BuildMeter guide · Energy & Electrical
Ridge Ventilation Planning Guide
Translate an entered attic ventilation ratio into exhaust net-free area and ridge length while checking intake balance and product ratings.
Direct answer
What to know first
Ridge vent is sized from required exhaust net-free area, not from attic perimeter or visible vent width. Use the project ventilation ratio, reserve a balanced share for low intake vents and divide the exhaust requirement by the product’s published NFA per foot or metre.
Open Ridge Vent CalculatorUse net free area, not visible vent dimensions
Vent manufacturers publish net free area (NFA), the open area available for airflow after screens and internal geometry. Total attic ventilation is commonly divided between low intake and high exhaust, but the required ratio and balance must come from the assembly and applicable requirements.
| Step | Calculation |
|---|---|
| Total NFA | Attic floor area ÷ entered ventilation ratio |
| Exhaust share | Total NFA × planned exhaust percentage |
| Ridge length | Exhaust NFA ÷ product NFA per length |
| Packages | Required length + cuts ÷ package length |
Worked example
An 1,800 ft² attic modeled at 1:300 needs 6 ft², or 864 in², of total NFA. At a 50% exhaust share, ridge exhaust needs 432 in². A ridge vent rated at 18 in² per linear foot needs 24 ft of active ridge before end and cut allowance.
Intake and air sealing matter
Ridge vent cannot compensate for blocked soffits. Baffles and wind dams keep insulation from blocking intake paths, while ceiling air sealing reduces warm moist indoor air entering the attic. Adding exhaust without intake can draw air from the house or from unintended openings.
Common mistakes
- Counting vent length instead of product NFA.
- Installing ridge vent with no soffit intake.
- Mixing powered exhaust and passive ridge vent without system analysis.
- Cutting the roof slot outside manufacturer dimensions.
- Ventilating an assembly designed to be unvented.
Sources and professional limits
Final field verification
Before using the ridge ventilation planning estimate as a purchase order, walk the project and compare the calculated scope with the installed conditions. Measure every roof plane, intersection, eave, ridge and penetration from the actual structure rather than a plan alone. Photograph or sketch unusual details and keep the measurements with the estimate so another person can understand how the quantity was produced.
- Check the roofing manufacturer instructions, local wind exposure and required laps before converting length into purchase quantities.
- Coordinate ventilation and water-shedding details so one component does not block drainage or create a concealed moisture path.
- Round only after all measured sections have been combined; rounding every small section independently can materially overstate the total.
- Recheck units, package coverage and the number of full stock lengths or containers required before checkout.