Niche Adaptive
Subfloor Moisture Barrier: What Goes Under a Floor
July 8, 2026 7 min read By Fort Worth Flooring Co
There are three different products people call a moisture barrier, they do different jobs, and using the wrong one — or two of them at once — causes more floor failures than most people would guess.
Where the moisture is coming from
Concrete is not dry. A slab passes water vapour upward continuously, year round, regardless of the weather and regardless of what is sitting on it.
That vapour is what breaks adhesive bonds under glue-down floors, swells laminate cores from below, and produces cupping in wood. The characteristic thing about it is that the floor above can be completely undamaged while the bond beneath it has failed entirely.
It is also why the age of the slab is no guide. A slab poured in 1975 is still passing vapour. It never stops.
The three products
Vapour retarder. A physical sheet — polyethylene film, or a film laminated into an underlayment — laid over the slab beneath a floating floor. It slows vapour reaching the assembly above.
Cheap, straightforward, and only usable under a floating floor. You cannot put a sheet down and then glue to it.
Moisture-mitigation primer. A liquid-applied epoxy rolled onto the slab. It bonds to the concrete, forms a barrier, and provides a surface the flooring adhesive can bond to.
This is the answer for glue-down assemblies. It is a real product with a real manufacturer warranty, and that warranty is tied to a specific relative humidity ceiling — which means it is specified against a measured reading rather than applied speculatively.
Acoustic underlayment. Not a moisture product at all. It is for sound and for minor cushioning. Many products combine it with a vapour retarder, and that combination is fine — but underlayment alone does nothing about vapour, and plenty of people believe otherwise.
The mistake: two layers
This comes up constantly and it does real damage.
A great many rigid-core planks now come with a pad already attached. If you lay a separate underlayment beneath one of those, you have two layers of cushion under a click joint that was engineered for one.
The floor then has more give than the joint tolerates. Over months of foot traffic the joints work, then separate. It looks like a product failure and it is an installation one.
The correct sequence when the plank has an attached pad: check whether that pad is rated for a concrete substrate. Many are not. If it is not, a thin vapour retarder sheet goes down — a retarder, not another pad — and the plank goes on top of it. If it is rated for concrete, nothing else goes down at all.
Two vapour retarders is its own problem: moisture that gets between them has nowhere to go.
Testing, because none of this is specifiable without it
The reference method is in-situ relative humidity probes to ASTM F2170. Probes are drilled into the slab, sealed, and left to equilibrate before being read. Calcium chloride testing to ASTM F1869 is used where a specification calls for it.
The equilibration is the part that has scheduling consequences. A probe read twenty minutes after it goes in has not told you anything. This is why slab testing is booked ahead of install week rather than done on the morning, and why a contractor who tests and installs the same day has recorded a number rather than taken a measurement.
The plastic-sheet-taped-down-overnight test is a rough indicator that something is wrong. It is not a reading and no adhesive manufacturer accepts it.
Why the reading gets kept
Almost every flooring manufacturer makes a documented pre-installation moisture reading a condition of their warranty on a concrete substrate. So does every mitigation primer manufacturer, tied to the specific relative humidity measured at the time.
A floor installed without that reading is a floor installed without warranty cover, whatever the box said. The test costs very little against the cost of the floor, and the record is the thing that makes the cover real.
The short version
Floating floor over slab: vapour retarder, either separate or built into the underlayment. One layer only.
Glue-down over slab: test first, and use a mitigation primer rated above whatever the slab reads.
Either way: measure rather than assume, and keep the number.