Roof Ventilation Calculator
Calculates residential attic ventilation balancing requirements under the International Residential Code 1:300 and 1:150 rules, determining balanced 50% intake (soffit vents) and 50% exhaust (ridge vents or box roof louvers).
Formula
Total NFVA (sq in) = (Attic Sq Ft ÷ Ratio) × 144; Intake = 50%, Exhaust = 50%
1. Total NFVA (Sq Ft) = Attic Floor Area ÷ Code Ratio (300 or 150) 2. Total NFVA (Sq Inches) = Total NFVA Sq Ft × 144 3. Balanced Intake NFVA (50%) = Total NFVA Sq In ÷ 2 4. Balanced Exhaust NFVA (50%) = Total NFVA Sq In ÷ 2 5. Continuous Ridge Vent Linear Ft = Exhaust NFVA ÷ 18 sq in/ft 6. Box Vents = Ceil(Exhaust NFVA ÷ 50 sq in/vent)
Inputs
- Attic Floor Area
- Building Code Ventilation Rule
- Exhaust Vent System
Worked example
Calculating balanced ventilation for a 1,500 sq ft attic using continuous ridge vent and vented soffits.
Outcome: Total NFVA needed is 5.0 sq ft (720 sq inches). Balanced at 50/50: 360 sq inches intake (soffits) and 360 sq inches exhaust (ridge). 360 sq in ÷ 18 sq in/ft = 20 linear feet of continuous ridge vent.
Why is attic ventilation so important in both winter and summer?
In summer, proper ventilation exhausts superheated air (up to 150°F) to lower AC bills and prevent shingle baking. In winter, it exhausts warm humid air from the house to prevent roof condensation, mold, and destructive ice dams.