calcs.homes
calcs.homes
Enter your attic floor area and pick the 1/150 or 1/300 ventilation rule to get the total net free area needed, split evenly into intake and exhaust, plus the ridge vent length that covers the exhaust half.
Estimate only — use each vent's rated net free area from its data sheet, not its physical opening.
Code sets the minimum ventilating area as a fraction of the attic floor area — 1/300 when intake and exhaust are balanced and the ceiling has a vapor retarder, otherwise 1/150. We convert that ratio to square inches (×144), then split it 50/50 between intake at the soffits and exhaust at or near the ridge. Dividing the exhaust figure by the ridge vent's rated net free area per foot gives the ridge vent length.
Net free area is the actual open area for airflow after the louver, screen, and baffle — always smaller than the vent's cut opening, and printed on the product. Keeping intake at least equal to exhaust matters: an exhaust-heavy attic pulls make-up air backward through the ridge or bath fans instead of the soffits.
Provide 1 square foot of net free ventilating area for every 300 square feet of attic floor, when intake and exhaust are balanced and the ceiling has a vapor barrier. Without those conditions the ratio is 1/150.
Yes, or slightly intake-heavy. If exhaust exceeds intake, the ridge or gable vents pull air from the path of least resistance — often back down other roof vents — instead of from the soffits.
Divide the exhaust net free area by the ridge vent's rated NFA per foot (about 18 sq in/ft for shingle-over vents). Most gable roofs have enough ridge length to cover it.