The ventilation requirement is 1/150. Meaning 1 square foot of ventilation for every 150 square feet of attic floor. The distribution is 1/2 a square foot of intake and 1/2 a square foot of exhaust. Another calculation that would be easier to work with is 1 sq ft of intake and 1 sq ft of exhaust for every 300 sq feet of attic floor.
There is an exception to the above rule. A 100% increase in attic floor allowance is given (from 150 sq ft to 300 sq ft) provided that two provisions are provided:
1. The distance between the ventilators in relation to their height is 3 feet. This does not mean 3 feet of distance between them. The distance could be for example 150 feet apart but the difference in elevation is 3 feet. The placement is 50% of the intake vents must be in the lower portion and 50% in the upper portion (not in the middle).
2. A vapor barrier must provided.
If both of the above provisions are met, then calculations are based upon the 1/300 rule. Meaning that for every 300 square feet of attic floor 1/2 a square foot of intake and 1/2 a square foot of exhaust is required. Or to base it upon 600 square feet of attic floor, 1 sq ft of intake and 1 sq ft of exhaust is needed.
