Attic ventilation calculator
Enter the attic's floor area. You get the net free vent area the code requires, how it splits between exhaust and intake, and how many feet of ridge and soffit vent that is.
Inputs
Result
How it is worked out
A 40 by 30 ft attic is 1,200 sq ft. At 1/150 it needs 8 sq ft, or 1,152 sq in, of net free area. Balanced, at 1/300, it needs 576 sq in: 288 at the ridge and 288 at the eaves. With a ridge vent rated 18 sq in per foot, that is 16 ft of ridge vent, and 32 ft of soffit vent at 9 sq in per foot.
Net free area is on the product
Every vent is rated for net free area, the open area left once louvers, screens and baffles are counted. A ridge vent might be 12 to 20 sq in per foot and a continuous soffit strip 4 to 10; the defaults here are typical, not universal. Use the number printed on the product or its data sheet.
Shingle makers generally want intake equal to or greater than exhaust, so a 50/50 split, or a little more at the eaves, is the safe choice. Never mix two kinds of exhaust, such as a ridge vent and gable vents, on the same attic; one can short-circuit the other.
The 2015 IRC let either condition alone reduce the area to 1/300. The 2018 and 2021 editions need both, with the vapor retarder required only in Climate Zones 6, 7 and 8.
Questions
How much attic ventilation do I need?
1 sq ft of net free vent area for every 150 sq ft of attic floor, or 1 per 300 if the vents are balanced between the ridge and the eaves (and, in the coldest zones, the ceiling has a vapor retarder). A 1,200 sq ft attic needs 8 sq ft, or 4 balanced.
What is the 1/300 rule?
IRC R806.2 lets the vent area drop from 1/150 to 1/300 of the attic floor when 40 to 50% of it is in the upper part of the roof, within 3 ft of the ridge, and the rest is low at the eaves. In Climate Zones 6 to 8 a Class I or II vapor retarder on the ceiling is also required.
How many feet of ridge vent do I need?
Half the required net free area divided by the vent's rating. For 576 sq in balanced and an 18 sq in per foot ridge vent, 16 ft.
How much soffit venting do I need?
At least as much as the exhaust, usually half the total or a little more. For 288 sq in at 9 sq in per foot of continuous soffit vent, 32 ft.
What is net free area?
The actual open area of a vent after its screen and louvers are deducted, in square inches. It is printed on every code-listed vent.
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