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Calculating Ventilation Area of Attic

Calculating Ventilation Area

OK, to determine how much ventilation area you need for your house, all you need to know is the square foot area of the attic to be ventilated. Let's use the following simple ranch house for an example.

The house measures 30 feet by 50 feet. There is a 22 foot by 26 foot attached non-heated garage. In reality, the garage doesn't figure into the calculations. That doesn't mean you shouldn't ventilate it, it just means that I will not include it in my calculations:

30 feet X 50 feet = 1,500 square feet

OK so now we know the square footage of the attic area to be ventilated. Now, let's figure out how much ventilation space we need. Remember, the minimum area requirement when using a continuous system is a ratio of 1:300. So, in our example, we will divide the 1,500 square feet by 300.

1,500 square feet / 300 = 5 square feet

See how simple that was! We know we need 5 square feet total of ventilation area. Based upon just about everyone's recommendations, 60 percent (or 3 square feet) of this should be in the soffit area and the remaining 40 percent (2 square feet) should be located at the ridge of the roof. Let's see how a standard ridge vent product will calculate out.

Standard ridge vent produces 18 square inches of ventilating area per lineal foot. Our roof is 50 feet long, so using a standard product we will get 900 square inches of ventilating space. Nine hundred square inches equals 6.25 square feet. So the ridge ventilation has exceeded the minimum requirement. That's great! But what happens if we choose to use those individual metal pot vents instead? The calculations before indicated that we needed 2 square feet of ridge or upper roof ventilation. Two square feet equals 288 square inches. A normal metal pot vent produces only 50 squares inches of free net area. So, we would need at least six metal vents in addition to the lower soffit ventilation to meet minimum ventilation requirements.

If you choose not to use continuous ventilation, guess what? The required ventilation area doubles! This means that you would not have any soffit ventilation. It then means that you would need a total of 10 square feet of ventilating area. How many metal pot vents you may ask? Believe it or not, you would need 29 of those individual metal pot vents to meet minimum requirements. Have you ever seen that many on a roof before? I know I haven't!

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7 Responses to Calculating Ventilation Area of Attic

  1. We have a sunroom with its own attic space. It's only 8×10. Do we need to ventilate such a tiny space? According to the math, it's .26 sq ft.

  2. How come you only calculated using the 1:300 rule? What about the 1:150 rule. There seems to be some situation when you use one or the other but you never discussed that. Could you discuss that?

  3. Michael Glaser is correct. The 1:300 is used when calculating equivalent roof/soffit venting. In that case, the resulting area should be applied to BOTH. When calculating a 60/40 scenario, 1:150 is used and then a factor of 0.6 is used to calculate the soffit vent area and 0.4 to calculate the roof vent (or NFA) area.

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