CSLB General Building (B)Finishes and Building EnvelopeMedium
A contractor is sizing attic ventilation for a single-story home with a gable roof and no vapor retarder installed at the ceiling. The attic floor area is 1,800 square feet. Per the 1/150 rule with ventilation split evenly between high (ridge) and low (soffit) vents, how much net free ventilating area is required at the soffit vents alone?
- A1,728 square inches
- B1,200 square inches
- C432 square inches
- D864 square inches
Show answer & explanationAnswer & explanation
Correct answer: D. 864 square inches
Total required net free area = 1,800 sq ft ÷ 150 = 12 sq ft = 1,728 sq in. Since ventilation must be split 50/50 between upper (ridge) and lower (soffit) vents, the soffit portion is 1,728 ÷ 2 = 864 sq in.
Why the other options are wrong
- A. This is the total required net free area, not the soffit-only portion.
- B. This value does not correspond to any correct division of the total.
- C. This is one-quarter of the total, not half.
1/150 Attic Ventilation Rule
Attics without a vapor retarder require net free ventilation area equal to 1/150 of the attic floor area, split between upper and lower vents.
- 1/300 ratio allowed if 50% of vents are high, 50% low, plus vapor retarder or specific conditions
- Convert square feet to square inches: 1 sq ft = 144 sq in
- Ventilation must balance intake (soffit) and exhaust (ridge)
Memory trick: One-fiftieth for balance, half goes up, half goes down.