Attic Ventilation Calculator
A vented attic is supposed to be a cold attic. Air comes in low at the eaves, picks up the heat and moisture that have leaked up from the house, and leaves high at the ridge. When that path is too small or only open at one end, the attic stays warm and damp, and the results show up as ice dams in winter, cooked shingles in summer, and mould on the underside of the deck in any season.
Ventilation from attic floor area
How much net free ventilating area an attic needs, split between intake and exhaust, and what that is in vents.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Net free area required The total opening the attic needs, before it is split between intake and exhaust.
- Intake, at the eaves Half the total. Low down, where the air comes in.
- Exhaust, at the ridge The other half. High up, where it leaves.
- Ridge vent needed If the exhaust is all ridge vent.
- Soffit vents needed If the intake is all individual soffit vents.
- What you have now
- Shortfall
Both halves meet the requirement. Worth checking that the intake is actually open: insulation pushed into the eaves blocks soffit vents completely, and a vent that is rated for 9 square inches and covered by batts is rated for nothing.
Short of intake, which is the usual problem and the one that does damage. Exhaust without matching intake cannot pull air from the eaves, so it pulls it from the house instead, through every gap in the ceiling — carrying moisture up into the attic, which is exactly what ventilation is there to prevent. Adding more exhaust makes this worse. Add intake.
Intake is adequate but exhaust is short, so warm air is collecting at the ridge with nowhere to go. This is the straightforward case: continuous ridge vent is usually the cleanest fix, and it can often go on during a re-roof at little extra cost.
Both halves are short. Put the shortfall in at both ends rather than whichever is easier to reach: ventilation works as a path from eave to ridge, and doubling one end of a path that is blocked at the other end changes nothing.
The ratio is the code minimum for a vented attic, and it assumes the vents are reasonably spread along the eaves and the ridge rather than clustered. It does not apply to an unvented or conditioned attic, where the roof is insulated at the deck and ventilation is deliberately absent — those are designed the opposite way round and adding vents to one is a mistake. Mixing types of exhaust also undoes the maths: a ridge vent working alongside gable vents will usually short-circuit and draw from the gables instead of the eaves.
Where the ratio comes from
Code sets the ventilating area as a fraction of the area of the space being ventilated. The base figure is 1 square foot of net free area for every 150 square feet of attic floor. It halves to 1 in 300 only when both conditions in the International Residential Code are met. First, the vents are balanced: between 40 and 50 per cent of the total is placed high, no more than 3 feet below the ridge, and the rest is in the lower third of the attic, usually at the eaves. Second, in climate zones 6, 7 and 8, a Class I or II vapour retarder is also fitted on the warm side of the ceiling. Outside those zones, balanced vents alone are enough.
A house with continuous soffit intake and a ridge vent usually meets the first condition, which is why the calculator opens at 1 in 300. If you are in zone 6, 7 or 8 without a vapour retarder, or you are not sure either condition applies, work to 1 in 150 and you will be right either way.
Net free area is not the size of the hole
This is the part that catches people out. A vent is a hole covered by a louvre and a screen, and both of those block air. Net free area is what is left after the blockage, and it is always smaller than the opening you cut. A 16 by 8 inch soffit vent is 128 square inches of hole and typically around 65 square inches of net free area.
The figure is published for every vent worth buying, printed on the packaging or in the data sheet. Never work from the size of the opening, and be wary of any vent that does not publish a rating at all.
A worked example
A 1,200 square foot attic at 1 in 300 needs 4 square feet of net free area, which is 576 square inches. Split evenly, that is 288 square inches low and 288 high. At 18 square inches per foot, the exhaust is about 16 feet of ridge vent. At 9 square inches each, the intake is 32 soffit vents — which sounds like a lot, and is exactly why continuous soffit venting is the usual answer on a house of that size.
The half that gets forgotten
Intake. Almost every under-ventilated attic we would describe has enough exhaust and not enough intake, and the reason is that exhaust is visible from the street while intake is tucked under the eaves where nobody looks.
It matters because ventilation is a path, not a collection of holes. If the ridge vent cannot draw air from the soffits, it draws it from the house instead, through light fittings, loft hatches and every gap around a pipe — pulling warm, humid indoor air up into a cold attic, which is the precise mechanism that ventilation exists to prevent. Adding more exhaust to an intake-starved attic makes the problem worse, not better.
The other common fault is free: insulation stuffed into the eaves, blocking soffit vents that are otherwise perfectly good. Baffles hold the insulation back and keep the channel open, and fitting them is a morning’s work.
Do not mix types of exhaust
A ridge vent working alongside gable vents will usually short-circuit. Air takes the easy route, so the ridge pulls from the nearby gable rather than the distant soffits, and most of the attic sees no air movement at all. Pick one exhaust strategy and commit to it. The same applies to powered fans added on top of a passive system.
When none of this applies
Unvented attics exist and are a legitimate design. The insulation goes at the roof deck instead of the ceiling, the attic becomes part of the conditioned space, and there is deliberately no ventilation at all. Adding vents to one is a mistake, not an improvement. If your roof is insulated at the rafters rather than the attic floor, this calculator is not describing your house.