Vapor Barrier Calculator
Calculate rolls of vapor barrier from crawl space, floor, or wall area.
Returns roll count for 6-mil and 10-mil poly sheeting with overlap allowance.
Vapor barriers (also called vapor retarders) are materials installed in walls, floors, crawl spaces, and ceilings to control moisture migration through building assemblies. Installing the correct square footage prevents mold, rot, and structural damage caused by condensation within wall cavities.
Area formula: Crawl Space Floor = Length × Width Wall coverage = Σ (Wall Height × Wall Width) for each wall section
Overlap and waste factor: Vapor barrier sheets must overlap 12" at seams and extend 6–12" up foundation walls. Standard waste factor: 10–15% for crawl spaces, 5–10% for walls.
Adjusted area formula: Material Needed = Coverage Area × (1 + Waste Factor)
Vapor barrier types and specifications:
| Type | Thickness | Permeance (perms) | Application |
|---|---|---|---|
| 6 mil polyethylene | 6 mil | ~0.06 perms | Crawl spaces (minimum code) |
| 10 mil polyethylene | 10 mil | ~0.04 perms | Heavy-duty crawl spaces |
| 20 mil reinforced | 20 mil | ~0.01 perms | Encapsulation systems |
| Kraft-faced insulation | — | 0.5–1.0 perms | Wall vapor retarder (cold climates) |
Perm rating: Lower perm = less moisture transmission. Class I vapor retarder: ≤ 0.1 perms. Class II: 0.1–1.0 perms. Class III: 1.0–10 perms.
Climate zone placement (IRC building code):
- Cold climates (zones 5–8): vapor retarder on interior (warm) side of insulation
- Hot-humid climates (zones 1–3): vapor retarder on exterior side or use vapor-open assemblies
- Mixed climates (zone 4): use smart vapor retarders that adjust permeance with humidity
Worked example: Crawl space: 45 ft × 28 ft = 1,260 sq ft floor area. Walls: 24" high perimeter = 2 × (45 + 28) × 2 = 292 sq ft Total coverage = 1,260 + 292 = 1,552 sq ft With 12% waste factor = 1,552 × 1.12 = 1,738 sq ft Standard rolls: 10 ft × 100 ft = 1,000 sq ft each → buy 2 rolls of 10-mil poly