Vapor Barrier
6-mil poly: floor + optional walls; 10% overlap on floor; 4×100 ft rolls.
About this calculator
This calculator sizes 6-mil polyethylene vapor barrier for a crawlspace, starting from floor area and optionally adding foundation wall coverage for a full encapsulation job. Length (ft) and Width (ft) carry equal weight on Poly ft² -- the floor formula multiplies them together (with a 10% overlap allowance for seams), so a given percentage change in either one moves the total sheet area by the same amount. The choice of Walls coverage has the largest single effect on the total: switching from "Floor only" to "Floor+walls" adds the full perimeter-times-height wall area on top of the floor figure, a bigger swing than a proportional change in either floor dimension alone.
Wall h (ft) only enters the calculation at all when Floor+walls is selected -- in floor-only mode it has zero effect on Poly ft², since the wall-area term is multiplied by zero. Rolls are sized from standard 4-foot by 100-foot (400 sq ft) stock, rounded up to the next whole roll. This is a material-quantity estimate only -- it does not account for seam taping, foam board insulation, or local code requirements for crawlspace encapsulation, which can add labor and additional materials.
Inputs
Results
Poly ft²
1,355.2
How to Use This Calculator
- Enter the crawlspace length and width in feet.
- Select whether you are covering the floor only or the floor plus walls.
- If covering walls, enter the crawlspace wall height in feet.
- Read the total poly sheeting area in sq ft (including 10% floor overlap) and the number of 4×100 ft rolls to purchase.
How the result changes with Length (ft)
| Length (ft) | Poly ft² |
|---|---|
| 22 | 677.6 |
| 33 | 1,016.4 |
| 66 | 2,032.8 |
| 110 | 3,388 |
What each input means
- Length (ft)
- Interior length of the crawlspace in feet.
- Width (ft)
- Interior width of the crawlspace in feet.
- Walls
- Choose whether to cover only the floor or the floor plus foundation walls for full encapsulation.
- Wall h (ft)
- Height of the crawlspace foundation walls in feet. Only used if covering walls.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersLength (ft) = 44, Width (ft) = 28, Walls = 0, Wall h (ft) = 3 = 4 input(s) provided
- Calculate Poly ft²Poly ft²1355.2 = 1355.2
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Do crawlspace length and width affect the vapor barrier area equally?
Yes -- Poly ft² is calculated as length times width (plus a 10% overlap allowance), so a given percentage change in either Length (ft) or Width (ft) moves the total sheet area by the same proportional amount. Neither dimension has more influence than the other in this formula.
Why does covering the walls make such a big difference in the total?
Selecting Floor+walls adds the full wall area -- twice the sum of length and width, multiplied by wall height -- on top of the floor area, which is a larger jump than changing either floor dimension by a similar percentage. Floor-only jobs skip this term entirely, so the wall coverage choice has the single largest effect on total sheet area.
Does wall height matter if I'm only covering the floor?
No -- Wall h (ft) is multiplied into the wall-area term, and that term is zero whenever Walls is set to "Floor only." Changing the wall height input has no effect on Poly ft² unless you also select Floor+walls coverage.
Why does the calculator add 10% to the floor area?
The 10% overlap allowance accounts for seams where adjacent sheets of poly must overlap for a continuous vapor barrier -- without it, the raw length-times-width area would understate how much material you actually need to buy to cover every seam properly.
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