Underlayments and Vapor Retarders Under Floating Floors
Why this matters
The thin layer under a floating floor is where three separate jobs get confused: moisture control, cushion and sound. A crew that treats "underlayment" as one product puts foam on a slab with no film, adds a second pad under a plank that already has one, or sells a condo owner a pad whose sound rating on the box has nothing to do with their building. Each of those voids a warranty or fails an association rule, and none of them shows until the floor is down.
What the layer is for, job by job
Moisture control. Over concrete, a vapor retarder film slows the moisture moving up out of the slab so it does not collect under the floor. It is a separate job from cushioning; some underlayments do both, and some do only one.
Cushion and support. A thin cushion lets the plank bed evenly over small surface texture, takes some of the hard click off footsteps, and supports the joints. It does not fix flatness: a cushion over a dip is a softer dip, and the joint over it still flexes. Flatness first. See related: Floor Flatness Tolerances by Floor Type.
Sound. Two different sounds, rated two different ways. STC (sound transmission class) rates airborne sound through a floor-ceiling assembly: voices, a television. IIC (impact insulation class) rates footfall and dropped objects heard in the space below. A floor can do well on one and poorly on the other, which is the subject of Why a Floor That Blocks Airborne Sound Can Fail on Footfall.
The layers on a slab
SECTION: FLOATING FLOOR ON A SLAB
wall |
| film run up the wall,
| trimmed after base is set
|
| ================================ plank
| -------------------------------- cushion
| .................. film A
| ...................... film B
| |<-lap->| tape along
| the lap edge
-----+-----------------------------------
concrete slab
The film goes down first, directly on the clean slab, with each sheet lapped over the last and taped along the lap edge as the maker says, and turned up the wall so moisture cannot get around the edge. The cushion goes on top of the film, butted and not overlapped. The plank floats on the cushion. Film thickness and lap width are per the maker; 6 mil polyethylene is a common call in floating-floor instructions.
The underlayment types
| Type | What it does | Where it fits, commonly |
|---|---|---|
| Foam (polyethylene or similar) | Cushion only; no meaningful moisture control on its own | Wood subfloors, where the maker allows it |
| Combination film-and-foam | Film and cushion in one roll, with a film flap that laps and tapes over the next row | Slabs, where the maker accepts it in place of separate film |
| Cork | Cushion and sound; not a vapor retarder | Under a separate film on slabs; sound-sensitive rooms |
| Rubber | Dense cushion, good on impact sound | Multifamily and sound-rated assemblies |
| Attached pad | A foam or cork layer bonded to the plank at the factory | The plank itself; usually nothing else but film where required |
Over a wood subfloor, many floating-floor makers do not want a polyethylene film, because it can trap moisture between the film and the wood; others allow or require one. This is a per-maker answer, and it changes with what is under the subfloor (a dry conditioned basement versus a crawlspace).
Over a slab, most makers require the film on every slab, at any grade, including an elevated concrete deck in a multistory building. The film is not a substitute for testing where the maker also requires a moisture test; it handles a normal slab, not a wet one. See related: Slab Moisture Testing for Flooring.
The attached-pad rule
A plank with a factory-attached pad usually must not get a second cushion under it. The reason is mechanical: two soft layers stacked let the plank deflect further under each footstep, and the click joint, which is designed to work at the deflection its own pad allows, opens, peaks or breaks. Most makers that sell padded planks say this in their instructions and treat a second cushion as voiding the warranty. What is usually allowed under a padded plank on a slab is the film alone. Where a maker lists a specific thin acoustic layer as compatible, that listing is the permission, and nothing else is.
Sound ratings belong to the assembly
An STC or IIC number describes a whole floor-ceiling assembly under test: the slab or joists and their thickness, the ceiling below or its absence, the underlayment and the floor finish together. The number printed on an underlayment roll comes from one such test assembly, usually a thick concrete slab with a suspended ceiling, and does not transfer to a different building.
The IBC addresses sound between dwelling units in multifamily buildings, as adopted by your jurisdiction, in the section on sound transmission (1206 in the 2021 edition, numbered differently in earlier editions); in substance it sets STC and IIC minimums of 50 for floor-ceiling assemblies, or 45 where field-tested. That applies to the building's construction; for a flooring replacement in an existing unit, what usually binds the installer is the condominium or co-op's own rule, which may be stricter and may require a tested assembly and approval before work starts. Get the rule in writing before quoting.
Worked example: a padded plank in a condo
A third-floor condominium unit, about 700 sq ft, on a concrete deck with a drywall ceiling attached to the underside of the deck in the unit below. The owner bought a rigid-core plank with a factory-attached foam pad and wants it installed over the existing slab after the carpet comes out. The association's rule requires a floor assembly with a field IIC of at least 55, documented by a test report on a comparable assembly, and board approval before installation. The 55 is the association's number, set above the code minimum.
What the box says. The plank carton shows an IIC figure in the 60s. Reading the maker's test report, that figure comes from a lab assembly on a thick slab with a suspended ceiling and resilient channels below. This building has a ceiling fastened directly to the deck, which transmits more footfall. The carton number does not answer the association's question.
The tempting fix. Adding a rubber or cork acoustic layer under the padded plank would likely raise the impact performance. It would also put a second cushion under a padded plank, which this maker prohibits. Ruled out.
What the shop actually does. Three steps:
- Asks the plank maker for a field or lab test report closer to this construction. The maker has one on a slab with a ceiling fastened directly to the deck, showing a field IIC of 51 for this plank. Against the association's 55, 55 minus 51 leaves 4 points short.
- Looks for an unpadded plank from the same maker, in a close look, with a listed acoustic underlayment and a report on a direct-fastened-ceiling assembly. One combination reports a field IIC of 57: 57 minus 55 clears the rule by 2 points.
- Submits that combination and its report to the board with the film, the underlayment and the plank named.
On the slab. Even on a third floor, the deck is concrete, and the maker requires a film under the underlayment on any concrete substrate. The film is lapped and taped, run up the walls, and trimmed after the base is set.
What it costs the owner. The owner has a padded plank already purchased that cannot meet the rule in this building. The shop's write-up says so in plain words, with the two report numbers, so the owner can take it up with the seller. The shop does not install the padded plank with a second cushion to make the numbers work, and it does not install the padded plank alone and hope the association never tests.
When the choice changes
- A radiant floor. Every layer between the heat and the room adds thermal resistance, and thick foam, cork and rubber add the most. Radiant system makers and flooring makers both limit the total resistance of what goes over the heat, and some flooring makers list only specific thin underlayments for use over radiant. Pick from that list. See related: Radiant Floor Heating for Flooring Installers and Carpenters.
- Heavy rolling loads. A thick, soft cushion under a kitchen with a rolling island cart or an office chair lets the planks deflect at the joints. Dense, thin underlayments suit those rooms better, and some makers name a maximum cushion thickness for that reason.
- A wood subfloor over a damp crawlspace. No underlayment fixes the moisture coming up from below, and a film on top of the subfloor can make it worse by trapping that moisture in the wood. Fix the crawlspace. See related: On-Grade, Below-Grade and Above-Grade Floors.
Where this goes wrong in the field
- Foam on a slab with no film, because the combination roll was out of stock. The floor looks fine for a year, then a row lifted for a repair shows mildew on the foam and the plank backs.
- Film taped on top of the cushion instead of under it, which traps whatever comes through the slab inside the cushion.
- Cushion overlapped at the seams, doubling the thickness in a line that shows as a ridge under thin plank.
References
- IBC section on sound transmission (1206 in the 2021 edition), as adopted by your jurisdiction
- Flooring and underlayment maker's installation instructions and acoustic test reports
- See related: Why a Floor That Blocks Airborne Sound Can Fail on Footfall
- See related: Slab Moisture Testing for Flooring
- See related: What Is Inside a Laminate or Rigid-Core Plank