Passive Solar Design

With the possible exception of using the sun to heat our water for washing and bathing, we will get more “bang for our buck” by using passive solar design principles than from any other energy-saving measure we take. Think of passive solar design as both a philosophy and a science. There is a science to it, and you need to understand the science to execute it properly, but the science will be meaningless until you “get” the philosophy.

The Philosophy

passive solar basicsThe philosophy of passive solar design is straightforward. When it’s cold, I want to open my home to the sun’s light and heat when the sun is out and hold on to as much heat as I can, so when the sun goes down I will be able to continue to enjoy the heat. And when the sun goes down, I want to hold onto every bit of warmth I can. When it’s hot, I want to vent as much heat as possible at night when the weather is cooler, and I want to shade my home as much as I can when the sun is out.

There’s no perfect design for all purposes. There’s not even a perfect design for your home, because good passive solar design is always a compromise between conflicting goals. Once you understand the conflicting goals and the advantages and disadvantages of each technique (that’s where the science comes in), you can make an informed choice about the design that’s best for you and your family.

No design will mean anything until you know where your house is and where the sun is. For those in the northern hemisphere, that means knowing where south is. You want to focus the glass (passive solar buffs love to call it “glazing”) on the south wall of the house and protect it from the summer sun with an overhang or cantilever. You want to minimize the glass on the east and particularly the west walls, and you want to beef up the insulation on the north wall.

Another set of compromises will focus around the extent to which the dwelling is airtight. Some advocate what we call a “ZipLoc Strategy,” in which every possible avenue for air exchange (not just windows and doors, but even electrical outlets and ductwork) is sealed. Others are much more permissive about ventilation and place a firmer reliance on effective thermal mass. In general, the colder the climate, the more appropriate it is to focus on keeping the dwelling airtight. We live in a hot, humid climate, so we naturally lean more toward ventilation.

The Science

Once you know the climate where you are living and the placement of your residence, you need to understand the science. The first challenge you will have is to identify the angle of the sun in the cold season and the warm season, and the problem is that the drawing above is misleading. If the sun would stay in one place all winter and then pop cooperatively to another place for all the summer, this would be much easier. Instead, Old Sol is a moving target. In the northern hemisphere, he reaches his low point in the sky on the winter solstice, December 21, the first day of winter. Then the sun tracks a little higher each day, right through the vernal equinox, March 21, the first day of spring, and reaches his high track on the summer solstice, June 21, the first day of summer. Then the process reverses, with the sun gradually following a lower track each day, through the autumnal equinox on September 21, and reaching his lowest point again on the winter solstice. And here’s a fun little kicker: for those in the extreme southern portions of the U.S., the sun actually tracks a little north of center around June 21, the summer solstice. In general, if you peg the winter sun as the height of the sun on January 15 and the summer sun as the height of the sun on July 15, you won’t be too far off.

Another technical challenge you will confront is your choice and placement of thermal mass. Until the technology is well-distributed and widely understood, we suggest building in some adjustability to your thermal mass, the ability to do some tuning after you’ve lived in the house for awhile.

Common Mistakes

Know this: passive solar is easy to bungle. Here’s our top list of mistakes people have made:

  • Too much glass on the south wall, particularly glass that’s exposed to the sun in hot weather. If you see a house with a magnificent expanse of glass, chances are it’s a bear to keep cool in all but the coldest of climates. And it probably doesn’t hold in heat well at night either. Looks great; sucks for living. Those who understand such things suggest that the square footage of glass on your south wall (actual glass, not framing) should equal 8-13% of the square footage in your home. So for a 1,000 square foot house (which is all most of us need to build), you would want between 80 and 130 square feet of glass on the south wall. That’s really not that much glass.
  • Too much glass on the other three walls. Glass on the east and west walls will take in summer sun as the sun rises in the morning (east wall) and sets in the afternoon (west wall). And unlike the glass on the south wall, there’s no effective way to shade it during the summer.
  • In general, building to maximize a view rather than to stay comfortable. If you’re south or east of that beautiful lake with the mountain behind it, we feel your pain. You want to open up your house to that gorgeous vista. Do it if you must; just understand that you’re buying yourself a real heating and/or cooling challenge as long as you live there.
  • Corollary: building to face a street when it forces you to turn a corner to the south. You can make passive solar sort of work when your house faces southwest or northeast, but it gets a lot more complicated. Do yourself a favor if you have the choice: pass on the southwest facing lot and go for the one that allows you to face south.
  • Making the house too big. Small houses are much easier and cheaper to keep comfortable than large houses.
  • Focusing too much on heating and not enough on cooling. In all but the coldest of climates, it’s easy to get too much of the sun’s warmth in the summertime, particularly from those west-facing windows.
  • Too little attention to the roof. The roof will take a heavy dose of each summer. It needs to be dark-colored in colder climates, light-colored or metallic in hot climates. And don’t forget insulation.