A new heated dwelling slab requiring a vapor retarder under 2021 IRC R506.2.3 has 4-mil polyethylene laid over the base course with butted joints. No exception or separately approved alternative applies. What does the prescriptive rule require?
- APolyethylene of any thickness with no joint overlap
- BRemoval of the retarder because it is above the base
- CTwo loose 4-mil layers automatically establish compliance
- A minimum 10-mil vapor retarder meeting ASTM E1745 Class A, with joints lapped at least 6 inches
Where to look it up
https://core-docs.s3.amazonaws.com/documents/asset/uploaded_file/3836/Lennox/3042182/2021_International_Residential_Code.pdf
Why D is correct
R506.2.3 requires a minimum 10-mil vapor retarder meeting ASTM E1745 Class A, with joints lapped at least 6 inches, between the slab and base course, or prepared subgrade when no base exists. The stated 4-mil material and butted joints fail that route. Adding thin loose layers does not itself establish the specified material compliance. The provision lists exceptions for certain unheated/accessory/exterior uses and official approval based on local site conditions; this question excludes them. The former 6-mil answer was not the 2021 requirement. See printed p.5-25. Source: https://core-docs.s3.amazonaws.com/documents/asset/uploaded_file/3836/Lennox/3042182/2021_International_Residential_Code.pdf