- Rule: 1 sq ft per 150 sq ft of attic
- Split: about half intake, half exhaust
- Never: mix exhaust vent types
- PNW job #1: moisture control
Most attics need 1 square foot of net free vent area for every 150 square feet of attic floor, split roughly evenly between intake vents low at the eaves and exhaust vents high near the ridge. A 1,500 sq ft attic needs 10 sq ft (1,440 sq in) total: about 720 sq in of intake and 720 sq in of exhaust. That 1:150 ratio is the baseline in the International Residential Code (R806.2). Code allows a reduced 1:300 only when the vents are placed in a specific balance, and we design to 1:150 on every roof because in the Pacific Northwest, ventilation’s main job is getting moisture out of the attic.
Ask five people how much attic ventilation a house needs and you’ll hear five different answers. Some say code only requires 1:300. Some say more vents is always better. Neither is quite right, and the wrong answer shows up years later as damp sheathing, moldy rafters, or shingles that wear out early. Here is how the math actually works, how we size vents on a real roof, and what to look for if you think your attic isn’t breathing.
The 1:150 rule, in plain terms
Building code sets attic ventilation as a ratio of net free area (NFA) to the attic’s floor area. Net free area is the actual open space air can move through once you subtract screens, louvers, and baffles. That’s why a vent that looks 12 inches wide might only be rated for a fraction of that.
- Baseline: 1:150. One square foot of NFA for every 150 square feet of attic floor (IRC R806.2).
- Reduced: 1:300. Allowed only when the ventilation is balanced: 40–50% of it must come from vents in the upper part of the attic, no more than 3 feet below the ridge, with the rest low at the eaves. (In the coldest climate zones there’s an added vapor-retarder requirement; that piece doesn’t apply in Portland or Clark County.)
So “code says 1:300” is half true at best. 1:300 is a reduction you earn with a well-balanced layout, not the default. We size to 1:150 because our attics fight moisture for eight months of the year, and the margin costs little compared with replacing rotted decking.
How to calculate the ventilation your attic needs
- Find your attic floor area. For a simple house, it’s roughly the footprint of the heated space below the attic: length × width. Leave out garages and porches with their own roof space unless they share the attic.
- Divide by 150. That’s your total net free area in square feet.
- Convert to square inches. Multiply by 144, since vent products are rated in square inches.
- Split it in half. Roughly half goes to intake at the eaves, half to exhaust near the ridge. If it isn’t even, err toward more intake; an exhaust vent without enough intake pulls conditioned air out of the house instead.
- Divide by the product’s rating. Every vent lists its NFA on the packaging or spec sheet. Divide your intake and exhaust totals by those ratings to get how many vents, or how many feet of ridge vent, you need.
Worked example: a 1,500 sq ft attic
1,500 ÷ 150 = 10 sq ft of net free area
10 × 144 = 1,440 sq in total
Split: 720 sq in intake + 720 sq in exhaust
If your ridge vent is rated around 18 sq in per linear foot (a common rating; check yours), 720 ÷ 18 = 40 feet of ridge vent.
Attic ventilation chart by attic size
| Attic floor area | Total NFA at 1:150 | Intake / exhaust (each) | Total NFA at 1:300 |
|---|---|---|---|
| 1,000 sq ft | 960 sq in | 480 sq in | 480 sq in |
| 1,200 sq ft | 1,152 sq in | 576 sq in | 576 sq in |
| 1,500 sq ft | 1,440 sq in | 720 sq in | 720 sq in |
| 2,000 sq ft | 1,920 sq in | 960 sq in | 960 sq in |
| 2,500 sq ft | 2,400 sq in | 1,200 sq in | 1,200 sq in |
| 3,000 sq ft | 2,880 sq in | 1,440 sq in | 1,440 sq in |
Figures are net free area. The 1:300 column applies only when vents meet the balanced-placement condition above. Roof shape, cathedral ceilings, and blocked soffits change the real-world answer, which is why we measure every roof.
How we size ventilation on a real roof
The math above gets you close. On an actual job, we measure the roof from aerial imagery, run the attic volume through a ventilation calculator, and size intake and exhaust from that, then confirm it in the attic. Two rules we never break:
- Intake can mix and match. Continuous soffit vent, individual soffit vents, and rooftop intake vents can work together to hit the intake number, which helps on homes with shallow or closed-in eaves.
- Exhaust can’t. Pair a ridge vent with box vents or a gable vent on the same attic and the exhaust vents short-circuit each other: one starts pulling air in, and the attic stops venting the way it should. Pick one exhaust type per attic space. Our ridge vent vs. box vent guide covers which one fits which roof.
Why ventilation matters more in the Pacific Northwest
Most national articles sell attic ventilation as a way to cut summer cooling bills. Here, that’s the minor benefit. The big one is moisture. Through a long, wet Portland and Vancouver winter, the house stays closed up while showers, cooking, and laundry push warm, damp air into the attic. When that air hits the cold underside of the roof deck, it condenses. Without enough airflow to carry it out, the attic stays damp for months.
That’s how a roof rots from the inside while the shingles still look fine from the street. Our winter roof maintenance guide covers what attic condensation looks like through the season.
Signs your attic isn’t ventilated enough
- Frost on nail tips poking through the roof deck on cold mornings.
- Damp, dark, or stained sheathing, or black mold spotting on rafters and the underside of the deck.
- A musty smell when you open the attic hatch.
- Insulation pushed into the soffits, blocking intake vents. This is one of the most common problems we find, especially after insulation upgrades.
- A wavy or soft roof deck, or shingles curling and losing granules years ahead of schedule.
- Ice dams during cold snaps, which are rare here but point to heat and moisture building up in the attic.
What it costs to fix attic ventilation
Many ventilation problems are fixed with targeted vent work, not a new roof. Typical local pricing:
| Work | Typical cost |
|---|---|
| Exhaust vent replacement | $150–$200 each |
| Smart or eyebrow intake vent | $400–$500 each |
| Bathroom fan vent through the roof (with ducting) | $400–$600 |
Pricing from Jobin Roofing jobs in Portland and Clark County. Your quote depends on roof pitch, access, and how many vents the attic needs.
If your roof is due for replacement anyway, that’s the best time to correct the ventilation. We size it as part of every re-roof, and getting it right protects both the new roof’s lifespan and its shingle warranty.
Not sure your attic is venting right?
We’ll check your intake, exhaust, and attic for moisture, show you photos of what we find, and tell you honestly whether it needs work. Serving Portland, Vancouver, and all of SW Washington.
Book a free ventilation check WA (360) 952-3998 · OR (503) 523-3449Frequently asked questions
How much attic ventilation do I need?
Plan on 1 square foot of net free vent area for every 150 square feet of attic floor, split about evenly between intake at the eaves and exhaust near the ridge. For a 1,500 sq ft attic, that’s 1,440 square inches total, or about 720 square inches of intake and 720 of exhaust. Building code allows a reduced 1:300 ratio when vents are placed in a specific high-and-low balance, but in the wet Pacific Northwest we design to 1:150 because moisture control is the main goal, and the extra margin costs far less than replacing rotted roof decking.
Is the attic ventilation requirement 1:150 or 1:300?
Both appear in the International Residential Code (R806.2). 1:150 is the baseline requirement. 1:300 is a reduction allowed only when 40–50% of the ventilation comes from vents in the upper part of the attic, within 3 feet of the ridge, with the rest low at the eaves. Colder climate zones add a vapor-retarder condition that doesn’t apply in Portland or Clark County. If your vents aren’t balanced that way, 1:150 is the number that applies, and it’s the one we use on every roof we install.
How many roof vents do I need?
Divide your attic’s required exhaust area by the net free area printed on the vent you plan to use. For a 1,500 sq ft attic at 1:150, you need about 720 square inches of exhaust. With a ridge vent rated around 18 square inches per foot, that’s about 40 feet of ridge vent. With individual box vents, divide 720 by the box vent’s rating instead. Size the intake the same way, and make sure intake is at least equal to exhaust, or the exhaust vents will pull air from inside the house.
Can you have too much attic ventilation?
Not in the usual sense; going somewhat above 1:150 is fine when intake and exhaust stay balanced. The real problems come from imbalance: lots of exhaust with little intake, which pulls conditioned air out of the house, or mixing exhaust types such as a ridge vent plus box vents, which short-circuit each other so part of the attic stops venting. Balance and a single exhaust type matter more than piling on extra vents.
Can I mix ridge vents and box vents?
No. Mixing exhaust types on the same attic space causes one vent to act as intake for the other, so air short-circuits across the top of the attic instead of flowing up from the eaves. Pick one exhaust type per attic. Intake is different: soffit vents and rooftop intake vents can be combined to reach the intake total, which is helpful on homes with shallow eaves.
Does attic ventilation lower energy bills?
Somewhat, mostly by reducing summer attic heat, but in Portland and Vancouver that’s the smaller benefit. Our cool, wet climate makes moisture the bigger concern: proper ventilation carries out the damp indoor air that condenses on the roof deck in winter, preventing mold, sheathing rot, and early shingle failure. Air-sealing the ceiling and keeping insulation out of the soffits do more for energy use than vents alone.
