. The dark tiles transfer more heat faster to the thermal mass slab. The thermal mass & tiles then slowly release the heat in the cooler evening.

When we first began building our passive solar house, I was taken aback when I found out that our 12″-thick concrete slab weighed about 100 tons! I was even more surprised to learn that the rest of the house’s building materials weighed only 25 tons. That news would have been shocking enough by itself. Its importance, however, becomes evident when you consider that the 125-ton thermal mass encased in a super insulated container can store and slowly release a tremendous amount of heat. The super insulation shell also keeps most of that heat inside the box that is your house, so the total mass that is trapped by it has no place to release the heat but inside the house, and only very slowly dissipating it to the outside.

With that kind of 125-ton mass to store and release heat, the need for a Trombe wallhas always been a mystery to me. Invented by Edward Morse in 1964, its almost as if it was heating strategy from a bygone era before they were insulating many homes and well before the notion of super insulated hoes was around. That’s a freestanding vertical concrete wall connected to the floor and ceiling inside the house that stores and releases heat. Simply put, with 125 tons of super insulated thermal mass, who needs it? I have seen two versions of a Trombe wall on a ranch style house. One is at the north side of the house where it picks up heated air, instead of direct sunlight falling on the wall, which would be much more efficient. A variant of this idea is to have clerestory windows mounted towards the top of a south-facing sloped roof, letting in the sun. The light can fall directly on the upper part of the Trombe wall. Direct sunlight will give you greater solar gain than ambient sunlight that first heats the air, which, in turn, will be absorbed by the Trombe wall.

The second version makes even less sense to me. It is a Trombe wall that is built three to five feet in from the south-facing windows that let in the sun. It creates a hall space between the living area and the south facing windows. The advantage is that you get the greater efficiency of direct sunlight heating the wall. One disadvantage is that the side of the Trombe wall facing the windows will heat up much more than the inside surface of the wall. Since over the course of the day the wall will not heat evenly, at night the side facing the windows will release more heat than the side facing the living area. Not a very efficient installation. The second disadvantage is that, in a house that lets in volumes of natural light, greatly increasing your living enjoyment and minimizing you electric lighting needs, you’re blocking that(along with a possible lovely view) with a huge wall. It seems especially silly if you already have 125 tons of super insulated mass working for you.

There is, however, another way to go. Dan Chiras says, on p. 29 of his 2002 book The Solar House, quoting Steven Winter Associates, one of the nation’s leading solar designers, “storage mass that is heated only indirectly by warm air from the living space requires roughly four times as much area as the same mass in direct sun to provide the same thermal effect.” While I was unable to confirm that quote when I called Steven Winter Associates (they could not find the reference), it’s still a compelling notion. I was able to verify this claim when I attended the Passive House open house in upstate New York. The house had a concrete floor. We took our shoes off when we entered because the grounds were so muddy. Over the three hours we were there the floor on the south side of the house that had direct sunlight falling on it became toasty warm. The north side of the house that had its floor in the shade was cool. You can try this yourself. Tell me your experience.

You can maximize this direct sunlight three ways. The first way is to design the house so as much floor as possible gets direct sunlight over the course of the day as the sun sweeps across. The second way is to arrange your furnishings to maximize the floor space exposed to direct sun. The third way is to use dark-color ceramic tiles over the concrete floor. When I called The Ceramic Tile Institute of America, I was told that dark ceramic tiles would absorb heat faster than light tiles. In addition, once absorbed, the heat releases from the concrete at the same rate no matter if the tile color is light or dark on the surface. So you gain and store heat faster and release it at the same rate.