Net Free Area Charts
Proper operation of attic ventilation systems requires a combination of exhaust through the ridge vent and intake through soffit vents. The most efficient operation is realized through a balanced system with equal amounts of ventilation along the ridge and soffit areas. When a balanced system cannot be constructed, soffit vents must provide more than 50% of the total required ventilation with the remaining ventilation provided through the ridge vent.
The ratio of Net Free Ventilation Area to horizontal Attic Area is used to calculate the minimum amount of total ventilation. The 1/150 ratio applies when uniform distribution of soffit and ridge vents cannot be achieved or when no vapor retarder is present. The 1/300 ratio applies when proper distribution of soffit and ridge vents is achievable or if a vapor retarder is present.