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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Vapor barrier needed = floor area + wall run-up, plus 15% for the 12 in laps at every seam: a 40 × 28 ft crawl space with 1 ft of run-up takes about 1,444 sq ft of sheeting, roughly $230 in 6 mil poly and seam tape. Enter your space for sheeting, roll runs, tape and cost at 6, 10 or 20 mil.
With these inputs:
| Result | Value |
|---|---|
| Sheeting required | 1,444 sq ft |
| Strips across the space | 3 runs |
| Linear feet of roll | 120 ft |
| Seam tape needed | 132 ft |
| Wall run-up area | 136 sq ft |
| Material cost | $232 |
Base area = (length × width) + wall run-up area · Total sheeting = base area × 1.15 for laps and trim · Runs = width ÷ (roll width − 1 ft overlap)
6 mil meets code for a vented crawlspace. For full encapsulation or if anyone will crawl on it, use 10 to 20 mil reinforced.
Toward the warm side in winter — the interior in cold climates. In hot-humid climates skip the interior barrier entirely; it traps moisture on the wrong side.
Floor area, plus the wall run-up, plus about 15% for overlaps and trimming. A 30 × 40 ft crawl space with 2 ft of wall run-up is 1,200 + 280 = 1,480 sq ft, or about 1,700 sq ft of 6 mil sheeting to buy, with 12 in overlaps taped at every seam.
The warm-in-winter side, in heating-dominated climates — which means toward the interior. In hot humid cooling-dominated climates the moisture drive is inward, so an interior barrier is the wrong side and can trap moisture in the wall. This is one of the few building details where the right answer genuinely reverses with climate.
Many modern assemblies deliberately use a vapour retarder rather than a barrier, so the wall can dry in at least one direction. A true barrier on both sides is the failure case: any moisture that gets in cannot leave. The question worth asking is not whether to block vapour but which way the assembly is able to dry.
No, and conflating them is common. A vapour barrier resists diffusion through materials; an air barrier stops bulk air movement through gaps. Far more moisture moves through a wall carried by moving air than by diffusion, which means the air barrier usually matters more — and a vapour barrier full of staple holes and tears is not doing that job.
About $0.14 per sq ft for 6 mil polyethylene, $0.30 for 10 mil and $0.55 for 20 mil reinforced encapsulation liner, plus seam tape at about $0.25 per foot. A 40 × 28 ft crawl space with 1 ft of wall run-up needs about 1,444 sq ft of sheeting including laps: roughly $230 in 6 mil or about $825 in 20 mil, material only.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.