Underlayment is the cheapest part of a new floor and the one most people get wrong. The right roll depends on two things: what the floor is, and what it is going over. Get those two right and the quantity is easy.
The short answer
- Laminate or engineered floating floor over plywood: standard 2–3 mm foam.
- Anything over concrete or below grade: a combination underlayment with a built-in vapor barrier, or foam plus a 6-mil plastic sheet.
- Upstairs rooms or condos: felt, cork, or a sound-rated foam to quiet footfall.
- Planks with attached padding: no separate underlayment at all; doubling up voids most warranties.
Once you know the type, the floor underlayment calculator turns your room size into rolls to buy.
Underlayment by floor type
| Floor | What to use | Notes |
|---|---|---|
| Laminate (click-lock) | 2–3 mm foam, or combo over concrete | Most manufacturers cap thickness at about 3 mm |
| Engineered hardwood, floating | Foam or felt; combo over concrete | Felt adds warmth and sound control |
| Luxury vinyl plank (LVP) | Thin 1–1.5 mm vinyl-rated pad, or none | Soft foam lets joints flex and unlock |
| Solid hardwood, nail-down | Rosin or asphalt-saturated felt paper | Never foam; it is a slip sheet, not a cushion |
| Tile | Cement board or an uncoupling membrane | Foam underlayment is not for tile |
| Carpet | Carpet pad, sold separately | See the carpet calculator |
Underlayment by subfloor
Concrete, basement, or slab-on-grade: moisture is the enemy. Use a combo underlayment with an attached vapor barrier, overlap the seams, and tape them. Plywood or OSB above grade: basic foam is fine; the subfloor just needs to be flat and dry. Radiant heat: check that the underlayment is rated for it; thick cork and some rubber pads insulate too well and block the heat. Existing vinyl or tile: most floating floors can go straight over it with foam, as long as it is flat and well bonded.
The types compared
| Type | Thickness | Best for | Cost per sq ft |
|---|---|---|---|
| Standard foam | 2–3 mm | Laminate over plywood, budget jobs | $0.30–$0.50 |
| Foam + vapor barrier (combo) | 2–3 mm | Anything over concrete | $0.50–$0.75 |
| Felt | 3 mm | Sound and warmth under engineered wood | $0.60–$1.00 |
| Cork | 3–6 mm | Condos and upper floors, sound ratings | $0.75–$1.25 |
| Rubber | 2–5 mm | Maximum sound control, gyms | $1.00–$2.00 |
How much underlayment do I need?
Measure the room's length and width in feet and multiply for the area. Add about 5% for overlap at the seams and trimming at the walls, then divide by the coverage printed on the roll. Standard foam rolls are usually 100 sq ft; combo and felt rolls come in 100, 200, and 400 sq ft sizes.
Worked example: a 200 sq ft room (say 12 × 17 ft) plus 5% is 210 sq ft. With 100 sq ft rolls that is 2.1 rolls, so buy 3; with one 400 sq ft roll you are covered with plenty left for the closet. For a 12 × 12 bedroom (144 sq ft) it is two 100 sq ft rolls. Rooms with lots of cuts, angles, or a diagonal layout should use 10% waste instead of 5%.
Mistakes that waste rolls or wreck floors
- Doubling up. Padding under planks that already have it makes the floor spongy and the joints fail.
- Overlapping foam seams. Butt the edges together and tape them; overlaps telegraph as ridges. Only the vapor-barrier flap on combo products is meant to overlap.
- Too thick. Anything over about 3 mm under a click-lock floor lets the joints flex; check the flooring manufacturer's limit.
- Skipping the vapor barrier over concrete. Trapped moisture cups laminate and grows mold under it.
- Running rolls the wrong way. Lay underlayment in the same direction as the planks so seams do not stack.
Pick the type from the tables above, then enter your room in the floor underlayment calculator for the roll count, and size the planks themselves with the flooring calculator.