CSLB C-39 Roofing ContractorFlashing, Ventilation and InsulationMedium
An attic has no vapor barrier installed and the contractor wants to avoid the more restrictive ventilation ratio. Under California building code provisions, which ventilation ratio applies to attics WITHOUT a vapor barrier?
- A1/300
- B1/150
- C1/600
- D1/450
Show answer & explanationAnswer & explanation
Correct answer: B. 1/150
Without a vapor barrier, code requires the more conservative 1/150 ratio (1 sq ft of net free vent area per 150 sq ft of attic space) because moisture is more likely to accumulate without a retarder controlling vapor drive.
Why the other options are wrong
- A. This reduced ratio only applies when specific conditions like a vapor barrier or balanced venting are met.
- C. Not a recognized code ratio for attic ventilation.
- D. Not a recognized code ratio for attic ventilation.
1/150 Ventilation Ratio
Attics lacking a vapor barrier, or not meeting balanced/high-low venting conditions, must be ventilated at 1 sq ft of net free area per 150 sq ft of attic space.
- More conservative than 1/300 due to higher moisture risk.
- 1/300 is allowed only when specific conditions are met (vapor barrier, balanced vents, or certain vent locations).
- Applies to both crawl spaces and attics under similar logic.
Memory trick: 'No barrier, no break — use 150.'