# Vapor Barrier Calculator

Calculate vapor barrier square footage for crawl spaces, basements, and floors. Accounts for seam overlap and wall run-up. Free vapor barrier calculator.

## What this calculates

A vapor barrier keeps ground moisture out of your crawl space, basement, or slab. This calculator figures out exactly how much material you need, including the overlap at seams and the run-up along perimeter walls. Enter your dimensions and roll size to get the total square footage and number of rolls.

## Inputs

- **Area Length** (ft) — min 0
- **Area Width** (ft) — min 0
- **Wall Run-Up Height** (in) — min 0, max 48 — How far the barrier runs up the wall (6-12 in typical for crawl spaces)
- **Seam Overlap** (in) — min 0, max 24 — Overlap at seams (6 in minimum, 12 in recommended)
- **Roll Width** (ft) — min 1, max 20 — Width of your vapor barrier roll (common: 10 or 12 ft)
- **Roll Length** (ft) — min 10, max 500 — Length per roll (common: 100 ft)

## Outputs

- **Floor Area** (ft²)
- **Wall Run-Up Area** (ft²) — Barrier running up the perimeter walls
- **Total Coverage Needed** (ft²) — Includes overlap waste
- **Rolls Needed**

## Details

Vapor barriers for crawl spaces should be at least 6-mil polyethylene, though 10-mil and 20-mil are increasingly standard for durability. The barrier covers the entire floor and runs up the foundation walls 6-12 inches, where it gets fastened and sealed. Seams must overlap by at least 6 inches and should be taped with vapor barrier tape.

For concrete slabs, a 10-mil or 15-mil vapor barrier goes under the slab before the pour. This prevents moisture from wicking up through the concrete, which protects flooring materials installed later. ASTM E1745 rates vapor barriers as Class A, B, or C based on puncture resistance and permeance.

Do not confuse vapor barriers with vapor retarders. A vapor barrier has a perm rating below 0.1 (like polyethylene sheeting). A vapor retarder has a perm rating between 0.1 and 1.0 (like kraft-faced insulation). Crawl spaces and slabs need a true barrier, not just a retarder.

## Frequently Asked Questions

**Q: How thick should a crawl space vapor barrier be?**

A: Use a minimum of 6-mil polyethylene for basic moisture protection. For full crawl space encapsulation, 12-mil or 20-mil reinforced barriers are better because they resist tears and punctures from foot traffic and gravel. The thicker material costs more but lasts much longer.

**Q: How much overlap do I need at seams?**

A: The minimum overlap at seams is 6 inches, but 12 inches is recommended for crawl spaces. All seams should be sealed with vapor barrier tape (not duct tape or general-purpose tape). At wall junctions, the barrier should extend at least 6 inches up the wall and be fastened with mechanical fasteners.

**Q: Do I need a vapor barrier on basement walls?**

A: It depends on your climate and waterproofing strategy. In most cases, a dimple membrane or drainage mat on the exterior wall is better than an interior vapor barrier. If you install an interior vapor barrier on basement walls, you can trap moisture between the barrier and the wall, which can lead to mold.

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Source: https://vastcalc.com/calculators/construction/vapor-barrier
Category: Construction
Last updated: 2026-04-08