How to install a crawl space vapor barrier

An unpleasant day's work that pays back more than almost anything else you can do to a house. Here is how to do it once.

This is a grim job and a genuinely high-value one. Plan for a full day, wear a respirator and knee pads, and take a work light rather than a flashlight.

What you need

• 10-mil or heavier reinforced barrier, floor area plus about 20%
• Polyethylene seam tape, more than you think
• A mechanical fastener strip or masonry fasteners for the wall termination
• A utility knife, a long straightedge, gloves, knee pads and a respirator
• A work light and something to kneel on

The steps

1. Step 1Clear the groundRemove debris, old insulation, any previous barrier and anything sharp. Rake the surface reasonably flat. Stones and roots puncture barrier from underneath over time.
2. Step 2Measure and plan the runsWork out the direction that gives the fewest seams, usually along the long axis. Plan to start at the far end and work back toward the hatch, so you are never crawling across fresh material.
3. Step 3Cut generously and lay the first runLeave enough at each end to run up the wall by 6 to 12 inches. It is far easier to trim than to patch.
4. Step 4Overlap every seam by at least 12 inchesMore on uneven ground. The lap direction matters less than its width and the tape that follows.
5. Step 5Tape every seam, fullyClean and dry both surfaces, then press the tape down along its whole length. This is the step that decides whether the barrier performs, and it is the one people rush.
6. Step 6Terminate at the wallsRun the barrier up the foundation wall and secure it with a fastener strip or masonry fasteners, sealing the top edge with tape or sealant.
7. Step 7Seal around piers and penetrationsCut, wrap and tape around every pier, post and pipe. These are the gaps that get skipped and they add up.
8. Step 8Check your work on the way outLook back along the whole space with the light low. Lifted laps and missed tape show up clearly at a shallow angle.

#1The barrier itself

10-mil reinforced crawl-space vapor barrier

10-mil reinforced is the thinnest material that survives being crawled on.

8.6/10★★★★★Our editorial score: 8.6 out of 10

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Specifications, as published by Generic

Type	Reinforced polyethylene ground vapor barrier
Thickness	10 mil, the usual floor for a crawl space you will ever crawl on
Use	Full ground cover, lapped and taped, run up the walls and sealed

Order 20% more than the floor area to cover laps, walls and the corner you misjudge.

What it gets right

• Reinforced against punctures
• Holds a taped seam
• Common roll widths

Where it falls short

• Heavy to drag into position
• Stiffer than thin sheeting

Specifications from Amazon listing (search).

#2The seams

Crawl-space seam tape

Purpose-made polyethylene seam tape, because general-purpose tape lets go within a year.

8.7/10★★★★★Our editorial score: 8.7 out of 10

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Specifications, as published by Generic

Type	Wide polyethylene seam tape for lapping vapor barrier sheets
Why it matters	An untaped lap is the first place the barrier fails

Do not substitute duct tape. It does not hold on poly in a damp space.

What it gets right

• Bonds to polyethylene properly
• Wide enough to bridge a lap
• Cheap for the performance it unlocks

Where it falls short

• Needs clean, dry surfaces

Specifications from Amazon listing (search).

The termite rule

Some jurisdictions require a visible inspection gap at the top of the foundation wall so that termite tubes can be seen. Covering that band can cause a problem at sale time or at a pest inspection. One call to the local building department settles whether it applies to you, and it is much easier to ask before the barrier is fastened.