Vapor Barrier Calculator
Rolls of 6-mil poly sheeting for under a slab, deck, or crawlspace.
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A 6-mil poly vapor barrier blocks ground moisture from wicking up into a slab, crawlspace, or under a deck. It's cheap insurance against mold, musty smells, and a damp floor — the key is full coverage with overlapped, taped seams.
DifficultyBeginner
TimeAn hour or two to roll out and tape a typical area
SkillsNo special skills — measuring and taping
PermitsOften required by code under slabs and in crawlspaces — confirm the mil thickness your inspector wants.
Tools you'll need
- 6-mil polyethylene sheeting
- Utility knife
- Sheathing/poly tape
- Tape measure
- Gravel or stakes to hold it in place
Step by step
- Prep and level the ground Remove sharp rocks and debris that could puncture the poly, and rake the base smooth.
- Roll out the sheeting Lay the poly across the whole area, running rolls the long way to minimize seams.
- Overlap and tape seams Overlap edges ~6 inches and tape every seam continuously so moisture can't track through.
- Run it up the edges For crawlspaces and slabs, lap the poly a few inches up foundation walls and tape or batten it in place.
- Protect it Patch any punctures with tape before pouring concrete or adding gravel on top.
Common mistakes
Seams not overlapped or taped
Moisture tracks through gaps. Overlap ~6" and tape every seam.
Using too-thin plastic
Painter's plastic tears. Use 6-mil (or thicker where code requires) poly made for vapor barriers.
Punctures left unpatched
Sharp gravel and foot traffic poke holes. Inspect and tape over any tears before covering.
Safety
- Plastic sheeting is slippery underfoot — watch your footing.
- Keep poly away from open flame and heaters.
Frequently asked questions
How many rolls of vapor barrier do I need?
Enter your area above — the calculator adds 15% for overlaps and returns rolls of 6-mil poly (each 10×100 ft = 1,000 sq ft).
How much should I overlap the seams?
About 6 inches, with every seam taped continuously.
What thickness of plastic should I use?
6-mil is the common minimum under slabs and in crawlspaces; some codes call for 10–15 mil — check locally.
Next steps for this project
Concrete Slab
How much concrete you need for a slab, driveway, or patio — in cubic yards and bags.
Gravel
How much gravel you need for a driveway, path, or base — in tons and cubic yards.
Insulation
Bags of fiberglass batts by area and R-value.
Excavation
Cubic yards of dirt to dig and haul, plus truckloads.