What under-slab vapor retarder means
An under-slab vapor retarder is plastic sheeting laid on the base before a slab is poured, to slow ground moisture rising into the concrete.
Also called visqueen, poly under slab.
Where it matters on a concrete job
Flooring glued to a slab without one can fail from moisture. Lap and tape the seams and protect it during the pour, since punctures defeat it.
What the model code says
- The code calls for polyethylene sheeting at least 6-mil thick over the granular base, with joints lapped 6 inches. (IRC 2021, Chapter 5)
Check which code edition your building department enforces for under-slab vapor retarder before you quote.
What it costs
Jobs that involve under-slab vapor retarder are priced in the concrete slab cost guide: about $5,100 typical, usually $2,800 to $7,350. The guide shows its sources and what moves the price. The concrete slab cost calculator prices a job line by line.
Work it out
- Free calculator: Concrete slab cost calculator
- Cost guide: Concrete slab cost
- Tools, templates and guides: Free concrete resources
Sources: 2021 International Residential Code, Chapter 5, Floors (ICC). Checked October 9, 2026.