Residential Building Inspector B1Footings and FoundationsEasy
An inspector is examining the placement of a concrete slab-on-ground. 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?
- A4 mil
- B8 mil
- C10 mil
- D6 mil
Show answer & explanationAnswer & explanation
Correct answer: D. 6 mil
IRC Section R506.2.3 requires a minimum 6-mil (0.006 inch) polyethylene or approved equivalent vapor retarder with a permeance of 0.1 perms or less, placed directly under the concrete slab.
Why the other options are wrong
- A. 4 mil is less than the IRC minimum requirement.
- B. 8 mil is thicker than required but acceptable; it's not the minimum.
- C. 10 mil is thicker than required but acceptable; it's not the minimum.
Slab Vapor Retarder Thickness
A vapor retarder placed directly under a concrete slab-on-ground must have a minimum thickness of 6 mil (0.006 inch) and a permeance of 0.1 perms or less to control moisture.
- Required directly under slabs-on-ground.
- Minimum 6 mil polyethylene (or equivalent).
- Permeance of 0.1 perms or less (Class I).
Memory trick: Six mil shield, the slab will yield.