Exhaust vents get installed; intake gets blocked with insulation and nobody checks. An unbalanced attic does not ventilate, and it takes years off the roof above it.
Reviewed by Michael ThompsonRoof ventilation moves outside air through the attic - in low at the eaves, out high at the ridge - so that heat and moisture leave rather than accumulating. It only works as a balanced system: exhaust without matching intake pulls conditioned air out of the house instead of drawing fresh air through the attic. The model building codes size it by net free area, typically 1 square foot of vent per 150 square feet of attic floor, reduced to 1:300 where intake and exhaust are properly balanced.
Understand what is being ventilated, and why
An unvented attic in summer becomes an oven that cooks the underside of the roof covering and drives heat into the ceilings below. In winter the problem inverts: warm moist air from the house rises into a cold attic, condenses on the sheathing, wets the insulation and feeds mould. Ventilation addresses both by keeping attic conditions close to outdoor conditions.
Watch forVentilation discussed only as a summer cooling measure. The winter moisture case does more damage to more roofs, and it is the one that shows up as a "leak" that appears without rain.
Intake at the eaves - soffit vents, and the baffles behind them
Air enters low, through continuous or individual soffit vents, or through vented drip edge where there is no soffit to work with. Behind each intake, a baffle should hold the insulation clear so the air path into the attic is open.
Watch forInsulation blown straight over the soffits. This is the single most common ventilation fault in existing houses, it is invisible from outside, and it renders every exhaust vent on the roof useless. It is also cheap to fix.
Exhaust high on the roof - ridge, box, turbine or powered
Air leaves at or near the ridge. Continuous ridge vent with an external baffle is the usual choice on a roof with a suitable ridge; box vents, turbines or gable vents serve roofs where a continuous ridge is not available.
Watch forMixing exhaust types on the same attic space - a ridge vent plus gable vents, or a powered fan plus a ridge vent. The higher-capacity opening starts pulling air in through the other one rather than from the soffits, and the attic short-circuits instead of ventilating.
Calculating net free area
Vents are rated in net free area - the actual open area for air, which is much less than the physical size of the vent because of louvres and screens. You take the attic floor area, apply the required ratio, split the result roughly evenly between intake and exhaust, and select vents whose published NFA adds up.
Watch forSizing by counting vents rather than by net free area. Two different ridge vents of the same length can differ substantially in NFA per foot, and a vent chosen on appearance can leave you short without anybody noticing.
Balancing - and why intake should never be the smaller half
Good practice is to have at least as much intake as exhaust, and many roofers deliberately run intake slightly higher. An intake-starved attic converts its exhaust vents into a mechanism for pulling heated, humid air out of your living space through every ceiling gap.
Watch forMore exhaust added to fix a ventilation problem. If the attic is short on intake, adding exhaust makes the imbalance worse. Diagnose the intake first, every time.
The unvented alternative, where it applies
Not every roof should be vented. Conditioned attics, cathedral ceilings with no cavity to run air through, and spray-foam assemblies at the roof line are designed as sealed, unvented systems. These are legitimate and code-recognised, and they follow completely different rules.
Watch forSoffit vents added to a spray-foamed roof deck because a roofer assumed all attics need venting. Introducing outside air into an assembly designed to be sealed causes condensation inside the insulation, and that is a serious and expensive problem.
Adding ventilation during a roof replacement is usually a modest line item - hundreds rather than thousands for ridge vent and soffit work. Retrofitting soffit intake on a house with none, or unblocking insulated soffits, costs more. Our cost guide has the detail.
See the full roof ventilation cost guide - by material, size and region →
Attic ventilation calculator → Enter your attic floor area and get the net free area you need, split between intake and exhaust.
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