Vapor barrier under a concrete slab

Why a vapor barrier under a concrete slab matters

Concrete looks solid and is not. It holds a network of capillaries left behind by mix water, and ground moisture wicks up through them continuously, day and night, for the life of the slab. A bare slab on damp ground can pass several pounds of water vapor per 1,000 square feet every day.

Outdoors nobody notices. Indoors that moisture arrives underneath whatever you installed on top, and the results are predictable: adhesives release, wood flooring cups, vinyl bubbles, coatings blister and peel, and carpet grows mold. None of those repairs cost less than the sheet would have.

Slabs that need a barrier under them

What thickness of barrier to use

Thickness is measured in mils, where one mil is a thousandth of an inch.

The failure mode is not the plastic wearing out. It is a boot, a rebar chair or a wheelbarrow wheel tearing a hole during placement, which is exactly what thin film does not survive. Paying for 15 mil instead of 6 mil on a 400 square foot slab is a difference of well under a hundred dollars.

Sand over the barrier: the argument settled

Older practice put a couple of inches of sand between the sheet and the slab, on the theory that it helped the slab cure evenly from both faces and reduced curling.

The problem is what happens to that sand. It sits below the slab and above the plastic, so any water in it, whether from the mix, from rain during construction or from a leak, has nowhere to drain. It stays there and feeds moisture into the slab for months, which is the opposite of what the sheet was installed to do.

Current guidance for slabs that will receive flooring is to place the concrete directly on the sheet. Crews who prefer the sand layer are not being careless; they are working from what used to be standard. If flooring is going down, the direct placement wins.

Installing a vapor barrier under a slab

1. Finish the base first. Compacted crushed stone, graded flat. The sheet does not fix a bad base.
2. Roll the sheet out over the whole area, running it up the perimeter forms so it reaches the top of the slab edge.
3. Lap seams 6 inches minimum and tape them with a purpose made seam tape, not duct tape.
4. Seal the penetrations. Every pipe, drain and conduit gets taped or booted. An unsealed penetration undoes the sheet around it.
5. Walk it and repair before the truck arrives. Once concrete is coming down the chute, the barrier is finished being fixable.
6. Place concrete directly on it. Use chairs to hold reinforcement up, not bricks or stones that can pierce the sheet.

What the barrier costs per square foot

At our default of $0.50 per square foot installed for 10 mil poly, the sheet is a rounding error next to the pour.

Compare that to the retrofit. Once the slab is down, adding moisture protection means an epoxy mitigation system, a sheet membrane or a floating floor assembly, and those run several dollars per square foot before anyone replaces the flooring that failed.

Vapor barrier under a concrete slab: the bottom line

Under a slab that will be enclosed, use 10 mil at minimum, 15 mil if flooring is planned, lap and tape the seams, seal the penetrations and pour straight onto it. Outdoors, skip it. The concrete slab cost calculator prices it alongside the concrete, base and reinforcement, and the concrete floor cost calculator does the same for interior floors where the finish depends on getting this layer right.

Curing matters just as much once the concrete is down, and the concrete drying time guide covers how long to keep vehicles and flooring off the new slab.