TradesHow this works
Enter the attic floor area and whether it qualifies for the 1/300 rule. The tool works out the net free vent area you need under both the 1/150 minimum and the 1/300 exception, splits it evenly between low intake and high exhaust, and estimates how many intake vents that is at the NFA you enter. Then it recommends the rule you can use.
Balance is the part people miss. Keep intake at least equal to exhaust, or a ridge vent can pull air from itself instead of drawing it up from the soffits. And read the NFA off the product, since a small louvered vent passes far less air than its size suggests.
Common questions
How much attic ventilation do I need?
The IRC minimum is one square foot of net free area for every 150 square feet of attic floor, called the 1/150 rule. You can halve it to 1/300 if the attic has a vapor retarder over the living space and balanced high-exhaust, low-intake venting. This tool works out both and recommends the one you qualify for.
What is net free area?
Net free area, or NFA, is the actual open area a vent lets air through after its screen and louvers, not the size of the hole it covers. It is printed on the product in square inches. A vent labeled 8 by 16 inches might have an NFA of only 50 to 75 square inches once you subtract the mesh and slats.
What is the difference between the 1/150 and 1/300 rules?
The 1/300 rule needs half the vent area, so fewer vents and less cutting. But you only earn it with two things: a Class I or II vapor retarder over the living space, and 40 to 50 percent of the venting up high as exhaust with the balance low as intake. Without both, you are on the 1/150 minimum.
How do I split intake and exhaust?
Split the required net free area 50/50, half as low intake at the soffits or eaves and half as high exhaust at the ridge or roof. Never let exhaust exceed intake. If a ridge vent has more area than the soffits feed it, it can pull air from itself and short-circuit the flow.
Does this replace a code check?
No. This is the IRC R806 rule of thumb to size venting during planning. Your local code, the roof design, and any unvented or conditioned-attic assembly can change what is required. Confirm the requirement for your project before you cut vents.
Sources: IRC R806.2: minimum NFA = 1/150 of vented attic area; 1/300 with a Class I/II vapor retarder + balanced high/low venting · Split NFA 50/50 intake/exhaust; exhaust never exceeds intake (ARMA, Benjamin Obdyke, Roofing Contractor). Engine version 1.0.0. Risk tier 2.