Underlayment & vapor barrier: what and how much
Between the subfloor and the finish sits a thin layer that does a lot of work — cushioning, quieting and, over concrete, blocking moisture. This guide explains underlayment vs vapor barrier and how many rolls of each to buy.
Two different jobs
Underlayment is a foam or felt sheet under floating floors (laminate, some LVP) that cushions footfall, smooths tiny subfloor imperfections and cuts noise. A vapor (moisture) barrier is a plastic film that stops moisture wicking up from a concrete slab into wood or laminate, where it would cause cupping, swelling and mold. They are not the same thing — though many products combine both into one sheet with an attached film, which is worth confirming before you buy a separate barrier.
When you need each
- Floating laminate / LVP → underlayment, unless the planks have it pre-attached (many do — adding a second pad can void the warranty).
- Any wood or laminate over concrete → a vapor barrier, always, because slabs are never truly dry.
- Nailed hardwood over a wood subfloor → usually a felt or rosin paper, not foam.
- Glue-down and tile → generally no foam underlayment; follow the adhesive/mortar spec instead.
The roll formula
Both products are sold by the roll, and both use the same arithmetic — area divided by roll coverage, rounded up:
rolls = ceil(area_sqft ÷ roll_coverage_sqft)
An underlayment roll typically covers about 100 sq ft; a vapor-barrier roll about 200 sq ft (it is thinner and wider). So:
- Underlayment for 200 sq ft: ceil(200 ÷ 100) = 2 rolls (underlayment calculator).
- Vapor barrier for 500 sq ft: ceil(500 ÷ 200) = 3 rolls (vapor-barrier coverage).
The underlayment, leveler & vapor coverage table lists the typical roll sizes.
Do not forget the overlap
Vapor barrier seams are overlapped and taped (commonly 6–8″) and run up the wall a couple of inches so moisture cannot sneak past the edge. That overlap is real coverage you lose, so the roll count above is a floor, not a ceiling — buy a little extra rather than piecing in a short strip. Underlayment seams are butted and taped, with much less overlap.
How it fits the rest of the prep
Underlayment and barrier go down after the subfloor is flat and prepped and before the finish floor. If you are leveling first, do that, let it cure, then roll out the barrier/underlayment. Then the flooring calculator tells you the boxes that go on top.
Reading an underlayment spec
Not all pads are equal, and the spec sheet tells you which job a product does. Two numbers matter most: thickness (commonly 1–3 mm — thicker is not always better, since too soft a pad lets a click floor flex and fail) and the IIC/STC sound ratings, which matter a lot in condos and upstairs rooms where a floor covenant may require a minimum. Some pads add an R-value for a little warmth over concrete, and some fold in a moisture film. Buy for the requirement, not the price — the cheapest pad can cost you a squeaky, hollow-sounding floor.
Getting the moisture barrier right over concrete
Concrete looks dry but is not: slabs wick ground moisture for years, and that vapor destroys wood and laminate from below. A proper barrier is a continuous 6-mil (or heavier) poly film with seams overlapped 6–8″ and taped, run a couple of inches up every wall so moisture cannot creep around the edge. That overlap and up-turn is coverage you consume, which is why the roll count from the formula is a minimum — buy an extra roll rather than piece in a short strip at the last wall. If you are unsure whether your slab needs it, a simple plastic-sheet or calcium-chloride moisture test settles it before the floor goes down.
The order of operations
Sequence keeps you from redoing work. The right order is: get the subfloor flat and cured → lay the vapor barrier (over concrete) → roll out the underlayment (unless it is pre-attached) → install the finish floor → hide the perimeter gap with baseboard or quarter-round. Do not level over a barrier, and do not stack two pads. Each layer has one job, and doing them in order means the floor above sits flat, dry and quiet.
Common missteps
- Double padding. Adding foam under planks that already have it makes the floor spongy and voids many warranties.
- No overlap on the barrier. Un-taped seams let moisture straight through — the film is only continuous if you make it so.
- Foam under glue-down or tile. Those systems bond directly; a pad belongs under floating floors, not troweled ones.
- Buying the exact roll count. With overlaps and offcuts, order a margin — use the underlayment and vapor-barrier calculators and round up.
Matching underlayment to the finish floor
The right pad depends on what goes on top. Under floating laminate, a standard foam underlayment is usual — unless the planks already have it attached. Under floating LVP, especially rigid SPC, use only a thin, firm underlayment rated for it (or none if pre-attached); a soft, thick pad lets the rigid planks flex at the joints and fail. Under nailed hardwood, the “underlayment” is typically a felt or rosin paper that quiets the floor and slows moisture, not a foam cushion. Under glue-down and tile, there is no foam pad at all — the finish bonds directly to the prepared substrate. So before you buy rolls, confirm the pad the manufacturer specifies for your exact floor; the wrong cushion can void a warranty as surely as no vapor barrier over concrete can ruin the boards.
These are labeled planning values. Roll coverage and whether a barrier is required vary by product and subfloor — confirm on the product spec sheet, do not double up on pre-attached pad, and follow the manufacturer’s moisture requirements over concrete.