How To Install a Moisture Barrier In Your Crawl Space
Updated: Aug 12
So your plumber crawled out from under your house and informed you that you're in need of a moisture barrier, also known as a vapor barrier, or maybe you got up the courage to battle the cob webs and crawl under there yourself and came to the same conclusion. Either way, now you're asking yourself "is this something I can do myself or should I hire a specialist to come out and take care of it for me?" First we'll go over what to look for when determining if: 1) you need a moisture barrier or 2) you need to replace your existing barrier. Then we can go over the process of installing a barrier and you can determine if it is a task you want to handle yourself or hire a professional.
So let's assume since you are reading this that you're not afraid of spiders or other creepy crawlies that may lurk in the dark corners of your crawl space. First things first, you need to get a nice bright flashlight and crawl, or if you're lucky and have a tall crawl space "duck walk", around the majority of the crawl space area.
The first thing you want to look for is the condition of your moisture barrier (if you have one). If the plastic is flaking, disintegrating, or torn up in multiple places, you guessed it.. you need a new one. If the installer of your barrier didn't stake it down properly and you have pest control, plumbers, HVAC, or anyone else crawling around down there, then most likely you will have areas where your barrier is bunched up and soil is now exposed. If it is a fairly new barrier you may be able to get away with straightening it back out to last another few years. I usually tell my customers that a good barrier lasts about 10 years. Though, I have seen 15 year old barriers that still look great and 5 year old barriers that desperately need replaced. Everything depends on how well the install was carried out and how much moisture the barrier is exposed to on a daily basis.
If you found your barrier in prime condition, hurray! Proceed to the next item on your honey-do list. But let's say you have determined that your barrier either needs replaced or you don't
actually have a barrier to begin with. The next step is all about prep work. You now need to locate and remove all of what I like to call, crawl space debris. This can be random pieces of wood, broken or full cinder blocks, plumbing scraps, old HVAC duct work, insulation hanging on the ground, and whatever else you may find laying around the crawl space floor. All this will need to come out prior to laying down a new moisture barrier. These debris need to be removed so that when you lay your barrier, you're not allowing an inevitable tear or puncture from the debris. You're also protecting your crawl space from attracting termites when you remove random wood pieces from the area. I will share a little secret I've learned with removing all this debris, go buy one of those long plastic snow sleds and drag it under there with you. You can load it up and drag it around fairly easily under there with you, drastically reducing the time it takes to remove everything in order to prep a nice, bare surface to lay a quality moisture barrier.
You've braved the eight-legged creatures and cleaned out those random pieces of, "what the heck is this doing down here?"- Congratulations! The prep work is done and like painting, it is the most pain-staking process of it all. Now comes the actual laying of the plastic barrier.
Things you'll need are: A roll of 6mm plastic sheeting big enough to cover the square footage of your crawl space, long landscape stakes, a rubber mallet, tape measure, and a carpet knife.
I'm all about efficiency, so I'll share my tried and true, step-by-step process with you all. First you want to measure your crawl space. I usually buy my plastic in a large 40ftx100ft roll. If you are buying a small roll from the store, just try and do something similar to this process. Say I have an area of the crawl space that is 24ftx40ft with pillars in the center running the forty feet. I would roll out my plastic and use my carpet knife to cut four 6.5ftx40ft sections, which gives you a good 6 inch overlap on each piece. Then while I'm still outside the crawlspace, I roll up the sections so that when I get underneath the house I can just start on one end and roll out the plastic in front of me, staking the edges of the plastic as I go. I stake roughly every ten feet and use a few extra stakes in high traffic areas like around the crawl door. Just an extra tip; I don't like to cut a section of plastic larger than eight feet wide, it just gets difficult to handle. You're better off cutting two 4ft pieces instead. You'll just repeat this process of rolling and staking with each section of cut plastic until you've covered the entire area.
And just like that, you're done! You can count this as your abdominal workout if you've been crawling around. It's truly not too bad of a process depending on how your crawl space is set up. If you have duct work or plumbing close to or on the ground, the process can become a bit trickier. Or if your crawl space is only a foot tall. These are all things you can take into account when you first get under there and examine the area.
Hopefully this has helped you build the confidence needed to tackle this project yourself. If not, I am sure a local specialist can help you out. Either way don't let your crawl space go without a moisture barrier or you could be calling a different kind of specialist to replace rotten floor joists and center beams.
Written by - Lee Sell