Implementing solar design principles can slash heating and cooling bills, reducing the size of the renewable energy system you’d need. This article, the first of three, covers siting, orientation, building shape, and room placement.
Solar installer Bristol Stickney wistfully remembers passive solar conferences in the 1970s that drew hundreds of building tradespeople—all excited about building houses that derived a big part of their space heating from the sun. He had to look only as far as Mesa Verde, the spectacular Pueblo settlement in southern Colorado, to see how long passive solar has been in use. Those solar advocates of 40 years ago thought the ideas were finally making it into the building mainstream.
But passive solar design never reached its full potential—politics, cheap utility energy, and missteps by early passive solar practitioners helped cause the passive solar movement to lose its zip. Now, when people talk about “solar,” says Stickney, an industry veteran based in Santa Fe, New Mexico, they usually mean rooftop PV modules that produce electricity, not the building practices that allow homeowners to reap free heating from the sun.
Despite its second-class status, passive solar works—and well. The passive solar vanguards were correct—even in northern climates, a well-designed passive solar house will reduce a home’s energy costs. And unlike technologies such as ground-source heat pumps or PV arrays, many passive-solar features add little or no construction cost.
Basic Building Blocks
Passive solar design includes practices that help keep buildings cool in summer as well as warm in winter—something that early solar designers struggled to master. “A good passive solar house provides comfort no matter what the weather,” Stickney says. “In the past, a lot of passive solar houses had too much glass and would overheat when it was sunny and the weather was mild. It would get too hot in the spring and fall, but not hot enough in winter.”
Designers have since learned how to deal more effectively with that challenge. But any good design is site-specific, since each building site has its own weather and temperature patterns, as well as a unique topography that affects heating and cooling. A passive solar house designed for the high deserts of New Mexico might be pretty uncomfortable if transferred to coastal Maine. The house design must match its site.
There needs to be a good understanding of regional approaches, says Mark Chalom, an architect in Santa Fe, and a long-time solar designer. “In New Mexico, we have 300 days of sun, and it is strong. We can design a home that can gain as much energy as it loses for the month of January. But winter in Maine is different—the cold climate is dominant and energy conservation is the main goal.”
Despite regional differences, there are a handful of strategies at the heart of passive solar buildings anywhere. Key design issues and strategies are the:
- Building site and how the house is oriented on the site
- Shape of the building and its thermal envelope
- Size, type, and location of windows
- Use of thermal mass to moderate interior temperature swings
- Design of roof overhangs that shade glazing in windows and doors.
Although the principles of passive solar design aren’t hard to understand, the details can be daunting. Computer modeling can help, although the software can be complicated to use. Hiring an energy modeler is another option—more expensive but also reliable in balancing the many factors that affect energy performance. Alternately, owner-builders can refer to a series of builder guidelines published by the National Renewable Energy Laboratory (NREL; see the “Solar Design by Region” sidebar), or the guidelines published by the New Mexico Solar Energy Association (NMSEA; see Web Extras).
For the best solar gain in winter in the northern hemisphere, most homes should have their longest side facing true south. True south is not the same thing as magnetic south. This deviation, called the magnetic declination, varies by location—from 17°W in parts of Maine to 10°E in the Southwest. For help, consult the National Geophysical Data Center’s website (bit.ly/NOAAdeclination).
NMSEA guidelines say the south-facing side of the building can be as much as 15° from true south without a significant loss in solar gain. Other designers say the south-facing wall can be within 30° of true south. Generally, the closer to true south, the better for passive solar gain during the winter.
Every orientation has its trade-offs. When the principal glazed wall is more southwesterly than true south, for example, windows will contribute significantly to winter heat, but also increase summer cooling loads because of intense afternoon sunlight. In some locations, a southeasterly orientation may be more favorable, allowing a morning warm-up without afternoon overheating in the swing months.
Make sure the home’s “solar window” is clear—no vegetation or other obstructions, such as fences or outbuildings, should be within 10 feet of the home’s south wall. Even deciduous trees, which drop their leaves in the winter, aren’t a good idea—the leaves may be gone but tree trunks and branches will reduce available sunlight enough to decrease solar gain. In summer, the sun is so far overhead that leaves aren’t effective in blocking sunlight anyway.
Time of year is an important consideration in calculating how vegetation affects both shading and solar gain. At the summer solstice, the sun rises at its most northeasterly point of the year, and sets at its most northwesterly point and reaches its highest point in the sky. Conversely, at the winter solstice, the sun’s path of travel is at its narrowest, and the sun is lower in the sky than at any other time of year.
Geographic location is key in determining how nearby buildings, trees, and other objects affect the amount of sunlight reaching the house. New Mexico solar guidelines, for example, say that a one-story building at least 17 feet from the house will not create winter shading, and a two-story building is fine as long as it’s at least 39 feet away (assume one story is about 10 feet tall).
But a house at a higher or lower latitude would be affected differently by nearby objects because the sun angles are different. At more northerly locations, the winter sun is at a low angle, so even far-away objects could cause shading. At more southerly locations, sun angles are greater, and such objects could be closer to the house without causing substantial shading issues.
Eastern and western exposures are another story. Trees to the west of the building site can be helpful in blocking afternoon sun during the summer to decrease cooling loads. And, because limbs will be bare in the early spring and fall, trees will allow some solar gain in the cooler shoulder seasons. Deciduous trees on the east side of the building also can permit a little solar gain in the morning during spring and fall.
But even the most thoughtfully chosen building site can change over time. Neighbors move in and may add trees, new buildings are constructed, and existing trees get taller—these are factors to consider before you build. While solar access is protected by local regulations in some areas, that’s not universal. With that in mind, the U.S. Department of Energy suggests looking for a lot that is deep from north to south and placing the house at the north end of the property.
Read more at: https://www.homepower.com/articles/home-efficiency/design-construction/passive-solar-home-principles