Vapor Barrier Calculator
Work out the square footage of poly vapor barrier for a crawlspace, wall or slab — including overlap — and the number of rolls.
How it works
bare area = length × width · with overlap = area × (1 + overlap%) · rolls = ceil(with overlap ÷ roll coverage)
Vapor barrier area starts as a plain length × width, but sheets have to overlap at every seam — typically 6 to 12 inches — and you lose some to trimming around piers and walls, so 10–20% is added on top. Divide the total by the roll’s coverage and round up. In a crawlspace the barrier also runs up the foundation walls, so measure that height into the width if you are lining the perimeter.
Worked example
A 30 × 24 ft crawlspace is 720 sq ft. With 15% for overlap and waste that is 828 sq ft. One 10 × 100 ft roll covers 1,000 sq ft, so a single roll does the job with margin to run up the walls.
Frequently asked questions
How much overlap do seams need?
Most crawlspace and under-slab specs call for 6 to 12 inches of overlap, taped or sealed. That is why 10–20% is added to the flat area — the overlaps and trimming around piers and pipes are real material you have to buy.
What thickness of poly should I use?
Six-mil is the common minimum, but crawlspace encapsulation and high-traffic spaces often specify 10 to 20 mil for puncture resistance. Thickness does not change the area — just the roll you choose and its price.
Does the barrier go up the walls?
In an encapsulated crawlspace, yes — it runs up the foundation walls and is sealed to them. If you are lining the perimeter, add the wall height to your width so the calculator includes that material.
Related calculators
Estimates only — add for waste and confirm every load-bearing decision against local building codes and a design professional.