Attic Ventilation Calculator

Attic ventilation is sized from the attic floor area, under the 1/150 rule or the 1/300 exception in IRC R806.2, and what the code counts is net free area rather than the size of the hole. Enter the floor area, the rule and the split between low intake and high exhaust, plus the net free area of one vent, and the result gives the square inches needed at each level and the vent count or lineal feet that covers them. A second mode checks the vents already on the roof. More exhaust than intake is the usual mistake, and the result says what that does to the attic.