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How much attic ventilation do you need?
Under-ventilated PNW attics rot decking and cook shingles from below. Enter your attic size for an estimate of the intake and exhaust your roof should have — the same net-free-area math a pro uses.
Straight answers
How much attic ventilation do I need?
A common code baseline is 1 square foot of net free ventilation area (NFA) for every 150 square feet of attic floor — reducible to 1:300 when the ventilation is balanced (roughly half intake low at the soffits, half exhaust high at the ridge) and/or a vapor retarder is present. This calculator does that math; a pro should confirm it for your specific roof.
What is net free area (NFA)?
NFA is the actual open area a vent provides for airflow, after the screening and louvers block part of the opening. Every vent product is rated for its NFA (usually in square inches), so you compare the NFA this calculator asks for against the rated NFA of the vents you're considering — not the vent's overall size.
Should intake and exhaust be equal?
Aim for balanced — roughly 50/50 intake (soffit) and exhaust (ridge), or slightly more intake. Too much exhaust without matching intake can pull conditioned air out of the house and even backdraft. This tool splits the requirement 50/50 as the target.
A number is a start. A look in the attic is the truth.
Our free inspection includes the attic side — half of what kills PNW roofs never shows from the street.