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Ventilation Calculator
Net Free Ventilation Area (NFVA) per IRC R806
Attic Dimensions
Attic Length
ft
Attic Width
ft
Strategy
Balanced intake/exhaust
Vapor retarder present
1:150 ratio required (R806.2)
Vent Types
Exhaust (upper)
Ridge Vent (18 sq in/ft)
Gable Vent (155 sq in/ea)
Roof Louver (50 sq in/ea)
Turbine Vent (95 sq in/ea)
Power Attic Vent (CFM)
Ridge Length
ft
Intake (lower)
Continuous Soffit (9 sq in/ft)
Individual Soffit (65 sq in/ea)
Safety Margin
Safety Margin
10%
Reset Calculator
Total NFVA
—
sq in
Exhaust Needed
—
Intake Needed
—
Ratio
—
Airflow Diagram
Exhaust
Intake
Intake
Balanced: 50% intake / 50% exhaust
IRC R806.2
·
Minimum area: 1 sq ft NFVA per 150 sq ft of attic floor area
IRC R806.3
·
1:300 exception with Class I/II vapor retarder and balanced ventilation
Ventilation Best Practices
Balance matters.
Unbalanced systems cause moisture problems. Aim for 50/50 intake/exhaust.
Never mix exhaust types.
Ridge + gable or ridge + turbine creates short-circuiting. Pick one exhaust type.
NFA does not equal gross area.
Net Free Area accounts for screens/louvers. Always use manufacturer listed NFA.
Insulation baffles.
Blown insulation can block soffit intake. Install baffles at every rafter bay.