The exposed ground is covered with a sealed vapor retarder that laps up and is fastened to the stem walls, the perimeter walls are insulated, no wall ventilation openings were installed, and there is no mechanical exhaust from the space and no supply of conditioned air into it. How should the inspector rule?
- AApprove it, because a continuous sealed ground cover carried up and fastened to the stem walls is by itself what qualifies a crawl space as unvented under this code.
The ground cover is half of the unvented package. The other half is a mechanism that keeps air moving and dry, and without it the sealed space simply holds whatever moisture reaches it.
- Write a correction, because an unvented crawl space also needs an air change method, either continuously operating exhaust or a supply of conditioned air sized to its floor area.
- CApprove it, because insulating the perimeter walls of the space takes the place of the air change that the unvented crawl space option would otherwise require.
Insulation is part of the exhaust-fan option rather than a substitute for it. Perimeter insulation changes the temperature in the space but moves no air, so the moisture still has nowhere to go.
- DWrite a correction requiring ventilation openings to be cut through the stem walls, because a crawl space built with no wall vents at all is not a condition that the code permits.
The unvented crawl space is a recognized option, so demanding wall vents rejects an assembly the code allows. The missing item is the air change method, not the vents that option trades away.
Why B is correct
An unvented crawl space requires both a sealed ground vapor retarder lapped up the walls and one of the listed air change methods, such as continuously operating exhaust with insulated perimeter walls or a supply of conditioned air sized to the floor area. Without either mechanism the space is incomplete and a correction is warranted.
What this question is testing
Whether the candidate knows that the unvented crawl space option is a package with two required halves, and can identify which listed method the installation was attempting to satisfy.
On the job
Sealed crawl spaces are now the default in humid regions and they work extremely well when finished, but the fan or the supply register is the piece most often left off, usually because it belongs to a different trade than the one who sealed the ground. On a final inspection, find the mechanical component before you look at anything else: a running exhaust fan, a duct with a damper, or a dehumidifier with a condensate line. If nothing is moving air, the assembly is not done.
Memory technique
Seal the ground and move the air; one without the other is a wet box.
Exam tip
Sealed ground cover plus an air change method: if the stem gives you only one, the answer is a correction.
Where to look it up
IRC Chapter 4, the unvented crawl space provisions at R408.3 and the list of acceptable air change methods.