What is the 1:150 attic ventilation rule?

You need 1 sq ft of net-free vent area for every 150 sq ft of attic floor. A 1,200 sq ft attic needs 8 sq ft of NFA. The ratio eases to 1:300 with balanced venting and a vapor retarder.

How do I split intake and exhaust venting?

Roughly 50/50 between low intake (soffit) and high exhaust (ridge). Keep at least as much intake as exhaust so ridge vents draw from the soffits rather than from inside the house.

What is net-free area (NFA)?

The actual open area of a vent for airflow after subtracting the screen and louvers — always less than the vent’s physical size. It is printed on the product and is the figure the ventilation math uses.

How many vents does my attic need?

Divide the required NFA (in square inches) in half for intake and exhaust, then divide each half by the per-vent NFA. For 1,152 sq in total at 65 sq in per soffit vent, that is about 9 soffit vents plus matching exhaust.

Attic ventilation math: the 1:150 rule, soffit and ridge

An attic needs to breathe: intake low at the soffits, exhaust high at the ridge. The 2021 IRC sets how much net-free vent area you need — here is the math and how to turn it into a vent count.

By Francesco Zinghinì

Why attics need ventilation

A ventilated attic stays close to the outdoor temperature and humidity. In winter that prevents warm, moist indoor air from condensing on the cold underside of the sheathing (which leads to rot and mold) and helps stop the ice dams that form when a warm attic melts snow that refreezes at the cold eaves. In summer it dumps heat that would otherwise bake the shingles from below (shortening their life) and drive up cooling bills. Ventilation works by intake low at the soffits and exhaust high at the ridge, letting outside air wash continuously up the underside of the roof.

Net-free area and the two ratios

Vents are rated by net-free area (NFA) — the actual open area for airflow after subtracting the screen and louvers, which is always less than the vent's physical size. The 2021 IRC section R806 sets the required NFA as a ratio of the attic floor area:

1:150 — 1 sq ft of net-free vent area per 150 sq ft of attic floor (the baseline).
1:300 — half as much is allowed when ventilation is balanced (40–60% of it up high) and, in many cases, a ceiling vapor retarder is present.

So NFA = attic_area ÷ ratio. The attic ventilation calculator works it through and splits intake and exhaust.

Worked example

Take a 1,200 sq ft attic at the 1:150 ratio. Required NFA = 1,200 ÷ 150 = 8 sq ft of net-free area. Convert to square inches (× 144): 8 × 144 = 1,152 sq in. Split it 50/50 between intake and exhaust: 576 sq in each at soffit and ridge. If each soffit vent provides 65 sq in of NFA, you need 576 ÷ 65 = 8.9, rounding up to 9 soffit vents — and matching exhaust up top. At the easier 1:300 ratio the same attic needs only 4 sq ft of NFA, so half as many vents.

The vent types

A balanced system usually pairs continuous intake with continuous exhaust. Soffit vents (continuous strip or individual) are the intake, low under the eaves. Ridge vents run along the peak and are the most common exhaust, giving even airflow across the whole roof. Alternatives for exhaust include gable vents (in the gable-end walls) and powered or solar attic fans. The key is to pick one exhaust strategy and pair it with adequate intake — mixing exhaust types often makes them fight each other.

Balance intake and exhaust

The 50/50 split matters. If exhaust exceeds intake, ridge vents or fans can pull make-up air from inside the house (wasting conditioned air) or from other roof vents rather than the soffits, short-circuiting the airflow so parts of the attic never wash through. If intake is short, the exhaust is starved and does little. Aim for at least as much intake (soffit) NFA as exhaust (ridge). Don't combine a ridge vent with gable or powered exhaust, either — the ridge vent can become an intake for the fan, pulling weather in. Keep insulation from blocking the soffits by using baffles at the eaves.

Reading NFA and spotting problems

Do not confuse a vent's overall size with its NFA — the package states the net-free area in square inches, and that is the figure the math uses. A continuous ridge vent lists NFA per linear foot; a soffit or box vent lists NFA per piece. Total the NFA of the vents you plan and confirm it meets the required intake and exhaust figures. Signs of inadequate ventilation include a scorching attic in summer, frost or damp sheathing in winter, recurring ice dams, and shingles that age prematurely — all reasons to check the math against what's actually installed.

Ventilation and insulation work together

Ventilation and attic insulation are partners, not alternatives, and it's easy to sabotage one with the other. Insulation slows heat moving between the house and the attic; ventilation flushes the attic itself with outside air. You need both, and they meet at the eaves, where deep insulation can smother the soffit intake vents. The fix is baffles (rafter vents) at each rafter bay that hold a clear air channel from the soffit up over the top of the insulation, so you can pile insulation deep and keep the intake open. In winter this combination keeps the attic cold and dry, preventing the warm, moist air that condenses on sheathing and the uneven roof warmth that drives ice dams; in summer it lets heat escape before it bakes the shingles and loads the air conditioner. Size the insulation for R-value with the depth calculator, size the vents for airflow here, and make sure baffles keep the two from fighting.

Code note — confirm locally

The ratios are cited from the 2021 IRC (R806). Editions and local amendments vary, and conditioned or unvented (sealed) attic assemblies follow different rules entirely — they are not vented to the outside at all. Treat this as a planning estimate and confirm with your local building official before you build. Vent work touches the roof — working on a roof is dangerous, so use fall protection or hire a professional.