How much venting does an attic need, and do attics need to be vented at all?
Short answer. Most attics need to be vented, and the common rule is about 1 square foot of net free vent area for every 300 square feet of attic floor, half low and half high. Attics designed as sealed, unvented spaces with spray foam on the roof deck are the exception and should not be vented.
Do attics need to be vented?
- Yes, if insulation is on the attic floor and the attic is outside the conditioned space, which describes most Georgia homes.
- No, if the attic is sealed with insulation directly under the roof deck and built to the code's unvented attic rules. Venting a sealed attic lets humid outdoor air bypass the insulation.
- It depends for cathedral ceilings and bonus rooms, which can be built either way but must be consistent.
Quick counts per 1,000 square feet of attic
At the balanced 1/300 ratio, every 1,000 square feet of attic floor needs about 480 square inches of net free area: about 240 intake and 240 exhaust. In practical terms that is roughly:
- About 5 box vents rated around 50 square inches, or
- About 13 to 14 feet of ridge vent rated 18 square inches per foot, plus
- Soffit intake of about 240 square inches, such as roughly 27 feet of continuous strip vent rated 9 square inches per foot.
Scale these for your attic, and check the ratings printed on the products you use.
Placement matters as much as amount
The 1/300 ratio only applies when 40 to 50 percent of the vent area is in the upper part of the attic, within 3 feet of the ridge measured vertically. Exhaust lower on the slope, or gable vents alone, call for the 1/150 ratio. See roof ventilation basics and ridge vent vs box vents.