Gutters & ventilation
Attic ventilation: the 1:150 rule and balancing intake and exhaust
An attic needs air moving from the eaves to the ridge. The code gives the amount; the balance between intake and exhaust decides whether it works.
Why attics are ventilated
A vented attic is meant to be outside the house: cold in winter, as close to outdoor temperature as possible in summer. Ventilation does three jobs. In winter it carries away moisture that rises from the house through ceiling gaps, before it condenses on the cold underside of the roof deck and rots it. It keeps the deck cold, which reduces the melting that feeds ice dams at the eaves. And in summer it lets out some of the heat that bakes asphalt shingles from below, which is why shingle makers tie their warranties to ventilation.
The rule
The International Residential Code (R806.2) sets the minimum net free area of vent openings:
- 1 square foot per 150 square feet of attic floor (1:150), as the basic rule;
- 1 per 300 (1:300) when at least 40 percent and no more than 50 percent of the area is exhaust vents in the upper part of the attic (at least 3 feet above the eave vents), or when a Class I or II vapor retarder is installed on the warm side of the ceiling in climate zones 6, 7 and 8.
Net free area is the open area of a vent after its screen, louvres and baffles: what air can actually pass through. It is printed on vents and their packaging and is always less than the vent’s size.
| Attic floor | 1:150 | 1:300 | Intake and exhaust each at 1:300 |
|---|---|---|---|
| 1,000 sq ft | 960 sq in | 480 sq in | 240 sq in |
| 1,500 sq ft | 1,440 sq in | 720 sq in | 360 sq in |
| 2,000 sq ft | 1,920 sq in | 960 sq in | 480 sq in |
| 2,500 sq ft | 2,400 sq in | 1,200 sq in | 600 sq in |
The attic ventilation calculator works this out for any attic, and the ventilation table lists more sizes with typical vent ratings.
Balance: why intake matters more
Air enters at the soffits and leaves near the ridge, pulled by the stack effect and by wind. If the exhaust area is larger than the intake, the exhaust vents look for air elsewhere, and the easiest source is the house below, through gaps around light fixtures, top plates and the attic hatch. That pulls warm, humid air into the attic in winter and cooled air out in summer: the opposite of the goal. At least as much intake as exhaust is the safe direction, and 50/50 is the usual design.
A 1,500 square foot attic
1:300, balanced: 720 square inches, 360 intake and 360 exhaust.
- Exhaust as ridge vent rated 18 sq in per foot: 360 ÷ 18 = 20 ft of vent, though most installers vent the whole usable ridge
- Exhaust as box vents of 50 sq in: 8 vents
- Intake as 16 × 8 inch soffit vents of about 56 sq in: 7 vents, about 11 ft apart along 80 ft of eave
From the ridge vent, box vent and soffit vent calculators.
One exhaust type per roof
A ridge vent combined with gable vents, or with box vents, short-circuits: the vent nearest the wind becomes an intake for the others, and the soffits stop drawing. The attic ends up ventilated near the top and stagnant at the eaves, which is exactly where moisture and ice dams form. Pick one exhaust type, close the others, and feed it from the soffits.
Keeping the intake open
Soffit vents are useless if insulation fills the rafter bays above them. Rafter baffles, chutes stapled to the underside of the deck in each vented bay, hold the insulation back and keep a channel open from the soffit past the top of the insulation. They must reach at least 2 inches above the finished insulation; on shallow pitches with deep insulation that can take two baffles per bay. The rafter baffles calculator counts them.
Vented vinyl soffit panels are another trap: many deliver only a few square inches of net free area per square foot of panel. Check the rating against the intake needed.
Powered fans
Powered attic ventilators move a lot of air but need intake to match, about 1 square foot of net free area per 300 CFM. Without it they depressurise the attic and pull air from the house, and can backdraft gas appliances. Building scientists are sceptical about their energy savings in air-conditioned houses for that reason. The attic fan calculator sizes one with its intake; in most houses balanced passive ventilation over a sealed ceiling is the better plan.
Ventilation and insulation together
Ventilation removes moisture and heat that get into the attic; air-sealing and insulation keep them from getting there in the first place. A leaky ceiling overwhelms any vent area, and an attic stuffed with insulation that blocks the soffits defeats it. The order of work is: air-seal the ceiling, fit baffles, insulate, then check the vents. Attic insulation by climate zone covers the second half, and ice and water shield covers the eave membrane that backs up ventilation in snow country.
Unvented attics
The code also allows unvented attics (R806.5), where insulation is applied directly to the underside of the roof deck and the attic becomes part of the conditioned space. They suit attics full of ductwork and equipment and complex roofs that are hard to ventilate, but the insulation type and thickness are specified by climate zone, and they are designed, not improvised. The rules on this page are for vented attics.
Ventilation and roof life
Shingle makers can deny warranty claims on roofs over unventilated attics, and poor ventilation shortens shingle life by years; the replacement planner takes it into account. How long roofs last lists the expected lives it shortens.