What a vapor barrier does
Warm indoor air carries moisture. In cold weather, if it reaches a cold surface inside a wall or ceiling, it condenses and can cause mold, rot and wet insulation. A vapor retarder on the warm side of the insulation slows this diffusion. In crawlspaces, a heavy polyethylene ground cover stops moisture rising from the soil.
Vapor retarder classes
Building codes classify materials by their permeance, in perms:
| Class | Permeance | Examples |
|---|---|---|
| I | 0.1 perm or less | polyethylene sheet, aluminum foil |
| II | 0.1 to 1 perm | kraft-faced batts, some “smart” membranes |
| III | 1 to 10 perms | latex paint, some house wraps |
The International Residential Code (R702.7) requires a Class I or II vapor retarder on the interior side of frame walls in climate zones 5 to 8 and marine zone 4, with exceptions for walls with enough exterior insulation. In hot, humid climates, an interior polyethylene sheet can trap moisture coming from outside and is generally not recommended.
How the calculator works
- Rolls = area × (1 + waste) ÷ (roll length × (roll width − overlap)).
- Tape = length of seams (area ÷ effective width) plus the perimeter, where the membrane must be sealed to the floor, ceiling and framing.
Example
1,200 ft² of walls and ceiling with 10 × 100 ft rolls, 6 in overlaps and 10% waste:
- each roll covers 9.5 × 100 = 950 ft² after overlaps;
- 1,320 ft² ÷ 950 = 1.39, so 2 rolls;
- about 266 ft of sealing tape for the seams and the 140 ft perimeter;
- 6-mil polyethylene at about 0.06 perm is a Class I vapor retarder.
Installation tips
- Overlap seams by at least 6 in and land them on framing; seal them with compatible tape or acoustical sealant.
- Seal around electrical boxes, pipes and windows: air leaks carry far more moisture than diffusion through the material.
- Use 6-mil polyethylene for walls, and 10 to 20-mil reinforced sheeting for crawlspace floors, run 6 to 12 in up the walls.
- Do not install two Class I layers on both sides of an assembly, which traps moisture.
- Smart membranes, whose permeance rises in humid conditions, let walls dry inward in summer while blocking vapor in winter.
Vapor control depends on climate and on the whole wall assembly. When in doubt, follow local code and a building-science guide for your climate zone.