A vapor barrier being installed. The words, "DIY Install Vapor Barrier Into Crawl Spaces" are overlain.

Adding a vapor barrier to a crawl space can prevent mold, mildew, corrosion, and more.

It can also stop various pests from making a home out of the crawl space.

Professional vapor barrier installations can be quite costly.

You’ll save a lot of time and money by DIYing the project, so let’s get into it.

To DIY-install a vapor barrier into crawl spaces, clean the space, layer butyl tape with a plastic vapor sheet, then pull the sheet to the bottom of the space. Use more butyl tape to secure the bottom of the vapor sheet. Add vapor tape outside the plastic sheet to prevent moisture from entering.

Throughout this post, we’ll explain the step-by-step process of installing a vapor barrier to protect a crawl space.

We’ll also provide some additional tips to keep moisture and pests out of the space.

Read on to learn more about this simple DIY project.

Gather the Necessary Materials for the Vapor Barrier

A box cutter, roll of butyl tape and a roll of vapor barrier plastic.
The tools you need for installing a vapor barrier are easily available in your local DIY store.

Installing a vapor barrier doesn’t require too many materials that you might not already have.

Furthermore, you don’t have to use any power tools.

You’ll need double-sided butyl tape, vapor tape, a box cutter, vapor sheets, and vapor barrier fasteners (also known as Christmas tree fasteners).

Let’s look at each of these materials to ensure you have everything ready to go.

  • Vapor tape: The Blue Summit Supplies Vapor Barrier Tape is one of the best vapor tapes you’ll come across. This 180-foot roll has a 3.78-inch width to secure any vapor barrier you can put in your crawl space. It works on any crawl space material, including layering it over wood, butyl tape, etc.
  • Double-sided butyl tape: Try TAPEM Butyl Tape if you want a long-lasting, strong bond. This butyl tape sticks to the crawl space, holding the vapor barrier in place for many years to come. The tape is 30 feet long by ¾ of an inch. It also includes a bubble roller that doubles as a tape cutter in a pinch.
  • Box cutter: You can use a box cutter or any other handheld knife to get the job done. Box cutters are the best choice because they won’t cut too deep into the butyl tape roll if you’re having trouble removing a piece. Always cut butyl and vapor tape horizontally to prevent the knife from damaging the rest of the roll.
  • Vapor sheets: If you want a high-quality, heavy-duty plastic vapor sheet, try the Aluf Plastics Plastic Sheeting. This 10’ x 100’ sheet is 6mil thick. There’s more than enough vapor barrier for an entire crawl space, so you shouldn’t have to buy multiple sheets. You can also choose a 4mil or 12ft wide option rather than 10ft.
  • Vapor barrier fasteners: The ELK 1” Vapor Barrier Christmas Tree Fasteners should be at the top of your list if you don’t have a set. These capped fasteners sit flush with the vapor barrier. They also prevent it from shifting, drastically reducing the chances of tears or loosening from the butyl tape.

Remove All Debris and Items From the Crawl Space

An immaculate crawl space in a home
Ensure you clean out the crawlspace before you start work installing the vapor barrier.

Anything touching the edges of the crawl space will prevent the vapor barrier from working.

There should be nothing between the barrier and the wall (other than butyl tape). Move everything to the center or get it out of the crawl space. This process includes leveling dirt if your crawl space doesn’t have flooring.

You should also take this time to clean the crawl space walls. Butyl tape needs a clean surface. It sticks to dust and debris, so there’s a high chance it’ll fall off if the crawl space isn’t ready to go.

Use alcohol wipes or an all-purpose cleaner to tidy the crawl space before installing the vapor or butyl tape.

Quick Note: If you put anything back in the crawl space when you’re done installing the vapor barrier, keep it away from the edges. Vapor barriers are often made out of plastic sheets or foam, both of which don’t last long when they’re under constant pressure.

Layer the Butyl Tape With a Vapor Barrier Sheet

A black vapor barrier being put into position.
The vapor barrier must be held in place by the butyl tape for a good seal.

The butyl tape should be at least half an inch wide. If it’s too narrow, you won’t be able to secure the vapor barrier properly.

Keep it in a straight line as much as possible. Cut the butyl tape, stretch it, the slowly pressure the vapor barrier against the tape as you go.

Take your time, and don’t forget to press and hold the barrier for a few seconds to adhere it to the wall.

The wall needs to be cleaned if you’re having trouble sticking the butyl tape in the crawl space.

Butyl tape needs to stick, compress, and bond for a little while before it works. This step is much easier if you have an extra pair of hands to help you out.

One person presses the vapor barrier to the tape while the other person applies the tape to the walls.

Some crawl spaces have windows and numerous entrances. If your crawl space has this unusual layout, read on.

Measure, Mark, Cut, and Seal the Entrances and Windows

A worker measuring a window frame.
It’s important to measure the window spaces accurately before cutting the vapor barrier.

You don’t have to cover entrances and windows with the vapor barrier. You can cut around them to ensure they’re accessible and don’t look tacky from the outside.

All you need to do is mark and cut the spaces once you tape the sheet near these spaces. However, leaving a bit of extra material to tuck around windowsills and door frames is essential.

Here’s how you can ensure your crawl space has a vapor barrier around each entry point and window:

  1. Place butyl tape over the window or doorframe, then attach a sheet of vapor barrier plastic. There should be enough butyl tape to cover the enter width of the frame. Keep the tape as close to the top of the frame as possible, ensuring enough room to stretch the vapor barrier far below the frame.
  2. Mark two inches inward from the frame with a pen, then cut the line. There should be enough extra material to tuck into the frame. Another way to do this is to measure the depth of the frame, then use the dimensions as a marker for how much extra material is needed.
  3. Secure the bottom of the vapor barrier with butyl tape and vapor tape. This process works the same as the previously mentioned method. The vapor barrier should be stretched snugly over the frame, ensuring it looks as flat and seamless as the rest of the crawl space you modified.
  4. Fold the extra vapor sheet into the frame, then apply vapor tape. It’ll look as if you framed the area with a vapor barrier, which is precisely what you’re going for. You can also use butyl tape inside the frame to double the bond with the vapor barrier. The frame barrier should be stretched, so ensure it’s not sagging or slouching.

Fasten the Plastic Barrier Throughout the Crawl Space

A diagram of a christmas tree fastener for holding a vapor barrier in place.
Christmas tree fasteners are just the thing for holding the vapor barrier in position. Image courtesy of Lowes.

Americover suggests placing Christmas tree fasteners throughout the barrier. You can use a rubber mallet to drive them through the vapor barrier.

Make sure the fasteners are facing directly toward the wall; they’re fragile and can break if hit at an angle. Furthermore, the threads won’t stay in the wall without proper application.

Although it might be tempting, avoid using metal screws when installing your vapor barrier. The screws rip the edges of the barrier, allowing vapor to get into the crawl space.

It can also cause the screws to rust, which you won’t experience if you use plastic Christmas tree fasteners (such as the ones mentioned in the materials section).

When you use these fasteners, you only have one chance per fastener. They poke holes in the vapor barrier when you hit them through the walls.

The flat plastic head secures the vapor barrier, but removing it will break the seal. The wide, flat cap is much more suitable than metal screws.

Seal All Cracks and Exposed Spaces

Air sealing around an electrical outlet using spray foam.
Air sealing is a critical part of ensuring insulation and vapor barriers do their jobs properly. Eliminating drafts around electrical outlets using spray foam can make a huge difference to the energy efficiency of a room.

Like controlling moisture with insulation, installing a vapor barrier requires seamless installation.

Any gaps between joins in the barrier will allow moisture into the crawl space.

You can cover the entire area, but if you forget a one-inch section, you’ll allow enough moisture into the crawl space to make it a waste of time.

On that note, you should take this opportunity to seal all the cracks in the crawl space.

Use silicone caulking since it’s naturally waterproof. Replace the caulking around each window and entrance if your crawl space has any.

Don’t forget to seal cracks in the bricks, cobblestones, wood, and other materials in the crawl space.

While a vapor barrier provides unparalleled protection, it’s also important to ensure that the outside of the crawl space is protected.

If anything sharp (such as a nail) or pressurized (such as excessive water flow) hits the barrier from inside the wall, it’ll rip. You’ll have to replace that section of the vapor barrier immediately.

Consider an Active Depressurization System

A wall mounted ventilation fan with a square grille covering.
Ventilation fans can serve to facilitate the active depressurization of crawlspaces to ensure gasses and vapors do not build up.

Crawl spaces are often ridden with various soil gases. These gases can be dangerous for your home.

In natural settings, soil gasses evaporate into the atmosphere without causing problems.

However, crawl spaces don’t have room for the gasses to escape, which is where an active depressurization system comes into play.

According to Cabin DIY, you can install an active depressurization system in your crawl space to remove soil gasses and debris from behind a vapor barrier.

This system uses a series of fans that draw stagnant air from the crawl space, ensuring nothing gets too toxic.

When used with a vapor barrier, an active depressurization system almost completely eliminates moisture and vapor in a crawl space.

Make a Drainage Slope Outside of the Crawl Space

A drainage slope next to a white wall.
A drainage slope can help to divert water away from your crawlspace and ensure the integrity of the vapor barrier isn’t impacted by water ingress.

Drainage slopes keep water vapor out of your crawl space because they prevent water ingress.

Crawl spaces are prone to excess humidity when the exterior water levels get too high.

Fortunately, making a drain slope is easy and only takes an hour or so. All you need is a shovel to get started.

Follow these instructions to protect your crawl space’s vapor barrier:

  1. Remove all debris within two to three feet outside the crawl space. You’ll need plenty of room to dig a slope but make sure you don’t turn it into a ditch. You might have to add extra dirt if the soil slopes toward the crawl space. However, you can use a drain pipe to keep the ditch.
  2. Dig a two-foot slope leading away from the crawl space. Family Handyman recommends digging the slope down one inch for every foot. This angle ensures that water will drain away from the crawl space without causing structural damage or turning into the ditch during the next rainstorm.
  3. Flatten the dirt with the slope’s angle, then consider adding a drainage pipe. Drainage pipes remove excess water from the area around the slope. Place the end of the pipe wherever you want the water to drain. You can install a drain pipe with one opening or get one with an exposed porous top to collect rainwater.

Final Thoughts

Vapor barriers are more than worth it. You don’t need to fork out a bunch of money to hire an expert, nor do you have to put it off for several years.

Installing a vapor barrier with the DIY method mentioned above takes a day or two, and you don’t have to wait for the crew to fit you into their busy schedule.

Further reading on crawlspace vapor barriers is available here.

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