Residential Building Inspector B1Footings and FoundationsMedium
An inspector is evaluating a slab-on-ground foundation where the concrete is poured directly on compacted fill. According to the 2021 IRC, which of the following is an exception to the requirement for a vapor retarder directly beneath the concrete slab?
- ASlabs supporting conditioned spaces only.
- BSlabs with a minimum thickness of 6 inches.
- CSlabs where a floor covering is not installed.
- DSlabs for carports, detached garages, and unheated accessory structures.
Show answer & explanationAnswer & explanation
Correct answer: D. Slabs for carports, detached garages, and unheated accessory structures.
IRC R506.2.3, Exception 1, states that a vapor retarder is not required for slabs for carports, detached garages, and unheated accessory structures.
Why the other options are wrong
- A. Incorrect. Conditioned spaces often have a greater need for vapor retarders to prevent moisture issues.
- B. Incorrect. Slab thickness does not generally negate the need for a vapor retarder.
- C. Incorrect. The presence or absence of a floor covering does not typically waive the vapor retarder requirement.
Slab Vapor Retarder Exceptions
Specific conditions under which a vapor retarder is not required beneath a concrete slab-on-ground, as outlined in the IRC.
- IRC R506.2.3 details vapor retarder requirements.
- Exceptions are typically for unconditioned or non-habitable spaces.
- Prevents moisture migration from the ground into the slab/structure.
- Common exceptions include carports and detached garages.
Memory trick: A vapor retarder is a must, unless it's a 'C-U-D' space: Carport, Unheated, Detached.