Residential Building Inspector B1Footings and FoundationsEasy
A builder is preparing to pour a concrete slab-on-ground for a new garage. The plans specify a vapor retarder directly beneath the slab. What is the minimum thickness required for this vapor retarder, according to the 2021 IRC?
- A15 mil
- B10 mil
- C6 mil
- D3 mil
Show answer & explanationAnswer & explanation
Correct answer: C. 6 mil
The 2021 IRC, Section R506.2.3, states that a vapor retarder beneath a slab-on-ground shall be a minimum of 6-mil (0.15 mm) polyethylene or approved equivalent.
Why the other options are wrong
- A. This thickness is not a standard minimum requirement for slab vapor retarders in residential construction.
- B. While acceptable, this thickness is greater than the minimum required.
- D. This thickness is generally insufficient for a vapor retarder under a concrete slab.
Slab Vapor Retarder Thickness
The minimum thickness of the vapor retarder material placed directly beneath a concrete slab-on-ground.
- IRC R506.2.3 specifies 6-mil (0.15 mm) polyethylene.
- Prevents moisture migration from the ground into the slab/structure.
- Must be continuous and sealed at seams and penetrations.
Memory trick: Slab needs a shield, 6-mil strong, to keep moisture where it belongs.