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Attic Ventilation Calculator

Attic area in, net free area out: the ventilation IRC R806.2 requires, split between exhaust and intake, compared to what the roof already has, with the ridge vent, box vents or soffit vents needed to close the gap and a copy-ready F9 note. Free, no sign-up, nothing leaves your browser. Built by the estimators at Estimate on Demand for one of the line items carriers leave off most often.

SF
Area of the space being ventilated (usually the house footprint under the roof). Not the roof area.
IRC R806.2: 1:150, or 1:300 where 40–50% of the vent area is in the upper third of the attic (at least 3 ft above eave vents) with the balance at the eaves, or in climate zones 6–8 with a Class I or II vapor retarder on the warm side.
%
Upper (exhaust) vents 40–50%; the rest is intake at the eaves. 50/50 is the usual target.
LF
From the roof report; limits how much ridge vent can be installed.

Required net free area

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Exhaust (upper) required—
Intake (eave) required—
Existing exhaust—
Existing intake—
Exhaust shortfall—
Intake shortfall—

To meet code (supplement quantities)

Ridge vent to add (limited by ridge length)—
Or box vents to add—
Continuous soffit vent to add—
Or individual soffit vents to add—
Exhaust is figured two ways so you can pick the line item that fits the roof; intake likewise. Do not mix ridge vent with gable or box exhaust on the same attic unless the manufacturer allows it; short-circuiting reduces the effective area.

F9 note (copy into Xactimate)

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Net free area (NFA) defaults are typical published values; the manufacturer’s NFA governs and is what the F9 note should cite. Whether ventilation-to-code is owed on a claim depends on the adopted code, whether the roof is being replaced (R908.4 applies existing-building rules), and the policy’s ordinance-or-law coverage. Inputs stay in your browser.

How the math works

The code ratio is area to area: the attic floor area in square inches divided by 150 (or 300 under the exception) gives the net free ventilating area required, in square inches. That total is split between upper (exhaust) and lower (intake) vents, 40 to 50 percent high and the balance at the eaves. Each vent type contributes its published net free area: roughly 18 square inches per linear foot for ridge vent, 50 for a box vent, about 110 for a 12-inch turbine, 9 per linear foot for continuous soffit vent, 56 for a standard 8-by-16 soffit vent. The shortfall on each side, divided by the NFA of the vent you intend to install, is the quantity for the supplement, with ridge vent capped by the ridge length available.

Ventilation FAQ

What does IRC R806.2 require?

Enclosed attics and rafter spaces must have cross ventilation with a net free area of at least 1/150 of the area of the space ventilated. The ratio may be 1/300 where 40 to 50 percent of the required area is provided by vents in the upper portion of the attic, at least 3 feet above eave or cornice vents, with the balance at the eaves; or in climate zones 6, 7 and 8 where a Class I or II vapor retarder is installed on the warm side of the ceiling. R806.1 requires the openings to be screened and protected against rain and snow.

What is net free area?

The unobstructed opening a vent actually provides after screens, louvers and baffles, in square inches. A 12-by-12 box vent has far less than 144 square inches of net free area, which is why the manufacturer’s published NFA, not the vent’s size, is what the calculation and the F9 note should use. The defaults here are typical published figures and are editable.

Why does the calculator split exhaust and intake?

Because the code and every manufacturer do. Exhaust without intake does not ventilate; it pulls conditioned air out of the house. A balanced system puts roughly half the required area at the eaves and half high on the roof. The tool reports the shortfall on each side separately so the supplement can add the right line item: ridge vent or box vents for exhaust, continuous or individual soffit vents for intake.

When is ventilation to code owed on a claim?

Where the adopted code requires it and the roof is being replaced, bringing ventilation to code is an ordinance-or-law item, owed under the policy’s code-upgrade coverage. Where the old roof already had adequate ventilation, replacing the vents like-kind is simply part of the roof. Manufacturer shingle warranties also require adequate ventilation, which is a second argument for the line. Our roof ventilation requirements post covers the details.

More free tools are on the tools page. Rather have the whole supplement written? EOD’s roofing supplement services: 10% of the increase, no increase, no fee, no minimums. Send us a claim.

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