Flooring Correct
What is a vapor barrier and when is it required?
A vapor barrier, more accurately called a retarder, slows water vapor moving up into a floor from below. Whether a job needs one is decided by what is underneath the room, not by the flooring category. Over a slab, a crawl space or an unheated space it usually belongs there. Over a conditioned room it usually does not.
Water reaches a floor two ways. It arrives as liquid, from a spill or a leak, and it arrives as vapor, drifting upward through concrete or through the air under a house. The second kind is what a vapor barrier is for. It is a layer, sometimes a sheet and sometimes built into another product, that slows vapor on its way up into the flooring, so the material above is not living over a supply of moisture that never runs out.
The decision most people are actually trying to make
The real question is rarely whether a vapor barrier exists as a concept. It is whether your specific room needs one added, whether it already has one built into a product you are considering, or whether adding one would be pointless or even counterproductive. Those are three different answers, and which one applies comes down to what is underneath the room rather than what flooring category you are choosing.
Option one: nothing extra, because the room does not need it
A room sitting over a heated and cooled living space has broadly similar air above and below the subfloor. Vapor is not being driven upward any harder than it is being driven down, so there is no one-way flow for a retarder to work against. This is the ordinary case for a main-floor bedroom or living room, and it is the case most hard flooring and carpet installs were designed around with no added layer at all.
Option two: it comes built into what you are already installing
A great deal of floating floor underlayment used under laminate and click together luxury vinyl plank arrives with a thin moisture film laminated to its back, so the vapor retarder is not a separate purchase at all. It is a property of the underlayment you pick rather than an extra step. Tile takes a different route: some jobs use a membrane under the thinset for crack isolation or uncoupling, and certain versions of that membrane also slow vapor, which is a decision made at the tile stage rather than as a flooring wide add on. Carpet generally does not carry a vapor barrier of its own over a healthy subfloor, and the pad underneath it is doing a different job entirely.
Option three: a specific membrane, chosen for what is under the room
A concrete slab, a wood subfloor over a vented crawl space, or an unheated space below are the cases where a retarder is doing real, ongoing work rather than sitting there unnecessarily. What form it takes, a taped sheet, a rolled on membrane or a moisture control adhesive, is governed by the specific product and its installation instructions rather than a single rule that covers every job. The full mechanics of that for wood flooring specifically, including why a fastened floor, a bonded floor and a floating floor each want a different form of it, are covered on our hardwood vapor barrier page, which is worth reading if wood is what you are considering over one of these conditions.
What a perm rating actually tells you
The word barrier oversells what these layers do. They are rated in perms, a measure of how much water vapor passes through a material over a set period, and the products used under floors slow that passage rather than stopping it dead. A lower perm number means less vapor gets through. That rating is printed on the data sheet for the sheet, membrane or adhesive you are looking at, and the flooring manufacturer's installation instructions are where the requirement for their product is written down. Two products both called vapor barriers on a shelf tag can sit a long way apart on that scale, which is why the data sheet is the thing to read rather than the name on the packaging. It is also why more layers is not automatically safer: a second vapor-resisting layer added where the room never needed one leaves whatever moisture does find its way in with no route back out, which the hardwood page works through in depth.
A description of the house will not settle any of this on its own. What is under the room, which install method fits the product, and whether a membrane is already doing the job get looked at together, and the reading happens before anything is ordered rather than after.
More on vapor barriers and moisture layers
Whether carpet pad does anything about moisture, when tile needs a membrane, and what a perm rating is measuring.
636-332-0654No, and the two are worth keeping separate. Pad under carpet is chosen for cushioning, for how the carpet wears and for how the room sounds underfoot, and it does nothing to slow vapor arriving from below. If what is under the room calls for a retarder, that is a distinct layer chosen for that job, and the pad sitting above it does not change the requirement in either direction.
Sometimes, in the form of a membrane under the thinset rather than a plastic sheet. Certain crack isolation or uncoupling membranes also slow vapor as a secondary function, and whether one belongs in your job depends on the slab and the tile manufacturer's instructions rather than a blanket rule.
Perms measure how readily water vapor moves through a material over a set period, so a lower number means less of it gets through. Two products sold under the same vapor barrier wording can sit far apart on that scale. The figure your job actually needs is not a universal one. It comes off the flooring manufacturer's installation instructions for the product going down, and the layer's own data sheet is where you check whether it clears that.
Bring a piece of the underlayment you are considering
If a sample already has a moisture film laminated to its back, bring it in and we will tell you exactly what it does and does not cover. We will also tell you what is underneath your room before recommending anything.
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