Passive Solar Home Heating Rules Of Thumb

The following are not rigid passive solar design rules, but rather guidelines to give you an idea of factors to consider when designing a solar home.

• Solar homes do best with their long axis in an east-west direction the majority of the glazing on the side of the building facing the equator (that is, the south side in the northern hemisphere and the north side in the southern hemisphere. For the rest of these tips, I’ll assume we’re in the northern hemisphere. South = toward the equator).

• A good passive solar design rule is to site the building so it has the most sun exposure possible between 9AM and 3PM.

• Windows will lose about 10% of available solar energy if they are angled up to 25 degrees away from true south. Any more than that, and the solar radiation collected drops greatly.

• Magnetic south (the south you see on a compass) is not true solar south. Magnetic south may as much as 22 degrees different from true south. Obviously, this can make a great deal of difference. It's something you'll need to account for in your passive solar design.

• The optimal ratio of square feet of window to square feet of floor space depends very much on local climate conditions as well as over all house design. If the south facing windows are too small, the house will never warm up. If they’re too large, the house could overheat. To give you an idea of a proper ration, in a climate where the average winter temperature is between 20 – 30 degrees F, the house should have between .18 and .38 square feet of south facing windows for every square foot of floor space.

• Keep thermal mass masonry between 4” and 6” thick and no more than 8” thick (greater thickness does nothing for thermal efficiency).

• Ideally, the light from one square foot of window strikes about 9 square feet of thermal mass. Light diffusing glazing helps with this.

• Every square foot of south facing window requires 150 lbs of masonry or 4 gallons of water to serve as thermal mass.

• Concrete floors serving as thermal mass should be a medium dark color.

• Thermal mass floors should not be covered with wall-to-wall carpet and only have as many area rugs on them as esthetically required.

• Non-thermal mass walls should be light colored to reflect light toward the thermal mass.

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