Venting products are normally rated with a Net Free Area (NFA). The NFA is the open area that exists for air to pass through. The key to effective venting is to balance air intake (at your eaves, soffits or fascias) and air exhaust (at or near your roof ridge). Ideally the NFA of the soffits should be equal or greater than that of the ridge vent and in no case should the amount of exhaust ventilation exceed the amount of intake ventilation. As a general rule, the space ventilated should have a minimum Net Free Area of 1/150*. This means that for each 150 square feet of attic floor space, 1 square foot of Net Free Area is required for ventilation. The ratio can be reduced to 1/300 if:
- There is a vapor barrier installed in the attic over the living space, or
- There is a difference of at least three feet in height between the intake (soffit under-eave vents) and the exhaust (Ridge).
To calculate NFA, take the attic square footage ÷ 2 = square inches of exhaust and square inches of intake NFA needed. This calculates the 2015 International Residential Building Code minimum (IRC Section R806 – Roof Ventilation 1) which states that 1 square foot of Net Free Area for every 150 square feet of attic floor space with the attic defined as length x width floor of the attic.
The total Net Free Area of the intake and exhaust must be balanced. Any excess Net Free Area should be at the intake.
* Check local building codes.