Solve backwards
How big a crawlspace will my vapor barrier rolls cover?
- 2 rollsThe example worked below
- Vapor Barrier CalculatorEvery figure is its own
You have
How much vapor barrier?
Worked by the Vapor Barrier Calculator at the settings below.
The settings it uses
- Seam Overlap Allowance (%)
- 10
What it will do
1,818 sq ft of crawlspace floor
169 m²
One crawlspace. Enough for a crawlspace about 43 × 43 ft.
Or several. About 1 crawlspace of the size the Vapor Barrier Calculator opens on: a crawlspace about 33 × 33 ft.
Check it forwards
- Floor or Wall Area
- 1,818 sq ft
- It asks for
- 2 rolls
- You have
- 2 rolls
The Vapor Barrier Calculator asks for 2 rolls at that size, and 3 rolls one step larger.
If you had more, or less
Around your amount
Each worked by the Vapor Barrier Calculator at the same settings.
Half
1 roll
909 sq ft
a crawlspace about 30 × 30 ft
Yours
2 rolls
1,818 sq ft
a crawlspace about 43 × 43 ft
Half as much again
3 rolls
2,727 sq ft
a crawlspace about 52 × 52 ft
Double
4 rolls
3,636 sq ft
a crawlspace about 60 × 60 ft
Common amounts
What each amount of vapor barrier covers
The sizes it is usually sold in, each worked by the Vapor Barrier Calculator at the settings above.
| You have | It covers | As one crawlspace | Average crawlspaces |
|---|---|---|---|
| 1 roll | 909 sq ft(84.5 m²) | a crawlspace about 30 × 30 ft | under 1 |
| 2 rolls | 1,818 sq ft(169 m²) | a crawlspace about 43 × 43 ft | 1 |
| 3 rolls | 2,727 sq ft(253 m²) | a crawlspace about 52 × 52 ft | 1 |
| 5 rolls | 4,545 sq ft(422 m²) | a crawlspace about 67 × 67 ft | 2 |
Ten feet by a hundred, one thousand square feet
The calculator fixes the roll at 10 by 100 feet, 1,000 square feet or about 93 m² — the standard 6-mil poly roll its source gives. A 12, 16 or 20 foot wide roll, a 50 foot roll, or a reinforced 10 to 20 mil barrier will not divide into this count, so with another roll read the area from the table and divide by its own coverage.
Seam laps take about a tenth
On a plain rectangular crawlspace the taped seam laps use roughly a tenth of a standard roll, the calculator's allowance. Wider overlaps, narrower rolls or a footprint that forces many short strips take more. Sheet turned up the walls and wrapped round piers is extra area rather than overlap, so measure it and add it — it shrinks the floor here.
A roll count is not a vapour design
The answer says nothing about the permeance, thickness or puncture class the barrier must meet, or which face of the insulation it belongs on; in an unvented crawlspace the wrong side traps moisture. Nor is a poly roll a radon or ground-gas membrane, which is a designed and tested system of its own.
How the answer is worked
Nothing here is a second formula. The Vapor Barrier Calculator runs forwards — a size in, a quantity out — and this page grows its own starting crawlspace in proportion until the quantity it asks for is what you have. Where it rounds up to whole containers, the answer is the largest size before the next one would be needed.
Where the calculator adds an allowance for cuts or breakage when you order, the same allowance comes off here, because it is the same calculator: a stack you already own finishes less than its face value. Where it adds none, neither does this page — the settings beside the answer say which.
Vapor Barrier Calculator
runs it forwardsEstimate how many rolls of 6-mil poly vapor barrier you need for a crawlspace or basement.
Work out what to buy insteadSolve backwards
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Solve backwards
Start from what is already in the yard
Paint, sheets, rolls, bags and pieces, each read the other way by the calculator that orders it.
The calculator's own figures
Nothing is estimated twice: the page runs the same calculator you would order with.
The largest size that fits
Where a calculator rounds up to whole containers, the answer stops before the next one.
Both unit systems
Gallons and litres, square feet and square metres, side by side.
Frequently asked questions
- How much crawlspace floor does 2 rolls of vapor barrier cover?
- At the Vapor Barrier Calculator's own settings, 1,818 sq ft (169 m²) — a crawlspace about 43 × 43 ft. That is about 1 crawlspace of the size the calculator opens on: a crawlspace about 33 × 33 ft. Every figure follows the settings, which you can change in the calculator.
- How many square feet does a roll of 6-mil vapor barrier cover?
- A standard 10 by 100 foot roll covers 1,000 square feet, about 93 m². With ten per cent spent on taped seam laps it covers about 910 square feet (84 m²) of crawlspace floor, before any sheet turned up the walls or wrapped round piers.
- Should a crawlspace vapor barrier go up the walls?
- In an encapsulated crawlspace, usually yes — it is run up the foundation walls and sealed, and wrapped round piers. That sheet is extra area on top of the floor, so measure the wall height it climbs and add it before working out how many rolls you need.
- Is thicker vapor barrier better in a crawlspace?
- Thicker and reinforced barriers resist punctures from foot traffic and stone far better than 6-mil poly, which matters in a crawlspace used for storage or access. They come in different roll sizes, so the count changes with the product even where the floor does not.