Passive Solar Design and Systems

Passive Solar Design and Systems

The term "passive solar" can mean a lot of things, but to keep things relatively simple, we're going to discuss the main form of passive solar energy—using (or rejecting) direct sunlight to reduce the amount of electricity or fuel you need to use to heat or cool your home. During the winter, the sun streaming in your windows will help warm the house and lower your heating bills; during the summer, your carefully designed roof overhangs and other features will help reject the sun and reduce your A/C bills.

New Homes and Major Re-Do's

picture of hearth wall made of block and tile, positioned in front of large window array If you have the opportunity to design and build a new home, including passive solar is a no-brainer. It costs very little extra to design for passive solar, but the energy savings over the lifetime of the house will be enormous. Similarly, if you can swing a major rehab of your existing home, paying attention to the dictates of passive solar design will be of great benefit. Here are some issues to consider.

* You want lots of windows on the south side of the house, with fewer windows on the other sides. The reason is that the sun arcs across the southern sky in the winter (in the northern hemisphere), and its angle is just right for the sunlight to come through all of those south-facing windows.
* You want lots of thermal mass—block, brick, and tile—on the south side of the house (inside the house), with much of it positioned so it can be struck directly by the sun in the wintertime. The thermal mass absorbs and stores the heat energy during the day and then radiates it back into the living space as night falls and the interior temperature begins to cool. Dark tile that has a rough surface will usually absorb more of the sun's energy than light-colored tiles or those with shiny, smooth surfaces. The amount of thermal mass one needs varies somewhat based on climate and application, but generally, the exposed area of thermal mass should be about six times the area of glass that receives direct sunlight.
* While sunlight coming through windows is a boon during the winter, it's a big negative during the cooling season. For this reason, you want to make sure your roof on the south and west faces have the proper amount of overhang. (See: Overhang Calculator Tool) The combination of the overhang and the sun's higher summer elevation should work to block most of the direct sunlight on the windows, but you may find that removable awnings or well positioned deciduous trees can also help.

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