Attic Ventilation Calculator

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.

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.

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.

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.

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.

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.

Fundamentally. A cold roof insulates at ceiling level and ventilates the space above; a warm roof insulates at the rafter line and some designs are unventilated by intent. The rules do not transfer between them.

Heat escaping into the roof space melts snow, which refreezes at the cold eaves and dams the water behind it. The fix is air-sealing and insulation at the ceiling, with ventilation keeping the roof deck cold.

For the cross-flow to work, yes — air enters low and leaves high. Ridge vents without soffit intake pull air from the house below instead, which is worse than no ventilation.

It is rarely the problem in practice, though excessive exhaust without matching intake actively causes one. The balance between intake and exhaust matters more than the total.

They hold a clear air channel between the insulation and the roof deck at the eaves, so insulation does not block the soffit vents. Blocked soffits are one of the most common attic ventilation faults.

It can make things worse by depressurising the attic and pulling conditioned air from the house through ceiling gaps. Passive ventilation with adequate intake is generally preferred.