Do hardwood floors need a vapor barrier?

What the layer is, and what it is not

Water moves into a floor assembly two ways. It can arrive as liquid, from a leak or a wet slab, and it can arrive as vapor, drifting up out of the ground or a damp space through everything above it. The layer people call a vapor barrier deals with the second one. It slows the passage of water vapor into the wood so the boards are not sitting on a permanent supply of it.

The more accurate word is retarder, because the ones used in floors slow vapor rather than stopping it dead, and that distinction turns out to matter later on this page. What the layer is definitely not is waterproofing. It will not hold back standing water, it does not fix a wet crawlspace or a slab with a moisture problem, and it is not a substitute for a subfloor that tests dry before the job starts. It is also a different thing from the underlayment or pad under a floating floor, which is there for sound and feel, though some products combine the two.

The real question is what is under the room

There is no universal answer for hardwood, which is why the question keeps coming up. Same product, four different houses, four different specifications.

• Concrete slab, on or below grade. Yes, something belongs between the concrete and the wood, because a slab moves moisture upward every day of its life. The general case for below grade rooms is covered on our basement flooring page, and the wood specific version is hardwood on a concrete slab. Both are worth reading before you specify anything.
• Vented crawlspace. Yes, and the more valuable sheet is the one on the ground down there, not the one in your floor. A continuous ground cover over the dirt cuts off the supply at the source and benefits the whole structure, not only the flooring. That is a house repair rather than a flooring item, and it is often the fix that actually solves the problem.
• Unconditioned basement or a garage below. Usually yes, treated like the crawlspace case, because the air under the floor is not the air in the room above it.
• A conditioned room below. No. A bedroom over a heated, cooled living room has the same air on both sides of the subfloor. There is nothing to retard, and adding a sheet there does no good and some harm.

How each installation method handles it

The method changes what the layer physically is, which is why a general answer is not much use until the install is decided.

• Nailed or stapled to a wood subfloor. The traditional approach rolls asphalt saturated felt or building paper over the subfloor with the edges overlapped, and the fasteners go straight through it. It slows vapor, it lets the assembly still breathe a little, and it quiets the floor as a side effect.
• Glued down. The adhesive commonly is the moisture control. A number of wood flooring adhesives are sold as a combined adhesive and moisture control membrane, applied at the spread rate the maker specifies and rated to a subfloor moisture level the maker specifies. Putting a separate sheet under a glue down would only stop the adhesive bonding to anything.
• Floating. A polyethylene film with the seams taped and the edges turned up at the walls, laid over the slab before the underlayment, or an underlayment that already has a film bonded to it. Floating is the usual route over a slab for exactly this reason.
• In all three, the number that governs is on the manufacturer's installation instructions for that specific floor and that specific adhesive. They differ product to product, so the instructions win over any rule of thumb.

Two barriers are worse than one

This is the part that surprises people, and it is the real meaning of trapping moisture. Wood has to be able to dry to at least one side. Sandwich it between two layers that neither of them let vapor through and whatever moisture finds its way in, from a spill, from a slow leak, from a subfloor that was not quite dry on the day, has no way out in either direction. It sits in the wood and does its work slowly.

So a sheet of poly laid over a wood subfloor with heated space beneath it is not a harmless extra precaution. It gives moisture from the room a cool surface to condense on and it stops the subfloor drying downward. More is not safer here. The right number of layers is the number the assembly actually needs, decided from what is under the floor.

The reading comes before the decision

None of this gets settled from a photograph or a phone call. The wood subfloor gets a moisture meter reading, a slab gets a concrete moisture test, and the flooring itself gets read too, because material and subfloor have to be close to each other before the first fastener goes in. That is the same check that makes acclimating the material meaningful rather than ceremonial. A striking share of the floors that go wrong turn out to have started going wrong underneath, before a single board was visible.

On our jobs that reading happens at the free in-home measure and again at the preparation step of the installation process, before anything is ordered or fastened. If the subfloor does not read the way it needs to, that gets said out loud and priced as prep rather than discovered on install morning.

Where it usually lands

Most main floor jobs over a wood subfloor with conditioned space beneath want felt or nothing at all. That is the ordinary case, and it is the case solid hardwood was built for. Slabs, crawlspaces and unheated spaces below are where the specification genuinely matters, and they are also where engineered construction starts earning its keep, since it is far less bothered by what little vapor does get through. If the whole moisture picture in your house is the thing you are unsure about, the Missouri climate question is the wider version of it. Or bring the details to 15570 Veterans Memorial Pkwy in Wentzville, or call 636-332-0654, and tell us what is under the room.