Attic ventilation calculator

How much attic ventilation do I need?

The International Residential Code (R806.2) requires a net free ventilating area of at least 1/150 of the vented attic space. A 1,500 sq ft attic needs 10 sq ft, or 1,440 square inches.

When is 1/300 allowed?

When 40–50% of the required area is in upper vents no more than 3 feet below the ridge, with the rest in the bottom third of the attic — and, in climate zones 6, 7 and 8, a Class I or II vapor retarder on the warm-in-winter side of the ceiling. The same 1,500 sq ft attic then needs 720 square inches.

How do I split it between upper and lower vents?

Under the 1/300 rule, 40–50% goes high (ridge or gable vents near the top) and the rest low (soffit or eave vents). For 720 in², that is 288–360 in² up high.

What is net free area?

The open area air can actually pass through, after screens and louvers. Vent makers list it on the product, usually in square inches per vent or per foot.

Which area do I measure?

The area of the vented space — usually the attic floor area, length × width of the ceiling below it.

Do ridge vents and soffit vents count together?

Yes, as a system. Air enters low through the soffit or eave vents and leaves high through the ridge or gable vents. The requirement is for the total, with the 1/300 option adding the rule on how the area is split between high and low.

Does insulation affect the vents?

Yes. The IRC (R806.3) calls for a clear air space of at least 1 inch between the insulation and the roof sheathing where eave or cornice vents are installed, so that insulation does not block the air path. Baffles at the eaves are the usual way to keep it open.

Does the calculator cover a cathedral ceiling or an unvented attic?

No. Rafter spaces in a vaulted ceiling follow the same net free area rule per space, but unvented (sealed) attic assemblies follow a different section of the code with its own insulation requirements. Ask your building department which applies.

How the calculation works

The code ties the vent area to the size of the space being ventilated. Take the attic floor area in square feet and divide by 150 to get the required net free area in square feet, or by 300 if the conditions for the reduced ratio are met. Multiply by 144 to convert to square inches, which is how vent products are rated. Under the 1/300 option, 40 to 50 percent of that area goes into upper vents and the remainder in the lower part. The result is the net free area, not the size of the opening on a vent catalog: a louvered vent with a screen passes only part of its gross opening.

Worked example

For an attic of 1,800 square feet, the 1/150 rule requires 1,800 ÷ 150 = 12 square feet, which is 1,728 square inches of net free area. If the conditions for 1/300 are met, the requirement is 1,800 ÷ 300 = 6 square feet, or 864 square inches. Of that, 346 to 432 square inches must be in the upper vents and the remaining 432 to 518 square inches in the lower ones.

Common mistakes

• Using gross vent size. Always use the net free area printed on the product. A vent with a large visible opening can have a much smaller net free area once the louvers and screen are counted.
• Only adding high vents. Ridge or gable vents with no intake at the eaves cannot move air through the attic. The 1/300 option requires the low vents explicitly.
• Blocking the soffit with insulation. Blown insulation pushed into the eaves can seal the soffit vents completely. Baffles keep the 1-inch air space.

Sources and related calculators

Code: International Residential Code section R806 (R806.2 minimum vent area, R806.3 vent and insulation clearance) as adopted in your jurisdiction. Related: roof calculator, insulation calculator and CFM calculator.

Code minimums for planning only. Your local building department decides which code edition and amendments apply. How we check our calculators.