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R o o f V e n t i l a t i o n
Page 3 of 5
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Calculating how much venting your attic needs is relatively simple.
All you need to know is the area of the attic floor. Include the garage, if you have one,
and the soffited overhang because heat gets trapped above them, too. A common rule of
thumb is the 1/300 rule, which means 1 square foot of net free vent area per 300 square
feet of attic floor space. Let’s look at an example. Say you have an 1,800 square
foot home with a garage that measures 20 feet by 22 feet. This will yield a total area of
2,240 square feet. You then divide this number by 300. |
| 2,240 ÷300 @ 7.5 |
| This tells us that we need
7.5 square feet of ventilation for the attic. Most attic vents are measured by square
inches so we need to convert the 7.5 square feet to square inches. This is done by some
simple multiplication. 1 square foot is equal to 144 square inches, so we multiply 7.5 by
144. |
| 7.5 x 144 = 1,080 |
| So we need 1,080 square
inches of Net Free Vent Area. Divide this by two and we see that we need 540 square inches
of intake ventilation and 540 square inches of exhaust ventilation. |
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| There is always a lot of
concern for what the best type of ventilation is. You have already read that you need both
intake ventilation and exhaust ventilation installed at an approximate one to one (1:1)
ratio. Now remember that the idea behind this is for maximum air circulation. Installing
more than 1 square foot of ventilation per 300 square feet of attic floor space will not
hurt anything – it’s a general guideline and code requirement in some areas.
Most roofing professionals will agree that the best type of ventilation is continuous
soffit and ridge ventilation. If a continuous exhaust vent and an equal or slightly
greater amount of intake vent is installed, then the attic will be ventilated for its
entire length. |
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