The under-floor space measures 1,750 square feet, the sheeting covers about 60 percent of the ground with the remaining 40 percent left exposed, and the installed vents total 240 square inches of net free area. The ratio is 1 square foot of net free area for each 150 square feet of under-floor area, or 1 square foot for each 1,500 square feet where the exposed ground is covered. What is the finding?
- AIt complies, because the reduced ratio calls for 168 square inches and 240 square inches are installed
This is the builder's position and it assumes the reduction has been earned. The sheeting has to cover the exposed ground, and a roll laid down the middle of the crawl leaves the perimeter soil evaporating into the space.
- It requires 1,680 square inches, because the reduced ratio is available only where the exposed ground is covered
- CIt requires about 773 square inches, prorating the covered portion and the exposed portion of the ground
Prorating is arithmetically tidy and has no basis in the provision. The reduced ratio is switched on by a condition being satisfied, not blended in proportion to how much of the ground was covered.
- DIt requires 840 square inches, applying 1 square foot of net free area for each 300 square feet
One square foot for each 300 is the attic reduced ratio. It has no application to an under-floor space, where the two figures are 150 and 1,500.
Why B is correct
The reduction depends on the exposed ground being covered, and it is a condition rather than a proportion. With bare soil over 40 percent of the space the full ratio applies, requiring 1,750 divided by 150, or 11.67 square feet, which is 1,680 square inches of net free area.
What this question is testing
Whether the candidate treats the reduced ratio as a condition to be satisfied rather than a proportion, and computes the full requirement when the condition fails.
On the job
Partial ground cover is extremely common, usually because the sheeting was laid before the plumber and the electrician finished crawling around under there. The judgment is whether the exposed ground is covered, and the honest answer at a perimeter full of bare soil is no. Builders push back because the vent count is so different, which is exactly the point: the cover is doing the work the vents would otherwise have to do.
Memory technique
Cover all the dirt or count all the vents.
Exam tip
Ratios switch on conditions. Partly satisfied is not satisfied.
Where to look it up
IRC Chapter 4, under-floor space, ventilation and the ground cover condition.