If a roof has 450 square feet of horizontal attic surface and no vapor barrier, how many net free vent square feet are required?

If a roof has 450 square feet of horizontal attic surface and no vapor barrier, how many net free vent square feet are required?

• A. 2
• B. 3
• C. 4
• D. 5

Answer: (B)

Ventilation requirements are based on providing a minimum net free vent area proportional to the attic floor area so moisture can escape and air can move. The standard rule used here is that the required net free vent area equals the attic floor area divided by 150. With 450 square feet of attic surface, that calculation is 450 ÷ 150 = 3 square feet of net free vent area. So you need at least 3 square feet of vent openings in total. You can achieve this with vents whose combined net free area equals 3 ft² (for example, a single 3 ft² vent or multiple vents adding up to 3 ft²). The key point is matching the 1/150 ratio to the attic area to meet the minimum ventilation, and arrange openings so the net free area is unobstructed by insulation and other materials.

Ventilation requirements are based on providing a minimum net free vent area proportional to the attic floor area so moisture can escape and air can move. The standard rule used here is that the required net free vent area equals the attic floor area divided by 150. With 450 square feet of attic surface, that calculation is 450 ÷ 150 = 3 square feet of net free vent area. So you need at least 3 square feet of vent openings in total. You can achieve this with vents whose combined net free area equals 3 ft² (for example, a single 3 ft² vent or multiple vents adding up to 3 ft²). The key point is matching the 1/150 ratio to the attic area to meet the minimum ventilation, and arrange openings so the net free area is unobstructed by insulation and other materials.