What is the difference between a moisture barrier and a vapor barrier?

A moisture barrier stops liquid water, the kind you can see and wipe up. A vapor barrier stops water vapor, the moisture in the air you cannot see. In most crawl spaces the same sheet of plastic does both jobs, so people use the names like they mean the same thing. They are close, but they are not identical, and the difference matters when you are sealing a crawl space.

Here is the plain version. Water shows up two ways under your house. As liquid, when groundwater pushes up through the dirt floor or seeps along the wall. As vapor, when damp earth gives off moisture into the air and that air sits in your crawl space. A barrier that handles one does not always handle the other well.

A vapor barrier is rated for how much vapor it lets through. That rating is called permeance, or perm. Lower perm means less vapor gets through. A true vapor barrier sits at 0.1 perm or below. (Source: DOE Building America.) A “moisture barrier” is a looser term. Some thin sheeting marketed that way slows liquid water but still lets vapor pass, so the air under your house stays damp. That is the trap. A floor looks dry while the crawl space stays humid, and humid air is what rots joists and grows mold.

In a crawl space we do not pick one or the other. We put down a thick floor barrier that handles both, then run barrier up the walls too. Our crawl space encapsulation uses a 20-mil floor vapor barrier and a 12-mil wall vapor barrier, with dimpled drainage matting under the floor barrier so any liquid water has a path to drain instead of pooling against the plastic. The seams get spray foam sealed, and a dehumidifier dries the air that is left. A sump goes in where water collects.

Why the matting matters. A barrier alone can trap liquid water underneath it. The dimpled matting gives that water somewhere to go, toward a sump, so the barrier handles vapor on top while the matting handles liquid below. Belt and suspenders. That is the part a cheap “moisture barrier” roll skips.

This is also why a barrier is not the whole fix. You can lay plastic over a dirt floor and still have a wet crawl space, because the air is still damp and the walls are still letting in groundwater. Sealing the space and drying the air is the job, not just covering the dirt. So if someone quotes you a few rolls of plastic and calls it done, that is the moisture-barrier-only shortcut, and in New England it does not hold up.

A little local context. Around here the soil at your foundation is disturbed backfill. That is the looser fill put back in when the house was built. It drains slowly and holds water, so the ground under and beside a crawl space stays damp longer than you would think. NH frost heave works on the walls too. Damp ground plus a closed-up crawl is how that musty smell ends up in the rooms above.

You do not always need a sump. If your crawl is damp from vapor but never sees standing water, the barrier, the sealing, and a dehumidifier may be all it takes. We will tell you that when we look. Adding a sump to a crawl that never floods is money you do not need to spend.

What this means for your home

If your crawl space smells musty, has condensation on the joists, or the floors above feel cold and damp, the fix is encapsulation, not a roll of plastic. Encapsulation seals the whole space, handles both liquid and vapor, and dries the air. A 12-mil wall and 20-mil floor vapor barrier do the heavy lifting, the matting drains the liquid, the foam seals the seams, and the dehumidifier keeps it dry.

One thing to keep straight. The Forever Dry System is our basement fix, with interior perimeter drainage, a sump pump, a wall vapor barrier, and a dehumidifier. It is for basements only. For a crawl space you want crawl space encapsulation, which is built different for that lower, tighter space. Do not let anyone sell you a basement system for a crawl, or call a few sheets of plastic an encapsulation.

Crawl space encapsulation in New Hampshire costs $3,000 to $25,000, depending on the size of the space, how much standing water you get, and whether you need a sump. Our encapsulation carries a 25-year warranty, and the wall vapor barrier is warrantied 25 years too. That warranty is transferable, but it voids if another contractor or owner touches the work, so keep it in the family.

A sealed crawl also keeps heat in. The Department of Energy says up to about a third of a home’s heat loss can come from an uninsulated foundation or crawl. (Source: DOE Building America.) Drier air, warmer floors, less of that musty smell. We have done this for thousands of homeowners across New Hampshire, Maine, and Massachusetts.

Ready to look at it?

We will come out, get into the crawl, and tell you whether you need full encapsulation, a sump, or just a barrier and a dehumidifier. The on-site inspection and written estimate are free, and we get you a quote within 24 hours.

Call us at 603-610-1770, or book your free inspection. Angel runs our waterproofing side, and he has seen every kind of wet crawl there is.

Related questions

Is a plastic sheet over the dirt enough for a crawl space?

Usually not. A loose plastic sheet slows some moisture from the ground, but it does not seal the walls, it does not dry the air, and it can trap liquid water underneath. A real fix seals the whole space with a 20-mil floor and 12-mil wall vapor barrier, drains liquid water with dimpled matting, foam-seals the seams, and dries the air with a dehumidifier. That is encapsulation, and it is what holds up in New England.

What perm rating counts as a real vapor barrier?

A true vapor barrier sits at 0.1 perm or below, meaning almost no vapor passes through it. (Source: DOE Building America.) Thin “moisture barrier” sheeting often has a higher perm, so it slows liquid but lets vapor through, and your crawl stays humid. We use a 20-mil floor and 12-mil wall vapor barrier rated to keep both liquid and vapor out.

How much does crawl space encapsulation cost in New Hampshire?

Crawl space encapsulation runs $3,000 to $25,000, depending on the size of the space, how much standing water you get, and whether you need a sump. A small dry crawl that just needs a barrier and a dehumidifier sits on the low end. A larger crawl that takes on water and needs full drainage and a sump runs toward the high end. We give you a written quote within 24 hours, and the work carries a 25-year warranty.

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