Best Way To Attach Foam Board and Vapor Barrier to Foundation

We’ve got another question from Chuck down in Georgia, and he was wanting to know what method we recommend to adhere foam board and vapor barrier to your block walls.

Crawl Space Foam Board and Vapor Barrier

Typically what we use is a fastener with our Hilti guns. You can also use a Christmas tree fastener for vapor barrier, or you can even get a ram set fastener if you want to mechanically attach.

If you’re against mechanically attaching foam board and vapor barrier to the wall because you’re concerned about putting holes in your foundation wall, you could try an adhesive. Our two-sided butyl tape would hold pretty well, typically.

Keep in mind that any adhesive is going to fail most cases over time. I have seen crawl spaces where they use a polyurethane caulk and it lasts forever, or at least it seems like it lasts forever. But eventually, all polyurethanes, all adhesives we believe will eventually fail. That is why we want to mechanically attach plastic and foam board to the walls every time.

Attaching Vapor Barrier to Foundation Walls

Lastly, don’t tape plastic to cinderblock. The tape doesn’t stick well to it because of loose granules that can fall off easily, and eventually, the tape will fall off as well. I’ve seen people do that time and time again. That’s not recommended at all. Two-sided butyl tape is going to be better than our white seam tape going up on the cinder block. We also recommend using Christmas tree fasteners in addition to the butyl tape to ensure the vapor barrier stays attached much longer.

If you’re wanting to maybe use a spray foam combination mixed with a Christmas tree fastener, check out the video below so that you can watch the Ninjas put up plastic after a spray foam application on a brick wall.

8 thoughts on “Best Way To Attach Foam Board and Vapor Barrier to Foundation”

  1. I read a article that the cat pee comes from the ph levels being to high in the soils and not the reinforced barriers.

  2. Encapsulating a crawl space with cinder block. Concern is, cracking the cinder block when fastening foam boards and vapor barrier. Should I be?

    1. Anytime you shoot a nail in something it can crack but the alternative of adhesives usually fail overtime. Our method of masonry tools, nails are designed to be used on block and concrete.

  3. What happens to butyl tape when you replace the vapor barrier in few years? Is old butyl usable again for the new vapor barrier or we would have to apply a new strip of butyl tape under or over that old butyl tape?

    1. Hi Josh, I don’t recommend using old butyl tape if you find yourself needing to replace the vapor barrier. Butyl tape is not like other tapes but it can still lose its effectiveness due to many reasons (temperature changes, moisture). Old butyl tape and waterproof seam tape should be replaced when installing a new vapor barrier. Hope that helps. Thanks for your comment.

  4. My radon mitigation guy wants to use Ramset nails and term bar to fasten vapor barrier (plus closed cell spray foam on the top) to my crawlspace foundation walls. Sounds like a solid permanent solution to me, but how long will 10-mil sheeting last? And when it ultimately needs replacing, how much more difficult will that be?

    1. If the 10-mil is good quality it should last for many many years and if it needs to be replaced it would be about the same difficulty if it was attached directly to the foundation wall or a pressure treated strip. I have seen many contractors do it the way you are describing and it seems to work very well.

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