Poly sheeting comes in wide rolls and is overlapped at every seam, so usable coverage is meaningfully less than the roll's stated area. Fifteen percent covers the laps and the turn-up at the walls.
Crawl space encapsulation
The sheet covers the entire ground surface and runs up the foundation walls by six to twelve inches, taped or mechanically fastened at the top.
Overlap, sealing, wall attachment and minimum thickness vary by local code and the encapsulation system. Heavier membranes usually tolerate service traffic better; follow the product instructions and local requirements.
Under a slab
A vapour barrier under a concrete slab stops ground moisture rising through it, which is what ruins flooring adhesives and causes damp in finished basements.
It goes directly under the slab, over the compacted stone. Punctures from rebar chairs and foot traffic need patching before the pour.
In walls it depends on climate
Wall assemblies do not have one universal placement rule. Climate, cladding, air leakage control, indoor humidity and air conditioning all affect whether a vapor retarder is needed and where it belongs.
Some assemblies need no polyethylene layer and some use variable-permeance membranes. Verify the complete assembly with local code guidance or a building professional; the roll count alone cannot make that decision.
Perm rating is the number that matters
Vapour retarders are classified by permeance. Class I is under 0.1 perms — polyethylene sheet and foil. Class II is 0.1 to 1.0 — kraft facing on batts. Class III is 1.0 to 10 — latex paint on drywall.
These are not interchangeable, and the right class depends on climate and on what else is in the assembly. A Class I barrier in the wrong place stops a wall drying and creates the problem it was meant to prevent.
Buy on the perm rating rather than on the word barrier on the packaging, which is used loosely.
Never two barriers in one assembly
A wall or a floor assembly needs to be able to dry in at least one direction. A low-perm layer on both faces traps whatever moisture gets in — and some always does — with nowhere to go.
This is how a polyethylene sheet installed inside a wall that already has foam sheathing outside ends up rotting the framing between them.
Before adding a vapour barrier, work out what the assembly already has. In a retrofit that is often the harder question, and getting it wrong is worse than doing nothing.