Guide · For the trade
Moisture barrier carpet pad: when to specify it, and what it does not do
A moisture barrier carpet pad is a standard cushion, usually 7/16 inch 8 pound rebond or a 3/8 inch synthetic fiber pad, with a thin polyethylene film bonded to its top face so spills, pet accidents and cleaning water stay in the carpet instead of soaking into the pad and the subfloor. Specify it under wall-to-wall carpet on a concrete slab, in any basement, and in a house with dogs. It does not stop water vapor rising through a wet slab, and it does not replace a moisture test before the carpet goes down.
This guide is for the designer writing the pad line of a carpet spec for a slab, a basement or a client with pets: what the film does and which way it faces, carpet pad for concrete, the best carpet pad for a basement, what to tell the installer, and the rug-pad version for a rug on a wood floor.
What a moisture barrier pad is, and which side faces up
Rebond cushion is chopped polyurethane foam bonded into a sheet; it is the standard residential pad because it is resilient and inexpensive, and it is also a sponge. A moisture barrier pad is the same rebond with a continuous polyethylene film laminated to one face, sold as a spill barrier or pet barrier pad. The film faces up, against the carpet backing, so anything that comes through the carpet sits on the film and is pulled back out by extraction. Pad seams are taped so the film is continuous; an untaped seam is a gutter straight to the subfloor. The pad keeps its weight and thickness, 7/16 inch 8 pound under a residential cut pile and 3/8 inch under a loop, and the film adds no thickness anyone can feel.
Does carpet trap moisture?
The carpet itself breathes; the pad is what holds water. A spill on a plain rebond pad soaks through the carpet, into the foam and onto the subfloor, where it dries slowly, smells, and in the case of urine comes back with every humid day. On a film-faced pad the liquid stays in the pile and the backing, the layer a hot water extraction cleaner can reach, and dries in hours. From below, the film keeps vapor from a damp slab out of the carpet, but that vapor collects under the pad, and a slab wet enough to do that is dealt with before any pad goes down.
Carpet pad for concrete: on grade, below grade and the test first
Concrete gives off water vapor for years, and a below-grade slab takes on more from the ground around it. Before carpet goes over any slab, the installer tests it: relative humidity probes under ASTM F2170 or a calcium chloride test under ASTM F1869. The mill's installation instructions set the limit, and a common one is 75 percent relative humidity or 5 pounds per 1,000 square feet in 24 hours. Under the limit, a moisture barrier pad is the right spec and the tackless strip is set with concrete nails or adhesive. Over the limit, the fix is a moisture mitigation coating on the slab, not a thicker film on the pad; carpet over a wet slab grows mold under it inside a season, and no pad changes that.
Film-faced rebond and synthetic fiber pads are the two to specify over concrete; natural felt and jute-backed pads hold moisture and stay on wood subfloors upstairs. A pad with the film on the bottom face keeps vapor out of the foam but lets spills in, so for a basement that is also a family room, specify the film up.
Best carpet pad for basement rooms
| Pad | Thickness and density | Under | Slab and basement |
|---|---|---|---|
| Rebond, plain | 7/16 in, 8 lb | Residential cut pile on a wood subfloor | No; absorbs and holds water |
| Rebond with moisture barrier | 7/16 in, 8 lb, film up | Cut pile on a slab, any room with dogs | Yes, after a passing moisture test |
| Synthetic fiber | 3/8 in, 32 oz | Loop, Berber, low cut pile | Yes; does not absorb, firm under seams |
| Felt and rubber, dense | 1/4 to 3/8 in | Wilton, natural fiber, stair runners | Rubber yes; natural felt no below grade |
| Waffle rubber | 3/8 in and up | Older residential cut pile | No; too soft for seams, breaks down with heat and damp |
The carpet above the pad matters as much below grade. Specify solution-dyed nylon or polypropylene, loop or a low dense cut pile, and keep wool for the floors above; a basement that runs humid is not its room. Tell the client the basement runs a dehumidifier to about 50 percent relative humidity, which is what the mill's warranty assumes.
Moisture barrier carpet pad for a client with dogs
Upstairs, on a wood subfloor, a carpet pad with moisture barrier is the pet spec. A dog's accident on a plain pad reaches the subfloor, and the smell in the plywood brings the dog back to the same spot; on a film-faced pad it stays in the carpet and an enzyme cleaner and extraction remove it. Pair it with a solution-dyed nylon cut pile and the room survives a puppy; the pet-friendly rugs and carpet guide covers the fiber and pile.
What to tell the installer
- Test the slab with ASTM F2170 or F1869 and send the reading before the carpet ships; over the mill's limit, stop and call for mitigation.
- Set tackless strip for concrete with concrete nails or adhesive, and seal the slab if the mill asks for it.
- Lay the pad film side up, butt the seams, tape every seam with the pad maker's tape, and keep the pad seams off the carpet seams by 6 inches or more.
- On a slab the pad is tacked at the strip and taped, not stapled; on a wood subfloor it is stapled every 6 to 8 inches along the strip and the seams.
- Power-stretch the carpet and seal both cut edges at every seam; the broadloom guide covers the seam plan.
- Ventilate 72 hours, and on a basement job confirm the dehumidifier is running before the furniture goes in.
Moisture barrier rug pads for rugs on wood and stone
The same question comes up under a rug. A felt-and-rubber rug pad holds the rug in place but does not stop a liquid; a pet accident on a 9x12 over an oak floor stains the oak through the pad. For a rug in a room with a young dog, or under a dining table over a wood floor, specify a rug pad with a film face or a solid rubber pad, 1/4 inch, cut 1 inch inside the rug's edge so it does not show. On stone or tile over a slab, a solid rubber pad is the right one for the same reasons the film-faced pad is right under carpet.
Questions designers ask
Does carpet trap moisture?
The carpet breathes; the pad holds water. A plain rebond pad soaks up a spill and holds it against the subfloor; a pad with a moisture barrier keeps the spill in the carpet, where cleaning reaches it.
Does the pad get glued down on a concrete slab?
Not in a stretch-in installation: the pad lies loose inside the tackless strip, tacked at the strip edge and taped at the seams, and the stretched carpet holds it. In a double-stick installation, on a long hall over a slab, the pad is glued to the floor and the carpet to the pad; specify a low-VOC adhesive under 50 grams per liter and the pad the mill approves for it.
Can a damp proof membrane go under the carpet?
A loose polyethylene sheet under the pad is not the answer: it traps vapor against the slab, wrinkles under the pad and voids most mill warranties. The vapor retarder belongs under the slab at the pour; on an existing slab the answer is a mitigation coating if the test fails and a film-faced pad if it passes.
Can a wet carpet pad be dried, or does it get replaced?
Replaced. Carpet can be lifted, dried with air movers inside 48 hours and reinstalled over new pad, but a soaked rebond pad does not dry evenly, holds the smell and grows mold in its foam. The carpet is the expensive layer; the pad is the one you change.
Is moisture barrier carpet padding worth it?
On a slab, in a basement or in a house with pets, yes; it is a small addition to the pad line of the spec and the difference between a cleanable accident and a subfloor that smells. In an upstairs bedroom with no pets, a plain 7/16 inch 8 pound rebond does the job.
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