How to Install Vapor Barrier in a Crawl Space: A Crawl Space Ninja DIY Guide
Installing vapor barrier can be really complicated in some crawl spaces.
If you asked me, “AJ, what’s the best way to install vapor barrier?” I’d probably tell you:
“Just roll it out.”
See? Easy.
...Well, mostly easy.
There’s science to everything, and your time is important. Crawl spaces have a funny way of making simple things complicated. You've got walls, floors, pillars, corners, plumbing, ductwork, and usually at least one thing down there that makes you say, “Well... that's going to be fun.”
So if you want the short answer to “How do I install vapor barrier?”:
Roll it out until you reach the end of your workable section, cut it, secure it, and repeat.
That's the basic idea.
But there's a right way to approach the job if you want to save yourself some time, minimize unnecessary seams, and avoid making the installation harder than it needs to be.
Before You Start: Measure Your Crawl Space
Before you start rolling out plastic, you need to know how much material you're actually going to need.
Measure the areas you're planning to cover, including:
- Crawl space floor
- Foundation walls
- Support pillars
- Other areas that need to be covered
And don't forget about overlap and waste.
Trying to buy the exact square footage of your crawl space is a great way to discover halfway through the project that you're short on material.
We've put together a complete guide to help you figure out how much material you'll need:
How Much Crawl Space Encapsulation Material Do I Need?
Once you've got your measurements and materials figured out, it's time to actually get the plastic in the crawl space.
Step 1: Start With the Walls
For a typical crawl space, we recommend working in this order:
1. Walls
2. Floors
3. Pillars
Starting with the walls gives you a logical place to begin before you start laying material across the floor.
For standard vapor barrier, work in manageable sections. Roll out a section of material, position it along the wall, cut it to length, and secure it before moving on to the next section.
You don't need to wrestle an enormous sheet of plastic around the entire crawl space at once. That's a great way to turn a relatively simple DIY project into a wrestling match you didn't sign up for.
If you're using an insulated vapor barrier like Insul-Barrier, the installation approach is a little different. You generally attach it as you work your way along the wall.
We've got a more detailed guide specifically for insulated vapor barriers here:
Crawl Space Insulation Vapor Barrier: What It Is and How to Install It
Step 2: Lay Out the Floor
Once the walls are handled, move on to the floor.
This is where planning your runs becomes really important.
Whenever possible, we want to use long runs of vapor barrier rather than cutting the material into a bunch of smaller pieces.
A good approach is to roll the vapor barrier out in a long run, then unfold it across the width of the crawl space.
The goal is simple:
Cover as much area as you can with as few pieces as reasonably possible.
Why? Because every time you add another piece, you've created another seam that eventually needs to be overlapped, secured, and sealed.
So if you can use one large piece instead of three smaller pieces, that's usually going to make your life easier.
Of course, crawl spaces aren't always nice and square. You may have to make cuts around walls, footings, plumbing, or other obstacles. Just keep the overall goal in mind: longer runs, fewer unnecessary seams.
Step 3: Save the Pillars for Last
Pillars are where the crawl space starts getting creative.
You might have a pillar sitting in exactly the spot where you really didn't want a pillar sitting. That's why we typically recommend doing them last.
Get your walls done. Get your main floor sections laid out. Then come back and work around your pillars.
This allows you to handle the large, straightforward areas first and save the more complicated cuts for the end.
Step 4: Overlap and Seal Your Seams
What we do is a 6 inch overlap, Here's where our original “just roll it out” answer gets a little more complicated.
The vapor barrier itself is only part of the system. How you connect your pieces matters too. Use Seam tape to cover your cuts and overlap, Use butyl tape around corners and edges to adhere it until you mechanically fasten
When two sections of vapor barrier meet, they need to be properly overlapped and sealed according to your installation method and the materials you're using.
This is where vapor barrier seam tape and butyl tape come into play.
Crawl Space Ninja® Exclusive Seam Tape | 4" × 180'
Crawl Space Ninja® Double-Sided Foundation Butyl Tape
Don't just put two pieces together and call it good.
The goal is to create a continuous moisture barrier, so your seams, transitions, and penetrations deserve just as much attention as the big pieces of plastic.
Okay, So Why 12-Mil?
Now that you know how to install it, let's talk about what you're actually installing.
You'll see vapor barrier sold in a bunch of different thicknesses, and this is where things can get a little confusing.
A mil is one-thousandth of an inch.
So 6 mil is only 0.006 inches thick. That doesn't sound quite as impressive when you put it that way.
At Crawl Space Ninja, we like 12-mil vapor barrier because it hits a really nice sweet spot for crawl spaces.
It's a solid piece of plastic that's durable, flexible, and practical to install.
And flexibility matters. You're working in tight spaces. You're going around corners. You're working around plumbing and ductwork. You're wrapping material around pillars. You need something that's tough enough to hold up but flexible enough to actually work with
Shop Crawl Space Ninja® 12-Mil Crawl Space Solid Vapor Barrier
But What About 20-Mil?
Here's the important part:
Higher mil doesn't automatically mean better. Most of the time it is a gimmick to make you think, "Wow, it's stronger"
And I'm not saying don't use 20 mil.
A solid 20-mil vapor barrier can be a great, extremely durable material.
What you want to pay attention to is if it is truly that thick or just two smaller mil's glued together
If you're shopping for a higher-mil vapor barrier, make sure you're getting a solid sheet, rather than multiple thinner layers laminated or glued together to reach the advertised thickness.
Why?
Because if moisture gets trapped between those layers, you've created a little moisture sandwich underneath your house. Over time, trapped moisture between layers can contribute to some really nasty odors. The infamous "cat urine smell"
To be clear, this concern applies to multi-layer systems. If you're using a solid 20-mil vapor barrier, that's a completely different situation.
The point isn't simply “12 mil good, 20 mil bad.” It's about making sure it's Solid, durable, and flexible You want a vapor barrier that can take the abuse of installation and continue doing its job without making your crawl space installation unnecessarily difficult.