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Brrr… Keeping warm with passive solar heat

O SOLAR PIONEERS! (Living Off the Grid—almost)

By Anna Moya Underwood

Direct passive solar heating should be easy. Passive solar (without wires or electricity) is different from active solar, which is all about electricity. Many old Mediterranean cultures used direct passive solar heating. The ancient homebuilder’s first hot discovery was to orient his or her small home to the south, and then to build or carve out a door or other opening to allow the southern sun in. The next discovery was that if one used some form of masonry — stone, adobe, bricks, pounded earth — as walls and floors, the masonry would hold and then release the latent heat of the sun during the night. Even south-facing caves must have been the most desirable! The third ancient idea was to shutter the door or window, or roll a stone in front of the cave opening at night to hold the captured heat inside. These ways of building passed from generation to generation and culture to culture because they worked.

Unfortunately, when we built our modern solar adobe 11 years ago, we made some serious errors in implementing the centuries-old direct passive solar heating. Following is a list of what not to do if you’re thinking about bringing the sun’s winter warmth inside your home to save fossil fuels, whether retro-fitting the home you have, or building new. Fortunately, our sunny New Mexico climate is perfect for keeping yourself warm without burning much gas, oil, or wood in the winter — if you do it right. Often doing it right, and lacking a culture that preserves old ways, demands research and expert advice.

1. While we did correctly orient our home precisely to the south, letting the long axis lie to the east and west, our south windows are too few in number for the size of the house that we’re trying to heat. Our house is 46 x 50, about 2000 square feet interior living space, excluding the thick adobe walls. The actual glazing (windows) square footage for direct solar gain should be, I now know, 7 or 8 percent of the functional floor space. The square footage of our windows is actually 57.5; they should total 70 or 80 square feet to heat even the south half (1000 sq. ft.) of the house. This area is ideally an open room; we do have a kitchen, dining, hobby, and living room without walls. So what happened? The windows were expensive, we thought we could safely reduce their size and cost; the design person I checked with was misinformed.

2. Two of the large windows on the south wall have an “e-film” within them that blocks the sun’s heat. This e-film and windows with it are popular in the South and Southwest because of the intense summer sun, and they are meant to keep houses cooler in the summer. My window salesman, who knew we were building a passive solar, did not understand that they would also keep houses cooler in the winter. For some reason I did not think it through either. It was only when I walked barefoot the first winter where the sunlight fell through the windows on the brick floor that I understood. The floor was warm near the second-hand French doors made with clear glass, and cool in front of the new, e-film windows. Luckily the four old French doors do transmit lots of sunlight and heat.

3. Our dark red brick floor is supposed to be our “heat tank,” one of the mass areas in the home that stores the sun’s energy. But because I am a romantic purist, eager to be living on tierra madre herself, the bricks are not on a concrete slab. My husband pleased me and put the bricks directly on “soil cement,” a magical mixture of soil, water, and powdered cement, tamped down with a compactor. However, without insulation, some of the heat absorbed by bricks goes right into the soil beneath them. A better plan would have been to use a concrete slab under the bricks with rigid styrofoam, gravel, lava rock, or other insulation beneath the slab. Indeed, a dark-painted and waxed concrete slab, with insulation beneath, makes a great heat tank for the sunlight coming through unimpeded windows of the right dimensions!

Even though our flawed direct passive solar heating is only partly effective, it still heats most of the south part of the house on sunny days. Still, we often need a back-up heat source, like a wood stove or other heater at night, when it is really cold. Heavy or insulated curtains over double or single glazed windows will help keep in the heat gained during the day. Triple-glazed windows will also profit from curtains, though not so dramatically.

Another consideration is body comfort. My husband is cold-natured, and at our average sunny indoor south-side temperature of 64 to 68, he is shivering, unless he is sitting right where the sun falls. Long johns under jeans, a sweater and/or a wool shirt keep me comfortable in that range. Everyone’s needs, especially those of children and seniors, are different.

Don’t forget to plan for the hot summers. You’ll need an overhang over your south wall to shade the less welcome summer sun. It passes higher in the sky than during the winter, and you can easily block it if you plan ahead.

If you want to retrofit an existing home with some solar heat, remember there are other ways besides direct solar gain. If your south wall does not have a window, designers can show you surprising ways to create “windows” to get southern sun in your house. Read books, get on the search engines, talk to and e-mail as many people in the field of passive solar energy as you can. The more questions you ask, the more time you spend in research, the fewer costly mistakes you’ll make.

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