Almost every conversation about attic ventilation ends with someone saying it looks adequate. That is an impression, not an assessment. The actual requirement is a calculation, it takes about twenty minutes, and doing it yourself frequently reveals that a system everyone assumed was fine is delivering a fraction of what it should.
Length times width of the attic footprint, in square feet. For a simple rectangular house this is straightforward. For a complex plan, break it into rectangles and add them.
Use the attic area, not the whole house floor area — garages, additions with their own separate attics, and vaulted sections are calculated separately because they are separate air volumes.
Call our worked example 1,500 square feet.
Two are permitted.
1:150. One square foot of net free ventilating area for every 150 square feet of attic floor. The default.
1:300. The relaxed ratio, available when the system is balanced — between 40 and 50 percent of the total in the upper portion, at least three feet above the eave vents, with the balance at the eaves.
Most properly designed modern systems use 1:300 because they are balanced. If your attic has, say, only gable vents and no soffit intake, you do not qualify for the relaxed ratio and should be calculating at 1:150.
Worked example at 1:300: 1,500 ÷ 300 = 5 square feet of net free area required. Convert to square inches, because that is how vents are rated: 5 × 144 = 720 square inches.
Balanced means roughly half high and half low. So approximately 360 square inches at the ridge and 360 square inches distributed across the soffits.
If anything, err toward more intake than exhaust. An intake-heavy system slightly pressurises the attic, which pushes air out through the exhaust. An exhaust-heavy system depressurises it, which pulls makeup air from wherever it can get it — and the easiest source is the heated, humidified house below, through ceiling gaps. That is the failure mode that turns a ventilation upgrade into a moisture problem.
This is where most calculations go wrong, and it is the reason systems that look generous perform poorly.
Net free area is the actual unobstructed opening after accounting for the screens, louvres, and baffles built into the vent. Insect screening alone can remove half or more of the gross opening. Louvred soffit vents deliver a fraction of their apparent size.
A rectangular soffit vent measuring 4 by 16 inches has 64 square inches of gross opening. Its published net free area might be somewhere in the region of 26 square inches — well under half. Calculate on the gross figure and you will conclude you have twice the ventilation you actually have.
Manufacturers publish NFA per unit, usually in square inches, and for continuous products such as ridge vent and continuous soffit strip it is given per linear foot. Use those published figures. Where a product is old and unidentifiable, a conservative approach is to assume roughly 50 percent of gross opening for louvred and screened vents, and treat that as an estimate rather than a number.
Exhaust. Measure the ridge vent's linear footage and multiply by its published NFA per foot. For box vents, count them and multiply by the published figure per unit. Include gable vents only if they are part of a coherent single strategy — and note that mixing ridge and gable vents causes short-circuiting, where the ridge draws from the nearby gable instead of from the soffits, leaving parts of the attic with no air movement.
Intake. Count soffit vents and multiply by published NFA, or measure continuous strip footage. Then — and this is the part that matters most — go into the attic and verify they are actually open. A soffit vent that is painted shut, buried under blown insulation, or blocked by a batt pushed into the eave contributes exactly nothing regardless of what it looks like from outside.
Put your two numbers against the requirement. The common results:
Exhaust adequate, intake short or blocked. By far the most frequent finding in Seattle, and the most consequential. Fix intake first — clear the vents, add capacity, install baffles to hold the airway open at the eaves. Adding more exhaust to this system makes it worse, not better.
Both short. Straightforward: add capacity to both sides, maintaining the balance.
Both adequate on paper, attic still damp. The ventilation is not the problem. Look at moisture sources instead: bath and dryer ducts terminating in the attic, unsealed ceiling penetrations, and indoor humidity levels. Ventilation removes moisture that has arrived; it cannot keep up with a source actively pumping humid air upward.
Because the alternative is accepting an impression. Armed with actual numbers — attic area, required NFA, installed intake, installed exhaust — you can evaluate any contractor's ventilation recommendation on its merits, and you will immediately recognise the most common bad advice in the trade, which is adding exhaust to an attic that has no functioning intake.
This post is part of our in-depth coverage of roof inspection in Seattle.
One square foot of net free area per 150 square feet of attic floor, or per 300 where the system is balanced with 40 to 50 percent high. A 1,500 square foot attic at 1:300 needs 720 square inches, split roughly evenly between ridge and soffits. Use published net free area, not the size of the opening.
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