Attic Ventilation Calculator
Calculate required attic net free vent area and the intake to exhaust split.
About Attic Ventilation Calculator
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.
Formula
Attic Ventilation
Net free area = attic floor area ÷ 300, or ÷ 150 without a vapour barrier
- net free area
- Actual open area after insect screening, not the vent's outer dimensions
- 50/50 split
- Half the ventilation at the eaves, half at the ridge
Worked example
With the default values below, Attic Ventilation Calculator returns Net Free Vent Area Required of 0.3167. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Attic floor areaminimum 0.1 m2
- 95 m2
- Ventilation ratioone of 1:300 with vapour barrier, 1:150 without vapour barrier
- 1:300 with vapour barrier
- Net free area per soffit ventminimum 1 cm2
- 250 cm2
- Ridge vent net free area per metreminimum 1 cm2
- 300 cm2
Results
- Net Free Vent Area Required
- 0.3167 m2
- Intake Area Required
- 1,583 cm2
- Exhaust Area Required
- 1,583 cm2
- Soffit Vents Required
- 7
- Ridge Vent Length Required
- 5.28 m
Step by step
- 1Values used
- 2Attic Ventilation
- 3Net Free Vent Area Required= 0.3167 m2
- 4Intake Area Required= 1,583 cm2
- 5Exhaust Area Required= 1,583 cm2
- 6Soffit Vents Required= 7
- 7Ridge Vent Length Required= 5.28 m
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Attic Ventilation calculated?
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.
Why does Attic Ventilation matter?
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.
What values do I need to enter?
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.