What is the required net free ventilation area ratio for attic spaces when both intake and exhaust ventilation are provided according to the California Residential Code?
Correct Answer
D) 1:300
California Residential Code Section R806.2 allows a 1:300 ventilation ratio when both intake ventilation (at eaves/soffits) and exhaust ventilation (at or near ridge) are provided with proper balance. Without balanced ventilation, the ratio increases to 1:150.
Why This Is the Correct Answer
CRC Section R806.2 permits a reduced ratio of 1:300 (one square foot of net free ventilation area per 300 square feet of attic floor) specifically when both intake ventilation (at eaves or soffits) and exhaust ventilation (at or near the ridge) are provided in a balanced system. This balanced approach is more efficient than single-point ventilation.
Why the Other Options Are Wrong
Option A: 1:450
1:450 is not a recognized code ratio for attic ventilation. This number does not appear in CRC R806 and is a distractor.
Option B: 1:150
1:150 is the more stringent ratio required when balanced intake-and-exhaust ventilation is NOT provided. Confusing the two scenarios is a common error — 1:150 applies to unbalanced or single-type ventilation systems.
Option C: 1:600
1:600 is not a standard code ratio. It is more lenient than any CRC-recognized threshold and would underventilate attic spaces.
Memory Technique
Think '3 for both': when you have BOTH intake AND exhaust, you get the 1:300 ratio. If you only have one type, you need twice as much ventilation (1:150).
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