By Bennett's Concrete and Excavation, Norwich, NY. Updated October 2026.
The short answer
Ground moisture moves up through concrete. Under a floor, that moisture can lift flooring, feed mold and keep a basement damp. The Residential Code of New York requires a vapor retarder under concrete floors on ground in homes (section R506.2.3), and NRMCA goes further, recommending one under every interior floor slab because nobody knows what flooring will go down later.
The model residential code text lists exceptions: garages, utility buildings and other unheated accessory structures, unheated storage rooms under 70 square feet, carports, and driveways, walks, patios and other flatwork not likely to be enclosed and heated later. Your code enforcement officer has the final word on your project.
Retarder vs barrier
People say 'vapor barrier', but most products are technically retarders. NRMCA explains the difference by permeance:
| Term | Permeance |
|---|---|
| Vapor retarder (per ASTM E1745) | Not more than 0.1 perms |
| Vapor barrier | Under 0.01 perms |
Either one slows moisture a great deal compared with bare stone.
6-mil or 10-mil?
The model code text calls for at least 6-mil polyethylene or an approved retarder, with joints lapped at least 6 inches. NRMCA recommends a minimum of 10 mils, meeting ASTM E1745. The thicker sheet holds up better to boots, wheelbarrows and stone during the pour. Torn plastic isn't doing its job.
Where it goes in the base
NRMCA's guidance, based on ACI 302:
- Where floors will get moisture-sensitive coverings like glued-down flooring, the concrete should be placed directly on the vapor retarder.
- A layer of granular fill over the plastic is acceptable only for interior slabs without moisture-sensitive coverings in humidity-controlled spaces. It must be at least 4 inches of crusher-run or 3 inches of crushed stone sand, and kept dry before the pour.
- A coarse stone layer of 6 to 8 inches is a capillary break, not a replacement for the retarder. NRMCA says to choke its top with about 2 inches of finer fill and put the retarder on that.
Laps, tape and penetrations
NRMCA calls for overlapping seams 6 inches and taping or sealing around pipes, posts, footing faces and the foundation walls. Plumbing stubs, floor drains and radiant tubing manifolds are where most holes end up. On the Holland Patent barndo floor we poured, radiant tubing went over insulation board before the concrete. See radiant floor heat in a slab.
Common mistakes
- Laying plastic days early and letting it get torn up by foot traffic and equipment
- Skipping tape at seams and around pipes
- Punching it full of holes with form stakes or grade pins and not patching them
- Putting wet sand or stone over it, which NRMCA says should be kept dry
- Using thin, cheap sheeting that rips under a wheelbarrow
A retarder with gaps lets moisture through the gaps.
The trade-off: finishing and curling
A slab on plastic can't lose water downward, so it bleeds more and dries only from the top. NRMCA's curling guide says that can increase upward curling at slab edges, and its plastic shrinkage guide warns that vapor retarders can raise the risk of early surface cracks. Crews handle it with a lower-water mix, careful timing on finishing, joint spacing no wider than about 24 times the slab thickness and good curing. It's a reason to plan, not a reason to skip the retarder indoors.
Garages, pole barns and shops
A detached garage or unheated pole barn falls under the code exceptions, but think about the future. If you might heat it, insulate it, store things on the floor or turn it into a barndo, put the vapor retarder in now. It can't be added later without tearing out the floor. Our pole barn floor guide and garage floor replacement guide go further.
What we do
On interior floors like garage floors, basement floors and pole barn floors, the vapor retarder goes over the compacted stone base, then the mesh, then the concrete. Ideally it's laid, lapped and patched just before the pour so other trades don't tear it up. If you're replacing a basement floor, it's the time to get one in.
