A 1,800-square-foot crawl space uses standard (non-vapor-barrier) under-floor ventilation openings. Using the common 1-to-150 ratio, what is the MINIMUM net ventilation area required?

Explanation

The baseline under-floor ventilation ratio is 1 square foot of net free vent area for each 150 square feet of crawl-space area. 1,800 / 150 = 12 square feet of net free area. Option a uses the reduced ratio that only applies with an approved vapor retarder. Option b overstates it. Option d divides by the wrong figure. Use 1:150 unless a code-recognized vapor barrier allows 1:1,500.