Calculate required attic net free vent area and the intake to exhaust split.
The ratio halves to 1:150 without a vapour barrier because more moisture reaches the attic. Splitting intake and exhaust evenly is what establishes the stack flow, and too much exhaust relative to intake pulls conditioned air out of the house. Net free area is typically half a vent's physical opening once screening is accounted for, so sizing on outer dimensions leaves the attic badly under-ventilated.
Attic Ventilation
Net free area = attic floor area ÷ 300, or ÷ 150 without a vapour barrier
Net free area = attic floor area ÷ 300, or ÷ 150 without a vapour barrier The ratio halves to 1:150 without a vapour barrier because more moisture reaches the attic. Splitting intake and exhaust evenly is what establishes the stack flow, and too much exhaust relative to intake pulls conditioned air out of the house.
Net free area is typically half a vent's physical opening once screening is accounted for, so sizing on outer dimensions leaves the attic badly under-ventilated.
This calculator takes 4 inputs: Attic floor area, Ventilation ratio, Net free area per soffit vent, Ridge vent net free area per metre. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.