I have a very low crawlspace that i finally have access to while the floors have been removed. The crawlspace is vented freely and Former owner attached unfaced polystyrene boards to the perimeter concrete foundation with just fastners. I'm going to encapsulate the crawlspace to seal against vermin, radon, and provide some extra amount of insulation. Humidity and water isn't a worry as we are in Albuquerque.

Are there any problems with running the vapor barrier over the insulation and attach/seal thru/against the foam board and or should i remove the foam board?

I'd rather add additional spray foam or something around the foam board gaps than remove it. I planned to get some extra long fastners so i would drill thru the foam and into the concrete and attach the vapor barrier with those.

I'll also be putting in an active submembrane radon system and a Humidity activated vent fan rather than a dehumidifier because wow those are expensive.


  • 1
    sound like a good plan
    – Traveler
    Commented Dec 10, 2022 at 19:22
  • Thanks! I've been doing a lot of research on the topic especially the radon angle as I'm from the gulf coast and Radon is not something we worry about at all. Commented Dec 10, 2022 at 20:38
  • 1
    We have to worry about it here in Arizona. Meanwhile I gave you some points
    – Traveler
    Commented Dec 10, 2022 at 20:49
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    In your last sentence, did you mean dehumidfier?
    – SteveSh
    Commented Dec 12, 2022 at 11:26
  • Lol yes. I'll chage that thanks! Commented Dec 12, 2022 at 14:55

1 Answer 1


You want the vapor against the source of the moisture. For a crawl space, that would definitely be the floor.

But the walls are a bit trickier. If you expect moisture to migrate into your crawl space from the ground outside the wall, then you want the vapor barrier against the wall - under the insulation. If you expect the moisture to try and migrate out of your crawl space to the ground outside the wall, then you want the vapor barrier on the inside of the insulation.

If you're not sure, or if the answer changes with the season, I would use XPS without any vapor barrier. XPS is semi-permeable and is relatively forgiving if it collects a bit of moisture, which is not the case with fiberglass batts.

  • Thanks I will take that into consideration, didnt know that xps was a bit vapor permeable. Thankfully moisture isn't an issue for us here, bone dry even during the monsoon season. Mostly I'm doing this for radon and rodent prevention with a side of insulation help after sealing up the vents. If the xps is a bit permeable that is kind of good for radon as it provides low volume perimeter path for air for the radon fan. Thanks! Commented Dec 12, 2022 at 15:55

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