Moisture & Vapor-Barrier Coverage
Size the moisture / vapor barrier over a slab or below-grade subfloor — how many rolls cover the area, before seam overlap.
Calculator
A vapor / moisture barrier over 500 sq ft in 200 sq ft rolls needs about 3 rolls. Over concrete or below-grade, a barrier keeps moisture out of wood and laminate. Overlap the seams — confirm the coverage and overlap on your product’s spec sheet.
A moisture or vapor barrier is a sheet — usually polyethylene film — laid over a concrete slab or a below-grade subfloor to keep ground moisture from wicking up into wood, laminate or engineered flooring, where it would cup, swell or grow mold. Any floor over concrete, over a crawl space, or below grade is a candidate; solid hardwood over a dry above-grade wood subfloor usually is not. Check what your specific floor and subfloor call for.
Like underlayment, a barrier is sold by the roll, so the question is how many rolls cover the subfloor — area divided by the roll's coverage, rounded up. The catch is overlap: barrier seams must overlap by several inches and be taped to actually block moisture, so the effective coverage is a little less than the label's flat number. Buy past the exact count. See the coverage table and the guide Underlayment & vapor barrier: what & how much.
Formula
rolls = ceil( area_sqft ÷ roll_coverage_sqft )
- area_sqft — the subfloor area to cover.
- roll_coverage_sqft — the roll's labeled coverage (often ~200 sq ft).
- ceil() — round up to whole rolls.
The count does not subtract the several-inch seam overlap and the run up the walls that a proper barrier needs, so it is the minimum — on a room that needs a lot of seams, buy an extra roll.
Worked example
A 500 sq ft basement, barrier rated at 200 sq ft per roll:
ceil(500 ÷ 200) = ceil(2.5) = 3 rolls
Two rolls cover only 400 sq ft, so you round up to 3 — and the third absorbs the overlap at the seams and the few inches you turn up the walls. Overlap and tape every seam per the product instructions; a barrier with open seams does not block moisture.
Vapor barriers in practice
Test before you assume. Whether a slab needs a barrier — and what kind — comes down to its moisture level. A calcium-chloride or relative-humidity test tells you what you are dealing with; a floor installed over a wet slab without protection is a warranty claim waiting to happen. Some floors come with an attached moisture backer or want a specific mil thickness of film, so read the install sheet rather than defaulting to any barrier.
Overlap, tape and turn up. The effective coverage is less than the flat roll number because seams overlap by several inches and the film often runs a few inches up the walls to be trimmed later. That is why buying to the exact ceil count can leave you short — round up when the room needs many seams. Confirm the roll's coverage and the required overlap on your product; both vary by brand and film thickness.
Barrier vs underlayment. They are not the same job. Some underlayments include a moisture film and double as a light barrier; plain foam or felt does not. If your underlayment already has an integral vapor film rated for your subfloor, you may not need a separate barrier — but do not stack two vapor layers, which can trap moisture between them.
Frequently asked questions
How many rolls of vapor barrier do I need for 500 sq ft?
With a 200 sq ft roll, ceil(500 ÷ 200) = ceil(2.5) = 3 rolls. Two rolls cover only 400 sq ft, and the third also handles the seam overlap and the run up the walls.
Do I need a vapor barrier over concrete?
Often yes for wood, laminate and engineered floors, because slabs wick ground moisture. Confirm with a slab moisture test and the flooring's install sheet — some floors require a specific barrier, some include one, and a few forbid a second layer.
Does the calculator account for seam overlap?
No. It returns whole rolls to cover the flat area. A real barrier overlaps seams by several inches and runs up the walls, so the effective coverage is lower — round up when a room needs many seams.
Is a vapor barrier the same as underlayment?
Not necessarily. Some underlayments have an attached moisture film that doubles as a barrier; plain foam or felt does not. Do not stack two separate vapor layers, though — that can trap moisture between them.
What roll coverage should I enter?
Use the square footage on your product's label, not a guess. Poly film rolls vary by width and length; 200 sq ft is a common default but far from universal, so overwrite it with your roll's figure.
How do I install it under a floating floor?
Roll it out over the clean, dry subfloor, overlap and tape the seams per the maker, and turn a few inches up the walls to trim after the floor is in. Then lay any underlayment and the floating floor on top.