Crawlspace Encapsulation

About: I'll try to fix or build anything.

Crawlspaces are a menace. They're dark, dirty, and humid. Bugs like to live in there. They're vented to the outside air in the summer (and in the winter if you neglect to close your vents) which can be very humid and this defeats the purpose of venting. Mold loves a dark humid environment with plenty of wood to eat. Today, with the help of my lackey friend Sunkicked, I'm going to show you how to put an end to this menace under our feet.

If you've never heard of crawlspace encapsulation a quick internet search will enlighten you... quickly. The basic principle is to completely cover the ground of the crawlspace with a heavy vapor barrier and even run it up the walls to block any moisture that might come in through the concrete blocks.

Step 1: Why?

I was in my crawlspace insulating pipes when I noticed mold on some of the floor joists. Being someone that watches Holmes on Holmes and Holmes Inspection I was immediate scared sh*tless. If you haven't watched one of those shows you can't begin to imagine how much they drill the fear of mold into you. You'll have nightmares. Moldy nightmares. Night mares about mold is what I'm trying to say. So I called a professional cleaning company to come out and have a look. They said it wasn't worth my money to pay them to clean it and that I could deal with it with a little cleaner and a sponge... and a proper mask.

I wanted to make sure there were no future problems and that's when I first read about encapsulation. I didn't know what it might cost though. To give you some idea why I chose to do it myself:

After a woman I work with heard about this she had someone come and do it for her and paid $8,000.

I paid about $300 to do this 30' x 25' space. That's a savings of nearly 97%. Boo. Ya.

I think I made the right choice.

Step 2: Needful Things

Vapor barrier. What the heck kind of vapor barrier? Definitely not that wussy 6 mil plastic you can get from the nearest home improvement store. What the encapsulation pros use is a thick vinyl material. I'm sure that to buy that material new would be exceedingly expensive and reduce your savings. That's why I got used billboard tarps from (actually I think I bought them through, but they were from these people).

This stuff is tough. They are three ply 15 -17 mil vinyl, tear resistant, mildew resistant, waterproof. They are made to last years outdoors exposed to UV and winds, so just imagine how long it will last in the dark under your house. Hell, people use these for pond liners!

And they have cool stuff printed on one side of them. Check out the pictures.

I estimated that I'd use 2 of the 14' x 48' models. They come in many different sizes.

Step 3: More Needful Things (How Awful Would That Movie Have Been?)

Slave labor. The one in orange was an escapee from the local nervous hospital. He called himself Lloyd, said he liked Elvis's hair and moonpies. The other one in grey was his co-escapee. He'd make ridiculous remarks like he invented the question mark and that Lady Gaga was the love child of Madonna and a shetland pony.

Other than that you'll need the usual stuff.
  • A Drill --- one capable of drilling into concrete with a concrete bit
  • Silicone caulk
  • Sharp utility knife
  • Tap Con or any other brand of Masonry screws ---- usually they're blue and come with an appropriate sized drill bit
  • Furring strips --- enough to go completely around the perimeter of your crawlspace; you can pre-drill these every 2 feet to save from doing it in the crawlspace
  • Extention cords
  • Knee pads
  • Dutch courage

Step 4: Doing It

My crawlspace is only accessible from inside the house so it helped to unfold the tarps on the lawn and then refold them to make it easier unrolling them in the crawlspace. Do this to best suit your needs. We folded the 14' side in half and then rolled the tarp up from the 48' end. We then folded this bundle in half making a 7' bundle to carry into the crawlspace. Once inside it was easy to unfold it and roll it out.

If you're up for doing a lot of cutting and going back and forth you can carefully measure your crawlspace, work out a little diagram, and cut it piece by piece.

* Use the drill to make holes in the concrete. Hold up the furring strips and use them as a template for your drilling.

* Apply a bead of silicone along the top of your concrete blocks, this will be a watertight gasket that will keep the humidity rising out of the Earth down under the barrier.

* Arrange the tarps so that once they are screwed in place behind the furring strips they will drape down the wall and onto the ground.

* Spread everything out evenly and cut away excess in as big of pieces as possible. These pieces we ended up using in patching holes where needed.

* You'll want to overlap any cuts by at least 1 foot. In many instances I ended up with 2 feet. You can use HH-66 to seal the joints, but I chose not to. I know that the water line running into the house has been in the ground since the early '60's and that the huge old cast iron drain will need to be replaced so I didn't want to have to cut and patch later. You can use housewrap tape if you wish, but my overlaps were so big that I didn't bother with either.

Step 5: Done

Yeah, done. Like it was so easy and took no time to do. Right. Actually, it wasn't all that bad. I'm very lucky that I have such a spacious crawlspace as well as a lackey friend who will work for cheap booze and a hamburger. If it it were shorter and required lots of belly crawling I'd definitely recommend going the route of careful measuring and cutting. As it was I think me and my helper in orange worked about 3 or 4 hours one day and then I finished up the rest in 2 or 3 hours the next day. Not bad, but keep in mind that it was only about a 30 by 25 space.

Was it worth it?

Oh heck yeah. I was amazed at how warm it kept it in the winter. I put a thermometer with a high and low memory down there during some of our coldest days that winter  single digits and low teens) and the temperature never got below 54 degrees. As an experiment I didn't reopened the vents for a year and haven't noticed any new growth of mold. If that trend continues then after this summer I may seal the vents permanently. I've done other work in the crawlspace and it's completely no problem at all to go in there. So clean and bright. Many times I've gone in there in pajamas because I've remembered a tool or something I'd left behind on the last trip in.

If you have moisture issues, it's definitely worth it. If you think you might have to hide from a massive government plot to kidnap you and experiment on your DNA, it's definitely worth it. If you live in an apartment you probably don't have a crawlspace. So who's house are you in? You're weird.



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    11 Discussions


    3 months ago

    Hate to bring up such an old post - I'm in the process of gathering materials to encapsulate mine as we speak! My biggest difference is I have a stone foundation.

    My real question comes from whether or not you continued to have moisture issues? We have a crawlspace connected to a basement. If I encap the crawl isn't the moisture just going to sit under the barrier and mold?

    1 reply

    Reply 3 months ago

    No, no moisture issues. It's not really a big crawlspace so it never seemed to need air conditioning in it. You're supposed to condition the air in encapsulated crawlspaces in case you weren't aware of that. Or put a dehumidifier in to keep it less than a certain humidity. I'm thinking less than 70% or 60%.

    The moisture will sit around under the barrier, but that's the whole point. It can't get through that barrier. If you have a pretty bad moisture problem you'll probably want to dig around the interior perimeter and put in drainage to a sump pump. Other than that you shouldn't worry about moisture under the barrier unless it's so bad it's pooling and filling up and making the barrier float!


    2 years ago

    I've been told not to take the plastic all the way up to the top of the concrete so that their cones will be visible for any inspection. Was that something you had heard, has your termite inspector said anything about it?


    2 years ago

    Great in theory but there's an oversight. The tarps are made from PVC which is quite toxic. I got a tarp from, but the smell/gas coming off it was nauseating. I ended up Googling PVC and Vinyl toxicity and a new world of plastic to avoid opened up. Too bad, I was stoked about reusing these tarps for my crawlspace.

    1 reply

    Reply 2 years ago

    I just wanted to point out to you that almost all modern plumbing utilizes PVC pipes and there have not been any reports of VC toxicity epidemics thus far. The vinyl chloride by itself is harmful, but I do not think you would be at risk by having pvc lining in your crawlspace, especially considering it has a low half-life and the amount of VC that vents off of PVC is not of any consequence because most harmful concentrations would be long gone by the time it was under your house.


    4 years ago on Introduction

    Living in New England, I have no experience with crawlspaces nor any need for this Instructable, yet I found it interesting and entertaining enough to read the whole thing. Thanks for such a well documented and well written piece! Now I want an excuse to vapor seal some space with reused vinyl tarps...

    1 reply

    Reply 4 years ago on Introduction

    Thanks! It's a good time . . . as long as you have a deep enough crawlspace. Tight ones might not be fun to work in.


    5 years ago on Step 3

    Lady Gaga and a Shetland pony. You just got 5/5 stars for that comment alone! Great Instructable by the way.


    7 years ago on Introduction

    Thanks for this instructable. I'm thinking of doing the same in my house. Can you give us an update? (i.e. did you seal the vents, did you eventually seal the seams, etc.)

    Also, I've heard different things about how to encapsulate the wall. Some say do it your way, putting the plastic up the entire length of the wall and screw with the firring strips. Others say to screw directly to the sill plate, with others saying, "DON'T YOU DARE!!" saying you could get wicking into the sill plate. Aaaannnndddd others still, saying to only bring the plastic to grade level and then put hard foam board on the walls. What is your take on these methods?

    2 replies

    Reply 7 years ago on Introduction

    I haven't opened the vents since I did this and this has been done for several years now (I'd had the pictures for the instructable for a long time and finally got around to putting them online). The temperature in the crawlspace only gets to about the mid-seventies in summer and mid-fifties in winter. Humidity can get as high as 50% or 60%. I think some encapsulation services recommend something in the mid forties, but it really isn't an issue down there. Since cleaning off the minor surface mold prior to doing the encapsulation I've not seen any hint of it returning. No condensation anywhere. It's a good thing to do to your crawlspace.

    I never did seal the seams and it's perfectly fine. Just leave a lot of overlap. At least a foot, two if your budget permits. That way you could easily pull it up and mess with incoming water lines or drains or sumps if the need arises.

    I wouldn't screw to the sill plate. I actually couldn't because there's about a 3 inch piece of galvanized metal sticking out all along the perimeter. I wouldn't be worried about the moisture getting to the sill, but termites. Moisture + Wood = Good Termite Time. Do the furring strip a few inches below your sill. Or glue it up with silicone. I did that in a few small areas and it's still hanging in there perfectly fine.

    If I'd not been such a cheapskate at the time I'd have put the 2" thick foam board all around and then covered it over as well. But the foam should be as good as keeping out moisture as the vinyl. It's your choice.


    Reply 7 years ago on Introduction

    Thanks for the reply. I'm about 95% sure that encapsulating is what I want to do. I'll probably bring the tarp up the wall 6"-12", seal it there and put the foam board around the sides and seal my overlaps. Thanks, also, for pointing towards the billboard tarps, I'll definitely be getting those. It might take a bit to actually get the project going, but I'll try to remember to put something here on how it went.