Wood & laminate
Underlayment: which type, how many rolls, and when you need none
Underlayment smooths small flaws, softens footfall, cuts noise and keeps slab moisture out. The right one depends on the floor above and the subfloor below, and sometimes the right one is none.
What underlayment does
A thin layer between the subfloor and a floating floor does four jobs. It bridges small flaws in the subfloor so the click joints are not stressed over a bump. It softens the floor underfoot. It reduces sound, both the hollow click in the room and the noise carried to the room below. And over concrete, a version with a film blocks moisture rising from the slab. Underlayment does not fix an uneven floor: it bridges flaws of a millimetre or two, not dips of a quarter inch, which need a leveling compound first.
The types
| Type | Thickness | Used under | Notes |
|---|---|---|---|
| Foam | 2–3 mm | laminate, floating engineered | cheapest; no moisture barrier |
| Foam with film | 2–3 mm | floating floors on concrete | the film overlaps and is taped |
| Felt or fibre | 3 mm | laminate, engineered | denser, better for sound |
| Cork | 3–6 mm | floating or glued floors | best for sound, costs more |
| Rosin or felt paper | thin | nailed solid hardwood | stops squeaks, not a barrier |
| 6 mil polyethylene | thin | any floating floor on concrete | a separate moisture barrier |
The flooring maker’s instructions decide which is allowed; many warranties require a particular thickness or rating.
When you need none
Many vinyl planks and some laminates come with a pad attached to the back. These are laid directly on the subfloor; a second underlayment under them makes the floor too soft, stresses the click joints and usually voids the warranty. The exception is a separate moisture barrier over concrete when the maker asks for one. Glue-down floors have no underlayment at all; they bond to the subfloor. Tile goes on backer board or a membrane, not underlayment. The vinyl plank calculator and the laminate calculator leave the underlayment out when you mark an attached pad.
Counting rolls
Underlayment is sold in rolls labelled with their coverage, usually 100 or 200 sq ft for foam, and with a width of about 3 to 4 ft. It is laid in strips across the room with the seams butted, not overlapped, and taped. Products with a film overlap the film and tape it.
A 20 × 15 ft room in laminate
- Floor area: 300 sq ft; with 5% for trimming, 315 sq ft
- Rolls of 200 sq ft: 315 ÷ 200 = 1.6, so 2 rolls
- Strips across the 15 ft width at 3.3 ft wide: 5 strips, 4 seams of 20 ft = 80 ft of seam
- Seam tape: 80 ft plus 10%, one 164 ft roll
The underlayment calculator counts the rolls, seams and tape for any room and type.
Over concrete: the moisture barrier
A concrete slab gives off moisture vapour, more in a basement or a new slab. A floating floor over concrete needs a vapour barrier: either a foam with a built-in film, or plain foam over a separate sheet of 6 mil polyethylene. The polyethylene is laid with seams overlapped about 8 inches and taped, and turned up the walls a couple of inches, then trimmed behind the baseboard. A new slab must be dry enough to floor over; makers give a test method and a limit.
The moisture barrier calculator counts polyethylene rolls with the overlaps and the turn-up at the walls.
Over wood subfloors
Over plywood or OSB, plain foam or felt is enough for a floating floor; a vapour barrier there can trap moisture in the wood. Under nailed solid hardwood, rosin paper or a felt paper reduces squeaks and keeps the boards from rubbing on the subfloor. It is laid in overlapping courses and stapled.
Sound ratings
In apartments and condos, the building often sets a minimum sound rating for floors. Two numbers matter: STC (sound transmission class) for airborne sound, and IIC (impact insulation class) for footsteps. Underlayment makers publish both, tested with a specific floor and ceiling assembly. A cork or dense felt underlayment raises the IIC of a floating floor the most. Check the building’s rules before buying; the rating has to be met by the whole assembly, not by the underlayment alone.
Carpet is different
Carpet goes over its own pad, a thicker cushion chosen for density and thickness rather than sound or moisture, and it is not interchangeable with flooring underlayment. The carpet padding calculator and the carpet padding guide cover it.
The short version
Floating floor on wood: foam or felt. Floating floor on concrete: foam with a film, or foam plus 6 mil poly. Attached pad: nothing, except a barrier on concrete if the maker asks. Nailed hardwood: rosin or felt paper. Glue-down or tile: no underlayment. The subfloor preparation guide covers what has to happen before any of it goes down, and the vinyl plank guide covers pads on vinyl.