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Crawl Space Vapor Barrier Calculator

How much polyethylene liner do you need under the house? Enter the floor size, wall upturns, and overlap to estimate total square footage, how many rolls to buy, and approximate seam tape.

Educational estimate. Not engineering advice, a cut list, a bid, or a substitute for local code, manufacturer instructions, or a licensed professional.

Floor size
Walls and piers

Default 0.5 ft (6 in.). The U.S. Department of Energy crawl space vapor retarder job aid calls for extending the liner at least 6 inches up the walls.

Assumption Default 4 sq ft per pier for a short wrap. Change it to match your pier size.

Overlap, rolls, and tape

Assumption Default 12%. The DOE job aid calls for at least 12-inch seams; 10–15% is a common planning band.

Assumption Seam tape length is estimated from a rectangular layout, not a measured seam map.

How to use it

  1. Enter length and width in feet, or floor area in square feet.
  2. Say whether the liner turns up the walls, and set the upturn height (default 6 inches).
  3. Add piers if you wrap them, and adjust the overlap percentage for your layout.
  4. Pick a roll size that matches the product you plan to buy, then calculate.

The numbers behind the estimate

Floor area is length × width. Wall upturn area is perimeter × upturn height when walls are covered. Pier wrap is the number of piers times the wrap area you enter. The total multiplies that subtotal by (1 + waste%). Rolls round up. Seam tape uses a rectangular layout estimate — labeled as an assumption — not a measured seam map.

Worked example

A 40 × 30 ft crawl space (1,200 sq ft) with a 6-inch wall upturn, no piers, and 12% overlap waste comes to about 1,270 sq ft before waste and about 1,422 sq ft after. On 1,200 sq ft rolls, that is 2 rolls, plus seam tape from the layout estimate.

Limits and safety

  • Irregular footprints, bunkers, and obstacles are not modeled.
  • This is a planning estimate, not a cut list or a structural assessment.
  • Do not block drainage paths with the liner. Standing water and active leaks need fixing first — see water in the crawl space and when not to encapsulate.

Also useful: Crawl space vapor barrier guide, Encapsulation guide, and Encapsulation process steps.

Sources

The U.S. Department of Energy’s Install or Repair Crawl Space Vapor Retarder job aid calls for at least 6-mil polyethylene, extending the liner at least 6 inches up the walls, and overlapping seams by at least 12 inches. Overlap waste percentages and pier-wrap defaults here are labeled planning assumptions you can change.