I have a large room in my house that is built over a crawl space. The floor of this room does not have any insulation installed in it. I live in upstate New York so it is my understanding that I should install a vapor barrier on the interior side of the insulation (the warm side). It seems like it would be very difficult to get the vapor barrier up against the floor from below.

Should I skip the vapor barrier? Or is there some other method I could use for this kind of retrofit?

  • 1
    What type of insulation are you thinking about using? I wonder if you get spray foam so it makes an air-tight seal if that would work as a vapor barrier.
    – auujay
    Sep 5, 2011 at 3:10

2 Answers 2


The vapor barrier for a crawl space belongs above the floor joists and below the sub floor. You pretty much have three alternatives here.

1) Remove all the flooring - including the subfloor - install the vapor barrier - and replace the flooring, except you'll pretty much have to destroy the flooring in the process.

2) From underneath, take 6 mil plastic and set it into each space between the floor joists. You can then either wrap the plastic tightly around each floor joist, stapling it in place and then taping over the staples, or you can cut the plastic so that it doesn't wrap the floor joists, and tape each cut edge.

3) Buy spray foam and use it as both the vapor barrier and insulation. For this application you would want the fire rated open cavity foam like what they sell here: http://www.tigerfoam.com/products.php


Super late to the discussion, but for those who are still using this as a reference, the important thing to note is that open cell spray foam insulation is not a vapor barrier. Use closed cell foam if you are intending to create a vapor barrier. Another solution that may be more cost effective would be to cut foil-faced EPS and place to underside of substrate (make sure it's a tight fit, foil face down). Tape edges to joist to ensure a seal and then add insulation. This isn't a 100% vapor seal, but better than no barrier at all. You could also look into liquid applied vapor barriers.

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