The MOTHER EARTH NEWS FAIR is on its way to my lovely little mountain town this very weekend. (If you're going to be there, you can catch my workshop on Net-Zero home design on Sunday afternoon!) In preparation, I was tasked by our marketing department to look through some of the various green homes we’ve built in the area and pull together the highlights of their green features. As usual, I wanted to list all kind of nerdy exciting details of heat transfer coefficients and Seasonal Energy Efficiency Ratings…and was told, with my marketing department’s usual patience, to cut a bit for simplicity and clarity. In doing that, one thing really stood out me. All these homes had one salient feature in common, one aspect to their design that really augmented their claim to green fame.
That feature is passive solar design.
Blogs and articles abound about passive solar design principles. If you’re a regular MOTHER EARTH NEWS reader, I’m sure you’ve heard of it. My aim today then, is not to explain it, but to celebrate it. It's darn neat to see how many of our houses have put passive solar design into practice! Especially here, in a southeastern climate, where cooling is just as important a consideration as heating, and high humidity can be a concern.
This home, located here in Asheville in the heart of the Blue Ridge Mountains, still has to contend with some hot summer days. Being built on a densely wooded lot helps greatly with the cooling side of things, as does the metal shingle roof, which is an Energy Star cool roofs product whose low emissivity helps keep the sun’s heat that it’s pelted with all day, out of the attic.
Yet the house is shaded by deciduous trees, which lose their leaves in winter. Following classic passive solar design principles, we made sure that the open living, kitchen, and dining area faced south, and we put an appropriate amount of window glass in that living room, three Marvin Integrity wood-ultrex windows at 6 foot wide and 4 feet tall, with a 2 foot deep overhang to shade them in summer but keep them un-shaded in winter. We used Marvin's "Low-E 180" glass coating that lets in 57 percent of the sun’s heat, as opposed to only 20 percent to 30 percent as is common with standard Low-E coatings. We made sure all that incoming solar heat was put to good use with an acid-stained concrete floor to act as a thermal battery. I remember standing in front of those windows on a 20 degree day in winter, after the house had been insulating but before the heating system had been installed, and feeling so comfortable.
This home, in warm and humid yet still occasionally chilly coastal North Carolina, uses "sun-tempered" design--what I like to think of as Passive Solar Lite. Sun-tempered design features some south facing glass and appropriate shading overhangs, but does not incorporate thermal mass. It can be a practical design strategy for those who want some passive heating benefit but do not want to have a concrete floor or use other interior thermal mass designs.
Pictured here with snow on the ground, this homeowner had to design for heating and cooling concerns alike. Expansive south-facing windows--deemed essential by the homeowner t to capture their view--were shaded with a nearly 4-feet deep overhang, keeping the ratio of un-shaded winter south-facing glass to floor area at 6 percent, the maximum that is recommended in a design that does not incorporate additional thermal mass. East and west facing windows, which can let in a considerable amount of low angle sun, were used sparingly, and shaded, when use at all, by deep covered porches. Energy Star certified windows, 2x6 thick walls and a layer exterior insulation helped this home far exceed the insulation values required by energy code for the area, holding heat inside in winter while keeping it out in summer. A high efficiency heat pump and air conditioning system, properly designed and commissioned, rounded out this home's practical energy design.
This home was designed for solar in every way. Passive solar, of course, with the usual contingent of south-facing glass, overhang shading, extra insulation, and thermal mass - but it was designed for active solar too, with a solar water and space heating system, and solar electric system. To hit their goals of being nearly net-zero in their energy use, these homeowners did all of this, tucked away in the Blue Ridge Mountains of Virginia, without installing an air-conditioning system. To passively keep the house cool use of light colored finishes, very minimal east or west facing windows, combined with a fan system designed to exhaust warm air at night through a high attic fan.
Crucial in all of these home designs was avoiding the overuse of glass on the south side. I have been far too many beautiful, well-intentioned passive solar homes in this area whose living areas became unbearable in spring, fall, and even winter, because of the large amount of glass used. Passive solar design techniques originally came out of the desert southwest—with high day/night temperature swings—and out of cold climates, with an intense focus on heating. It is a design principle that can work great here in the southeast; too, it just takes a little bit of different thought.
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