Attic Ventilation Calculator
Intake must exceed exhaust, not the reverse.
Intake must exceed exhaust, not the reverse. Free ventilation area for a roof space, split between intake at the eaves and exhaust at the ridge.
1:300 is common where a vapour barrier is present, 1:150 where it is not
Free area, not the physical opening — grilles and mesh reduce it substantially
Free ventilation area at 1:300
3,333 cm²
2,000 cm² at the eaves · 1,333 cm² at the ridge
Intake should exceed exhaust, which is the detail most often reversed. Exhaust without matching intake pulls air from the easiest available source — which is the heated house below through gaps in the ceiling — rather than through the roof space, so it makes the moisture problem worse while appearing to address it. Note also that free area is not the same as the physical opening: grilles, mesh and louvres can reduce it by half or more, and the manufacturer's stated free area is the figure to use.
How the Attic Ventilation Calculator works
Free ventilation area for a roof space, split between intake at the eaves and exhaust at the ridge. Intake should exceed exhaust — the reverse pulls air from the heated house below rather than through the roof.
Also known as: loft ventilation area calculator · how much roof ventilation do i need · 1 in 300 ventilation rule · soffit vent area calculator
Intake must exceed exhaust
The convention splits ventilation area between intake at the eaves and exhaust at the ridge, weighted towards intake — commonly 60/40. That weighting is not arbitrary and reversing it causes an active problem.
Exhaust without matching intake takes air from the easiest available source. In a house that is the heated space below, through gaps around light fittings, loft hatches and service penetrations, so warm moist air is pulled into the roof rather than out of it.
Which makes an under-vented eaves detail worse than no ridge ventilation at all. It converts a passive roof space into one that actively draws moisture from the house, while appearing to address the problem.
Free area is not hole size
Free area is the actual open area after grilles, mesh and louvres, and it can be half the physical opening or less. Manufacturers state it because the difference is large enough to matter.
Using the hole dimensions instead overstates ventilation substantially, and it is a common error because the hole is the thing you can measure. The stated free area is the figure the ratio applies to.
Baffles are the other half of the detail. Insulation pushed into the eaves blocks the intake path entirely, and blocked soffits are among the most common ventilation faults. Baffles hold a clear channel between insulation and roof deck.
Why the roof space needs ventilating at all
Moisture rises from the house below and condenses on cold roof timbers. Over time that causes rot and reduces the performance of insulation that gets damp, and in cold climates it contributes to ice damming at the eaves.
Ventilation carries that moisture away and keeps the roof deck cold, which is what prevents snow melting and refreezing at the eaves. Air-sealing the ceiling addresses the same problem from the other end and is usually the higher-return measure.
None of this transfers to a warm roof, insulated at the rafter line — some warm roof designs are unventilated by intent, and applying cold-roof rules to one is a design error rather than a conservative choice.
Where to go next
The Attic Ventilation question rarely arrives on its own. These are the ones that usually come with it:
- Insulation Calculator — The first hundred millimetres does most of the work.
- Rafter Length Calculator — The run is half the span less half the ridge.
- Dehumidifier Size Calculator — The rating collapses in a cold room.
- Ring Size Calculator — Finger measurement to US, UK, EU and Japanese sizes.
Frequently asked questions
How much attic ventilation do I need?
Commonly one square unit of free area per 300 of attic floor area where a vapour barrier is present, or 1:150 where it is not. The applicable ratio is set by local code.
Why should intake exceed exhaust?
Because exhaust without matching intake takes air from the easiest available source, which is the heated house through gaps in the ceiling. That makes the moisture problem worse while appearing to address it.
What is free area?
The actual open area after grilles, mesh and louvres, which can be half the physical opening or less. Manufacturers state it, and using the hole size instead overstates ventilation substantially.
Why does a roof space need ventilating?
To carry away moisture that rises from the house below. Trapped moisture condenses on cold roof timbers, causing rot and reducing insulation performance, and in cold climates it contributes to ice damming.
Can insulation block soffit vents?
Easily, and it is one of the most common faults. Insulation pushed into the eaves blocks the intake path entirely, which is why baffles are fitted to hold a clear channel.
Does a warm roof need ventilation?
A warm roof, insulated at the rafter line, is a different assembly with different rules — some designs are unventilated by intent. Applying cold-roof ventilation rules to a warm roof is a design error.
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Related calculators
Insulation Calculator
The first hundred millimetres does most of the work.
OpenRafter Length Calculator
The run is half the span less half the ridge.
OpenDehumidifier Size Calculator
The rating collapses in a cold room.
OpenRing Size Calculator
Finger measurement to US, UK, EU and Japanese sizes.
Open