Each vent is rated at 50 square inches of net free area. No ground cover will be installed, so the ratio is 1 square foot of net free ventilating area for each 150 square feet of under-floor area. What is the largest under-floor area those eight vents may ventilate?
- About 417 square feet, since 400 square inches is 2.78 square feet and that times 150 gives 417
- BAbout 833 square feet, applying 1 square foot of net free area for each 300 square feet
The 1-to-300 ratio belongs to attic ventilation, not to under-floor spaces. Borrowing it here doubles the area the vents appear to cover.
- CAbout 4,167 square feet, applying 1 square foot of net free area for each 1,500 square feet
The 1-to-1,500 ratio applies only where an approved ground cover is installed, and the problem says there will not be one. It is the right number for the wrong condition.
- DAbout 60,000 square feet, since 400 square inches of vent multiplied by 150 gives 60,000
Multiplying square inches by 150 skips the conversion to square feet and inflates the result by a factor of 144. The size of the answer is the tell: no crawl space covers 60,000 square feet.
Why A is correct
Eight vents give 400 square inches, which is 2.78 square feet of net free area. At 1 square foot for each 150 square feet of under-floor space, that serves 2.78 times 150, or about 417 square feet.
What this question is testing
Whether the candidate converts vent ratings to square feet before applying the ratio, selects the ratio matching the stated ground cover condition, and recognizes an answer that is off by the conversion factor.
On the job
Working the requirement backwards is how a builder decides how many vents to buy, and how an inspector sanity-checks a crawl space at a glance. The habit worth keeping is converting to square feet before touching the ratio, because vent ratings come in square inches and every ratio in the ventilation provisions is square feet to square feet.
Memory technique
Square inches over 144, then times the ratio.
Exam tip
Convert first, then apply the ratio. Both sides of the ratio are square feet.
Where to look it up
IRC Chapter 4, under-floor space, the required ventilating area.