Vapor Retarder
The sheeting laid under a concrete slab before it is poured, to slow ground moisture entering the slab from below. It shares most of a name with the barrier you roll onto a finished floor, and the two are so often confused that a permanent condition gets treated as a specification question.
What it is, and where it sits
A vapor retarder is sheeting laid over the prepared base before a concrete slab is poured. Its job is to slow ground moisture moving up into the slab from below. It is a construction detail, decided and installed before the concrete existed, and once the pour is done it is permanently out of reach.
That last point is the one worth carrying, because it separates this from almost everything else on a floor. Preparation can be redone. A product can be changed. The retarder cannot: whatever went down under that slab is what you have.
Retarder, not barrier
The word is deliberate. Practical sheeting slows vapor rather than stopping it, and vapor also travels around it as much as through it, by way of laps that were never sealed, punctures from stakes and chairs, and edges that stopped short of something.
You will still hear it called a barrier on every jobsite, and that is not worth correcting out loud. It is worth knowing, though, because the word barrier quietly suggests that the slab is sealed from underneath. A slab is never quite that, and floors get specified on the assumption that it is.
Two different things share the name
On a coatings job the word barrier is doing double duty, and the two meanings sit on opposite sides of the concrete.
Below the slab is the retarder described above. Above it is a moisture vapor barrier, a product applied to a cured slab as the first part of the floor system. They are not two grades of one product. They are in different places, they were installed by different trades at different times, and only one of them is still a decision you get to make.
Keeping them apart is what stops a permanent condition being mistaken for a specification question. A missing retarder is not solved by choosing a heavier coat, because the two are not on the same axis.
Why it matters when you are the one coating the floor
A slab on grade with nothing effective underneath it is connected to ground moisture permanently. The supply does not run out, so it does not dry down and waiting is not a plan. That is the mechanism behind osmotic blistering, and it is why blisters on those floors cluster at low spots and slab edges and come back in the same places after a recoat.
A surface applied barrier is sometimes the right answer, within its rated limit. What it cannot be is an answer to any reading at all, which is why a result in the top band should send you looking for the source rather than shopping for a heavier product. If a slab is reading that wet, the useful question is where the water is arriving from, not what to put over it.
What you can actually observe
Almost never the retarder itself. It is under the concrete, and the honest sources are the drawings, somebody who was present for the pour, or a core.
What you can read is the slab, and the tell that points downward is persistence rather than height. A slab holding its own mix water dries as time passes. A slab that reads high, gets addressed, and reads high again is being resupplied from somewhere, and the ground is the usual candidate. Start with an actual moisture test rather than the age of the slab, which tells you nothing about what is beneath it.
No figures are published here. Perm ratings and retarder classes belong to a product data sheet and to the building code that applied when the slab was poured, and the moisture thresholds this site does publish live with the calculator that computes them. A number on this page would invite exactly the shopping behavior the page argues against.
Frequently Asked Questions
What is the difference between a vapor retarder and a vapor barrier?
On a floor job the words usually point at two different things in two different places, which is the whole reason the confusion is expensive.
A vapor retarder is sheeting that went down on the fill before the concrete was poured. It is under the slab, it was installed by whoever built the building, and it has been there since the day the slab existed. A moisture vapor barrier, in coatings usage, is a product you apply to the top of a cured slab as part of the floor system.
They are not two grades of the same thing. One is a construction detail below the concrete and the other is a coat above it, and only the second is still available to you when you are standing on the floor deciding what to put down.
Why is it called a retarder rather than a barrier?
Because it slows vapor rather than stopping it, and the name was corrected to say so.
No practical sheeting is a true zero. Water vapor moves through it slowly, and it also moves around it: through laps that were not sealed, punctures from stakes and rebar chairs, and edges where the sheet stopped short. The name says what the material does, which is slow the rate at which vapor arrives rather than end it.
"Barrier" survives in everyday jobsite speech and you will hear it constantly for the under slab sheet. It is worth knowing what it actually does, because "barrier" invites the assumption that the slab is sealed from below, and a slab is never quite that.
Can I tell whether a slab has one?
Usually not by looking, which is the practical difficulty with the whole subject. It is buried under the concrete, and by the time you are on site the only honest answers come from the drawings, from somebody who was there for the pour, or from a core.
What you can do is stop guessing from the slab age. An old slab tells you nothing about what is under it, and a young slab is a different conversation entirely. Test the slab and let the reading say what it says.
If it is missing or failed, what are my options?
Narrower than people hope, and it is better to say so early than after a floor has been laid.
A slab on grade with nothing effective under it is connected to ground moisture permanently. That supply does not run out and it does not dry down with time, so waiting is not a plan. A surface applied barrier is sometimes the right answer within its rated limit, but it is being asked to hold back a source that has no end, and any product has a limit past which it is the wrong tool.
This is exactly why a reading in the top band should send you looking for the source rather than shopping for a heavier coating. If the number is that high, the useful question is where the water is coming from.
Does a moisture reading tell me about the retarder?
It tells you about the slab, and the retarder is one of several reasons a slab reads the way it does.
A reading is a measurement of the concrete now. A high one can mean a young slab still giving up its own mix water, a leak, drainage running toward the building, or ground moisture arriving from below through a retarder that failed or was never there. Those have very different outcomes, and only the first improves on its own.
The tell that points at the ground is persistence. A slab that dries with time was holding its own water. A slab that reads high, gets treated, and reads high again is being resupplied.