Vapor Barrier Calculator

How it works

The calculator lays sheets of poly along the crawl space or slab, overlapping them at the seams, adds the liner run up the walls, and divides the total sheeting length by the length of one roll. Seam tape is counted from the length of all seams.

Covered width = Width + 2 × Height up the walls
Covered length = Length + 2 × Height up the walls
Sheets = ⌈ (Covered width − Overlap) ÷ (Roll width − Overlap) ⌉
Pieces per sheet = ⌈ (Covered length − Overlap) ÷ (Roll length − Overlap) ⌉
Sheeting length = Sheets × (Covered length + (Pieces − 1) × Overlap) × (1 + Extra ÷ 100)
Rolls = ⌈ Sheeting length ÷ Roll length ⌉
Seams = (Sheets − 1) × Covered length + (Pieces − 1) × Covered width
Tape rolls = ⌈ Seams ÷ Tape roll length ⌉

⌈ ⌉ means rounding up. Under a slab the height up the walls is zero. If one roll width covers the whole width, there is one sheet and no side seam.

The worked example

A 40 × 26 ft crawl space has 1,040 sq ft of floor. With 14 ft wide rolls and a 6 in overlap, two sheets cover 14 + 13.5 = 27.5 ft, so two 40 ft sheets do it: 80 ft of sheeting, one 14 × 100 ft roll (1,400 sq ft). The one seam between them is 40 ft long, well within a 180 ft roll of tape.

Run the liner 2 ft up the walls and the sheeting has to cover 44 × 30 ft. Two sheets now cover only 27.5 ft of the 30, so it takes three sheets of 44 ft – 132 ft of sheeting, two rolls – and 88 ft of seams. That jump is why the area shortcut is not enough.

Roll sizes

Poly sheeting is sold by width × length. Stego lists StegoCrawl Wrap 15-mil in 14 ft × 100 ft and 5 ft × 200 ft rolls and StegoHome 10-mil in 12 ft × 75 ft and 14 ft × 105 ft rolls; the international StegoHome sheet gives 4.3 × 32.0 m. Those are the example sizes here – replace them with the roll you are pricing. A wider roll means fewer seams and less tape.

Overlap and tape

Stego’s instructions say to overlap all seams a minimum of 6 inches (152 mm) and seal them with tape, and its crawl space guide notes that some jurisdictions call for a 12 inch overlap. The overlap is taken off every roll width except one, and one overlap is added for each end-to-end join. Stego Tape comes in 3.75 in × 180 ft (95 mm × 55 m) rolls. Sealing the liner to the walls, piers and pipes uses other accessories and is not counted here.

Code and design

This calculator does the material arithmetic only. Whether a crawl space needs a liner, which class and thickness of vapor retarder to use under a slab, how far it goes up the walls and how the edges are sealed come from your local code, the plans and the product instructions; Stego’s instructions are based on ASTM E1643, the standard practice for installing vapor retarders under concrete slabs.

Worked example

Common mistakes

• Dividing the floor area by the roll area. Every seam overlap and every strip of liner run up the walls uses sheeting, so you need more than the floor area suggests – sometimes a whole extra sheet.
• Laying sheets across a long crawl space. Sheets laid along the length need fewer seams and less tape.
• Forgetting the overlap where one roll ends and the next begins. The calculator adds one overlap for each end-to-end join.
• Taping only the seams. The edges at the walls and around piers and pipes need sealing too, usually with other products (termination bar, double-sided tape, pipe boots).
• Using a lighter sheet or a smaller overlap than the code or plans call for to save a roll.

Assumptions and limits

• The crawl space or slab is a rectangle. Sheets run along the length; the roll width goes across.
• In a crawl space the liner runs the height you enter up every wall, so the sheeting covers (length + 2 × height) × (width + 2 × height); the extra at the corners is folded, as is usual.
• Sheets = ⌈ (covered width − overlap) ÷ (roll width − overlap) ⌉, or 1 when one roll width is enough. A sheet longer than one roll is joined end to end with the same overlap.
• Rolls are the total sheeting length divided by the roll length, rounded up; offcuts are assumed to be reused.
• Tape covers the side seams and end joins at the tape roll length you enter. Piers, columns and penetrations are not modelled.

Frequently asked questions

Sources

• StegoCrawl Wrap 15-Mil Vapor Barrier – product page — Stego Industries. Roll sizes 5′ × 200′ and 14′ × 100′ (US example default 14 × 100 ft).
• StegoCrawl Crawl Space Encapsulation – Detailed Installation Instructions — Stego Industries. "All seams/joints should be overlapped a minimum of 6 inches … some jurisdiction requirements call for a 12″ overlap at seams"; seams sealed with StegoCrawl Tape; height up the foundation wall is often set by local code (cites 2021 IRC R408.3, not less than 6 in); based on ASTM E1643.
• StegoHome Wrap 10-Mil Vapor Retarder – Data Sheet — Stego Industries. Under slab: "Overlap all seams a minimum of 6 inches and seal using StegoHome Tape or Stego Crete Claw Tape"; rolls 12′ × 75′ (900 ft²) and 14′ × 105′ (1,470 ft²).
• StegoHome 10-Mil (0.25 mm) Vapour Retarder – International Data Sheet — Stego Industries. Rolls 1.8 × 45.7 m, 3.7 × 22.9 m and 4.3 × 32.0 m (metric example default 4.3 × 32.0 m); "Overlap all seams a minimum of 152 mm (6")".
• Stego Tape – International Data Sheet — Stego Industries. "Stego Tape is 95 mm x 55 m (3.75" x 180')" (tape roll defaults 180 ft / 55 m); "Overlap Stego Wrap 152 mm (6") and seal with Stego Tape."
• NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.

Method reviewed November 5, 2026