Residential Building Inspector B1Footings and FoundationsHard
An inspector is evaluating a slab-on-ground foundation where the concrete is poured directly over a granular fill. The plans do not specify a vapor retarder. According to the 2021 IRC, under what conditions is a vapor retarder NOT required directly under a concrete slab-on-ground?
- AWhen the concrete mix includes a waterproofing admixture.
- BWhen the slab is constructed over a capillary break (e.g., granular fill).
- CWhen the slab is used as a finished floor in a habitable space.
- DWhen the slab is supporting a non-habitable accessory structure.
Show answer & explanationAnswer & explanation
Correct answer: D. When the slab is supporting a non-habitable accessory structure.
IRC Section R506.2.3 requires a vapor retarder under concrete slabs-on-ground that serve as the finished floor for habitable spaces. However, an exception is provided for slabs supporting non-habitable accessory structures (e.g., utility sheds, detached garages), where a vapor retarder is not explicitly required.
Why the other options are wrong
- A. Waterproofing admixtures can reduce permeability but are not typically accepted as a standalone substitute for a vapor retarder under a slab in habitable spaces by the IRC.
- B. While granular fill acts as a capillary break, it does not negate the requirement for a vapor retarder under habitable spaces, as moisture can still migrate as vapor.
- C. A vapor retarder IS required when the slab is a finished floor in a habitable space to prevent moisture migration.
Slab Vapor Retarder Exceptions
Specific conditions under which a vapor retarder is not required directly beneath a concrete slab-on-ground.
- Primarily required for habitable spaces.
- Exceptions often apply to non-habitable structures.
- Specified in IRC Section R506.2.3.
Memory trick: Habitable Needs Barrier, Sheds Do Not.