The overlap is the whole story
Every seam costs you the lap twice over: once as material that is doubled up and covering nothing new, and once as a foot of tape. A roll of liner covers its own width minus one lap, and that single subtraction drives the roll count more than anything else on the form.
On the default crawl — 40 ft by 30 ft, a 12 ft roll, a 12 in lap and the sheet run 12 in up the wall — the numbers come out like this. Net coverage per run is 11 ft. The width to cover is not 30 ft but 32 ft, because the liner climbs a foot on each of the two side walls. That needs 3 runs. Each run is 42 ft long for the same reason. So the field takes 126 linear feet of roll, which is 1,512 sq ft of material laid over 1,200 sq ft of dirt before piers or waste.
| Line | Figure | Where it comes from |
|---|---|---|
| Floor area | 1,200 sq ft | 40 × 30 |
| Net coverage per run | 11.00 ft | 12 ft roll − 12 in lap |
| Width to cover | 32.00 ft | 30 + 1 ft up each side wall |
| Runs | 3 | 32 ÷ 11, rounded up |
| Length of each run | 42.00 ft | 40 + 1 ft up each end wall |
| Field liner | 126.0 lin ft | 3 × 42 |
| Six pier wraps | 76.0 sq ft | 6 × (5.33 + 1) × (1 + 1) |
| Pier wraps off the roll | 6.3 lin ft | 76.0 ÷ 12 ft roll width |
| Plus 10% waste | 145.6 lin ft | (126.0 + 6.3) × 1.10 |
| Material to buy | 1,747 sq ft | 145.6 × 12 |
| Per sq ft of floor | 1.46 sq ft | 1,747 ÷ 1,200 |
1.46 square feet of liner for every square foot of dirt. Drop the wall run-up to zero and it falls to 1.39. Going to a wider roll does not automatically help, and this is worth seeing: a 20 ft roll with the same 12 in lap covers 19 ft per run, so the 32 ft width takes 2 runs — but two 20 ft runs is 40 ft of width laid over 32, and the ratio goes up to 1.61. A wider roll wins when the crawl width is close to a whole number of net run widths and loses when it is not. Check yours both ways before ordering, and remember that a 20 ft roll is heavy and has to go through a hatch that is often 18 by 24 inches.
What the seam count does to the tape
Three runs give two seams. At 42 ft each that is 84 ft. Six piers add a collar the girth of the pier plus a vertical seam up the wrap, which on 16 in block piers is 6 × (5.33 + 1) = 38 ft. Total seam length 122 ft. One tape pass plus the same 10 percent waste is 134.2 ft, which is one 180 ft roll with room to spare — but only just, and a second seam pass on the underside would need two.
The tape line is the one people underbuy. It is also the line that decides whether the barrier does anything at all, because an untaped lap in a crawl does not stay lapped: it curls, it gets walked on, and a year later there is a foot-wide strip of open dirt down the middle of the floor. The caulk calculator handles the sealant side of the same job if you are sealing the top edge against the wall rather than fastening it.
What this page will not tell you
It will not tell you what thickness of material to use, whether your crawl may be sealed, or whether a vapour retarder is required where you live. Those are code and jurisdiction questions and they genuinely differ between neighbouring towns. It also has nothing to say about drainage: laying plastic over ground that has standing water on it after every storm moves the water sideways rather than removing it, and that is a job for grading, a drain or a pump first. Work out what is actually coming in with the water entry organizer before buying plastic.
Once the barrier is down, the useful next question is how much water it stopped. The ground moisture calculator puts a number on the evaporation off the exposed soil, and it is usually a much larger number than people expect.
Questions people ask
How much vapor barrier do I need for a 1,200 square foot crawl space?
More than 1,200 square feet, and the amount depends on the roll width and the lap. On the default settings here — a 12 ft roll, a 12 in overlap and the sheet run a foot up the walls — a 40 by 30 crawl takes about 1,747 square feet of material, or 1.46 square feet bought for every square foot of floor. A wider roll lowers that ratio because fewer seams means fewer laps. Running the liner up the wall raises it, because the run-up is added to both dimensions of the sheet.
Why does the calculator add the run-up to the length as well as the width?
Because the sheet is one piece. If a run climbs 12 inches up the end wall at both ends of the crawl, that run is two feet longer than the floor it covers. The same happens across the width, but there it shows up as extra runs rather than extra length. Set the run-up field to zero if the liner will stop at the base of the wall and you are terminating it some other way.
How much overlap should I use at the seams?
The liner manufacturer states it, and it is worth reading rather than guessing, because different products and different tapes assume different laps. Twelve inches is a common figure and is what the form opens with. The reason it matters to your order is arithmetic rather than performance: on a 12 ft roll, going from a 6 in lap to an 18 in lap changes net coverage per run from 11.5 ft to 10.5 ft, which on a 32 ft width is the difference between 3 runs and 4.
Do I have to wrap the piers?
That is a detail question about the system you are installing and the person who specified it, not something a calculator decides. What the page does is count the material if you do: each wrap is the girth of the pier plus one lap, by the wrap height plus one lap. Six 16 inch block piers wrapped a foot up come to 76 square feet, which is only about six linear feet off a 12 ft roll but adds 38 feet of seam to tape, and the tape is the part that runs out.
Is the material cost the whole cost of encapsulation?
No, and it is usually the smaller half. The cost line on this page covers liner, tape and wall termination at the prices you type in. It does not cover labour, and labour in a crawl space is slow: everything goes in through a hatch, nothing can be carried upright, and cutting around ducts and piers takes far longer than the same cut in an open room. It also excludes drainage work, a dehumidifier, insulation and any repair to what the barrier is being laid over.