Moisture barrier and vapor retarder

Somewhere under a new floor, a decision gets made about whether moisture coming from below is stopped completely, slowed down, or left alone. Two things settle it together: what sits underneath that particular room, and what the manufacturer of the product you picked prints in its own installation guide. This page is about the layer itself, what each one is, the forms it arrives in, and where it lands under the materials that are not wood.

Barrier or retarder: the real difference

A moisture barrier is built to stop vapor almost entirely. Continuous polyethylene sheeting, lapped at every seam, taped, and turned up at the walls, is the common form. A vapor retarder is a lighter-duty layer, a coated paper or an asphalt-saturated felt, built to slow the rate of transmission rather than halt it. It buys time and reduces the load on whatever sits above it without pretending to seal anything. The trade uses the two words loosely, and plenty of products sold as barriers are honestly retarders, which is one reason the installation guide matters more than the label on the roll.

Neither one is better in the abstract. A full barrier under a room that only ever sees ordinary indoor humidity is overbuilt, and it can trap moisture in an assembly that needed to be able to dry. A retarder under a slab that is actively releasing vapor is undersized for the job. The correct layer is the one matched to the actual condition, which is why this gets decided room by room instead of once for the whole house.

Continuity, not thickness, is what makes a barrier work

A barrier only performs as one continuous, unbroken plane. A tear, a seam that was never taped, an edge that stops short of the wall: none of those make a weaker barrier, they make an opening, and vapor finds an opening the way water finds a low spot. That is why the sheet goes down with generous laps, why the seams get taped rather than butted, and why the perimeter turns up behind the base instead of stopping at the floor line. How thick the sheet has to be, which class of material qualifies, and how the seams get treated are all specified by the flooring manufacturer's installation guide for the product going on top, not by a general rule and not by a number quoted on a website. Getting that plane continuous is part of what the preparation step of our installation process is for.

The forms the layer actually arrives in

People picture a roll of plastic, and that is only one of them. Under a finished floor the same job gets done several different ways, and which one is even available usually depends on how the floor above it is being installed.

• A separate sheet. Polyethylene film rolled over the slab, seams taped, edges turned up at the walls, with the underlayment and the floor going on top of it.
• A film already bonded to the underlayment, so one roll handles cushioning, sound and vapor at once. Common under floating floors, and the reason two different products get confused for each other.
• A liquid-applied membrane, rolled or troweled onto the slab and left to cure into a continuous film before anything goes over it.
• A moisture control adhesive. On a glue-down floor the adhesive handles the bonding and the vapor retarding in one product, which is why a separate sheet under a glue-down would only stop the adhesive bonding to anything.
• Nothing at all, which is a legitimate specification over dry, conditioned space rather than a corner being cut.

Under vinyl, laminate, tile and carpet

Rigid core vinyl is unbothered by vapor itself, which is why it is the usual answer over a slab, but the layer still comes up, because vapor arriving at the underside of a sealed plank has nowhere to go and can collect there. Laminate has the most to lose: its core is a wood-based board, and a wood-based board that takes on moisture swells at the edges and does not come back. Carpet and its pad are breathable rather than sealed, so above-grade carpet over a dry wood subfloor rarely calls for a plastic layer at all, while below grade it becomes a real question. What the pad and the underlayment are actually there for is its own page.

Tile is the exception to the whole sheet idea. Tile is bonded to what is beneath it with mortar, so a loose plastic sheet between the two would simply prevent the bond from forming. Where moisture has to be managed under tile it gets managed with a membrane bonded to the substrate and then tiled over, which is a different product and a different step from rolling out poly.

The wood case, which has its own page

Wood is the one material where an extra layer can do real harm, because wood has to be able to dry to at least one side and sealing it top and bottom traps whatever gets in. That argument, and the four different cases underneath a wood floor, are settled on our page on vapor barriers under hardwood, which is the one to read if the floor going down is wood. The concrete-specific version, including the slab moisture tests by name, sits on hardwood over a concrete slab. Neither argument gets re-run here.

Where this gets decided for your room

The condition under the room settles it, and that condition gets identified in person rather than assumed. What a slab is doing underneath a floor and whether the surface needs leveling before anything goes down get checked on the same visit, because they are the same look at the same subfloor. Bring the sample you are leaning toward, or the box it came in, to the free in-home measure, and the layer decision gets matched to what that maker actually asks for rather than to a rule of thumb. Our installation process is built around checking instead of guessing.