The best crawl space vapor barriers
Bare soil evaporates into your house all year. Covering it properly is the cheapest large improvement available under a floor.
We earn a commission if you buy through our Amazon links, at no cost to you. We do not bench-test these products and we never take free product — how we pick · affiliate disclosure
Soil under a house is never really dry, and it releases water vapor continuously. Because warm air rises through a building, that vapor does not stay down there — it moves up into the rooms above, carrying the musty smell with it. A sealed vapor barrier is the single most effective thing you can do in a crawl space.
But it is a vapor barrier, not a water barrier. If liquid water collects down there after rain, a sheet of plastic over it traps a pond and changes nothing. Deal with the water first.
Choosing a thickness
| Thickness | Suitable for | Notes |
|---|---|---|
| 6 mil | Under a slab, never walked on | Tears under a knee. Not for a crawl space floor |
| 10 mil reinforced | Standard crawl space | The practical minimum where anyone enters |
| 12–16 mil | Crawl spaces used for storage | Noticeably tougher, easy to justify |
| 20 mil | Full encapsulation, frequent access | Expensive, near-indestructible, heavy to handle |
Quick picks
| # | Product | Best for | Score | Price |
|---|---|---|---|---|
| 1 | 10-mil reinforced crawl-space vapor barrier10-mil reinforced sheeting, which is the thinnest material that survives being crawled on. | A crawl space anyone will ever enter | 8.7 | Check price on Amazon #ad we earn a commission · Amazon shows the current price |
| 2 |  Crawl-space seam tapePurpose-made seam tape — the difference between a covered crawl space and a sealed one. | The seams, which are where it fails | 8.6 | Check price on Amazon #ad we earn a commission · Amazon shows the current price |
Scores are ours and editorial, not lab measurements — see how we pick. Tap a row for the full write-up.
10-mil reinforced crawl-space vapor barrier
10-mil reinforced sheeting, which is the thinnest material that survives being crawled on.
#ad we earn a commission · Amazon shows the current price
| 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 |
Six-mil sheeting is cheap and it tears the first time somebody crawls over it, which means the barrier is compromised the day it is installed. Ten mil reinforced is the practical floor, and the step up to 12 or 20 mil is worth it if the space is used for storage or inspected regularly.
Buy enough to lap generously. Twelve inches at every seam plus six to twelve inches up the walls is the standard, and running short halfway through is a miserable way to spend a Saturday.
What it gets right
- Reinforced, so a knee or an elbow does not open a hole in it
- Thick enough to lap, tape and hold a seal over time
- Widely available in roll widths that suit typical joist bays
Where it falls short
- Heavier and stiffer to drag into position than thin sheeting
- Needs mechanical fixing up the walls
Specifications from Amazon listing (search).
Crawl-space seam tape
Purpose-made seam tape — the difference between a covered crawl space and a sealed one.
#ad we earn a commission · Amazon shows the current price
| Type | Wide polyethylene seam tape for lapping vapor barrier sheets |
|---|---|
| Why it matters | An untaped lap is the first place the barrier fails |
An untaped lap curls, lifts and lets vapor straight past the barrier. Taping every seam roughly doubles what the same material achieves, and it is the step most often skipped because it is tedious.
Do not substitute duct tape. It does not bond to polyethylene reliably and it lets go within a year or two in a damp crawl space.
What it gets right
- Formulated to stick to polyethylene, which most general-purpose tapes do not
- Wide enough to bridge a lap properly
- Cheap relative to the performance it unlocks
Where it falls short
- Surfaces must be clean and dry or it will not bond
- Not a substitute for mechanical fixing at the walls
Specifications from Amazon listing (search).
Installing it so it lasts
- Deal with standing water firstIf water pools after rain, you need a pit and a pump or a drain to daylight before any plastic goes down. See crawl space sump pumps.
- Clear and rake the groundRemove debris, fallen insulation, old vapor barrier and anything sharp. Rake the surface reasonably flat.
- Treat any active mold on the joists firstAccess gets significantly worse once the barrier is down and you are trying not to damage it.
- Lay with 12-inch lapsWork from one end, overlapping generously. Resist the urge to economize on laps at the far corner.
- Tape every seamClean, dry surfaces, firm pressure along the whole length. This is the step that makes the rest of the work count.
- Run it up the walls and fix itSix to twelve inches up the foundation wall, sealed with tape or a mechanical fastener strip. Leave the top of the wall visible for termite inspection where that is a local requirement.
Questions people ask
What thickness vapor barrier for a crawl space?
10 mil reinforced is the practical minimum anywhere people enter; 12 to 20 mil is better for a space used for storage. Six-mil sheeting tears too easily to be worth the labor.
Do I need to tape the seams?
Yes. Untaped laps curl and let vapor around the barrier. Purpose-made seam tape roughly doubles the performance of the same sheeting.
Should the barrier go up the walls?
Yes, six to twelve inches, sealed to the foundation — unless local termite inspection rules require a visible gap at the top of the wall. Check first.
Will a vapor barrier fix standing water?
No. It stops vapor, not liquid water. Standing water needs a pump, a drain or corrected grading before the barrier goes down.
Sources
Read next
The best crawl space encapsulation kits
Good value for the materials, and consistently missing the three things that decide whether encapsulation works.
The best crawl space dehumidifiers
Nobody empties a bucket under a house. In a crawl space the condensate pump is not a convenience feature, it is the feature.
The best crawl space sump pumps
The constraints down here are clearance and access, not horsepower. The best pump is the one that fits the pit you can actually dig.
The best basement dehumidifiers with a pump
A dehumidifier manages the moisture already in the air. It does nothing about where that moisture is coming from — which is a feature, not a criticism, as long as you know it.
Is water in the basement bad?
Occasional dampness and standing water are different problems with different timelines. Here is how to tell which one you have.