Is a vapor barrier required under a concrete slab?
A 6-mil polyethylene sheet or approved vapor retarder is required under concrete slabs, except for specific unheated areas (IRC R506.2.3, 2012 edition).
The code requires a 6-mil (0.006 inch) polyethylene sheet or an approved vapor retarder between the slab and the base course or prepared subgrade, with joints lapped not less than 6 inches.
Exceptions include garages, utility buildings, small unheated storage rooms under 70 square feet, driveways, and patios. A building official may also approve omitting the barrier based on specific site conditions.
This answer is based on Section R506.2.3 of the 2012 IRC. Section numbers and details can change in later editions and through state or local amendments, so confirm the edition your jurisdiction enforces with your building department before you build.
Related answers
- How wide and thick does a footing need to be? — IRC §R403.1
- How deep do exterior footings have to be below the ground? — IRC §R403.1.4
- How deep below the frost line do footings need to go? — IRC §R403.1.4.1
- How thick does a foundation wall need to be? — IRC §R404.1
- What is the anchor bolt spacing for a sill plate on a foundation? — IRC §R403.1.6
- How big does a crawl space access opening have to be? — IRC §R408.4
Need this scoped to your jurisdiction?
Adopted edition and local amendments vary by city and county. Ask Donnie checks your jurisdiction and cites the exact adopted section.
Ask Donnie →General information based on the International Residential Code (a model code). This is AI assistance to verify — not legal advice. Confirm the adopted edition and any local amendments with your local building department (AHJ) and a licensed professional before you build.