roof ventilation

Old 02-09-01, 12:09 PM
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We are planning on adding insulation to the attic in our ranch home in Youngstown, Ohio.
The soffits under the eaves have no vents.
How do I determine the number of vents needed, and how far to space them apart?
I plan on using 4" x 16" vents with 1/8" screen and louvers.
Old 02-09-01, 04:24 PM
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Location: Arlington, WA
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First, figure out the square footage of your attic. Divide that number by 150. This gives you the minimum square feet of attic ventilation that you need. Divide that amount by 2, because half of the ventilation needs to be at the bottom (soffit vents) and the other half needs to be at or near the top, in the form of gable, ridge, or roof vents (the 12" square 'mushrooms' or similar. Using the vents you are talking about, you would need 12 for a 1500 sq.ft. house -- 6 per side, assuming the house is a rectangle.

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