Attic Ventilation Calculator

Attic Ventilation Calculator

Proper attic ventilation prevents moisture buildup (which can lead to mold and wood rot) and reduces summer heat buildup that stresses your roof and cooling system - this calculator applies the standard building code ratio to size your required vent area.

Required Vent Area = Attic Floor Area / 300 (with a proper vapor barrier)
Required Vent Area = Attic Floor Area / 150 (without a vapor barrier)

Example

1,500 sq ft attic floor area, with a proper vapor barrier installed:

Required Vent Area = 1,500 / 300 = 5.0 sq ft (720 sq in) of net free vent area

Why the Ratio Doubles Without a Vapor Barrier

A vapor barrier limits how much moisture rises from living spaces into the attic in the first place - without one, moisture load on the attic is considerably higher, so building codes commonly require double the ventilation area (1/150 instead of 1/300) to compensate.

Balancing Intake and Exhaust

This calculated vent area is typically split roughly 50/50 between intake vents (soffit vents, positioned low on the roof) and exhaust vents (ridge vents or gable vents, positioned high) - this balanced split creates effective natural airflow through the attic space, rather than concentrating all ventilation at one height, which can create dead air pockets elsewhere in the attic.