How to Install Vapor Barrier in a Crawl Space to Control Moisture and Ground Gases

Lay down that vapor barrier like a pro to keep your crawl space dry

The exterior of a house with blue siding
Photo: irina88w / iStock / Getty Images Plus / Getty Images
The exterior of a house with blue siding
Photo: irina88w / iStock / Getty Images Plus / Getty Images
SKILL LEVEL
Intermediate
COMPLETION TIME
12 hours
COST
$50–$100
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What you'll need:
TOOLS
  • Metal garden rake or landscaping rake
  • At least one work light
  • Headlamp (optional)
  • Extension cord(s)
  • Utility knife
SUPPLIES
  • Clear plastic sheeting (6-mil)
  • Tape made for vapor barrier installation
  • Garbage bags (for plastic scraps and debris removed from the crawl space)

If you’ve enjoyed a dry crawl space up to this point, congratulations. If not, well, you’re here to learn how to install a vapor barrier in the crawl space. Odds are that you've already been into the belly of the beast dealing with water leaking through your foundation or some other problem. Well, even if your crawl space isn’t a dank dungeon with a really low ceiling, you may still want to consider installing a vapor barrier, because moisture still rises from the ground and can cause condensation and rot in your floor joists and subfloor. So let’s walk—or crawl—through the steps for how to install a vapor barrier in your crawl space.

Getting Ready to Install a Vapor Barrier

As you’ve seen, the crawlspace is called that for a reason. It’s tight, low, cramped, dark, and you’ve got to crawl if you want to get around down there. To make this project easier, get some good knee pads and a headlamp. You’ll want to run an extension cord or two and set up at least one work light so you can see what you’re doing. If you want to reduce the shadows, set up another work light from another angle. Also, if you’ve got a lot of cobwebs in the floor joists, go through with a shop vac or a broom to clear out as many as possible before you’re crawling around and dodging them as you work. 

If you’ve already done battle down below, you know where the crawl space entrance is, so you’re all set. If you’ve not ventured into the land of hands and knees, find out where the crawl space access is and maybe take a flashlight to get a peek at what’s going on in there. It’s also important to understand that installing a vapor barrier differs from crawl space encapsulation.

  1. Clean the Crawl Space

    A crawl space about to be cleaned
    Photo: Ethen Dell / iStock / Getty Images Plus / Getty Images

    This is a good time to remove any cobwebs so you can focus on the task at hand instead of always feeling like you have a spider on your neck. Besides having to drag it around down there, a shop vac is ideal for this because you’re going to be able to suck up cobwebs and anything hanging out in them.

    You’ll want the ground as clean as possible, so take a garbage bag and pick up any debris. The most likely things you’ll find are chunks of concrete or mortar from when the house was built, but you never really know what you’ll find down there. The ultimate goal is a clean floor without debris that can poke through your vapor barrier.

  2. Dry the Crawl Space

    Remove any standing water with a pump or buckets. Run a dehumidifier, monitoring and emptying when needed. Even if the crawl space seems dry, it doesn’t hurt to run a dehumidifier down there for one or two days to be sure it’s as dry as possible before installing the vapor barrier.

  3. Level the Floor

    Take the rake and level out the ground. If the ground slopes significantly and you can’t get it level without hauling in fill, then just make the surface as smooth and flat as possible for the plastic.

  4. Lay Out Plastic Sheets and Cut to Size

    Vapor barrier laid in a house’s crawl space
    Photo: Grandbrothers / iStock / Getty Images Plus / Getty Images

    Start at the opposite side of the crawlspace if possible, so you can work your way out and minimize how much you have to crawl around on the plastic after it’s in place. Roll out lengths of plastic (6-mil or thicker) from wall to wall, cutting each piece of plastic, leaving about 6 to 8 inches of extra along the walls. Overlap each run of plastic at least 12 inches and tape each seam until you’ve covered the entire floor.

  5. Seal Seams Along Walls and Support Piers

    Using double sided tape for installing vapor barriers, seal the plastic to the base of the walls all along the perimeter. Seal the extra plastic around any support piers.

Ensure Proper Ventilation to Control Moisture in the Crawl Space

Congratulations, you’ve installed a proper vapor barrier in your crawl space! Although the plastic won’t stop 100% of the moisture, you can help keep the crawl space dry by making sure the crawl space is properly ventilated.

This may mean calling a mason or a handyperson to install one or more vents in the top of your crawl space wall. The goal is to allow for cross ventilation to carry moisture out of the crawl space. A pro can also guide you on whether to keep your crawl space vents opened or closed, among other tips for keeping things dry.

Benefits of Installing a Vapor Barrier

The benefits of installing a vapor barrier and controlling moisture in your crawl space really include preventing mold, mildew, and rot or water damage to the floor joists and subfloor above the crawl space. Keeping the crawl space dry goes a long way in knocking down those dank crawl space smells.

If you have water leaking through the foundation into the crawl space, a vapor barrier will not remedy that. You’ll need to contact a basement waterproofing company to assess the problem and provide an estimate for fixing it.

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