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Adding Rigid Foam Insulation on Top of Existing Vapor Barrier

kvng | Posted in General Questions on

Currently sitting near the SF Bay in climate zone 3. I have a ventilated crawlspace that someone went to the trouble of sealing (up to the rim joist), but didn’t insulate. I’d like to seal and condition the whole thing but all the details I’ve looked at show the insulation behind the poly barrier (against the wall). Am I doomed to pulling the barrier off the walls (with all the attached adhesive, fasteners, etc) and attempting to sandwich foam board behind it? Or, is there some way to layer it on top that won’t create a moisture nightmare?

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  1. brian_wiley | | #1

    Hi Kevin,

    I really don’t think that prevents any problems if the poly is well detailed. See Martin Holladay’s response to a very similar question:

    Additionally, I have the same set up albeit in cz5, and have had no issues with the R-15 EPS on the walls over Stegocrawl liner. Attached is a screen shot of the install details (top version) provided by their engineer.

  2. kvng | | #2

    Thanks Brian, that's heartening. I'm a little curious about the applicability of Martin's answer, as he seems to be mainly reassuring the person about moisture in the concrete.

    "You're worried about the conditions on the outside of the barriers you install. Don't worry. Those conditions are different from the interior air -- that's why you installed barriers. You want the interior air to be good."

    I'm also wondering if having the insulation inside the poly under heating conditions might leave the water-proof barrier cold enough to create condensation from any moisture in the crawlspace. But maybe that would be either minimal or a smaller problem that other things caused by excess moisture in a closed crawlspace.

  3. brian_wiley | | #3

    You're right; Martin's answer didn't address that specifically. I suppose that it was the lack of addressing it that I found interesting as the situation the original poster describes was so similar to your own: 6-mil poly over the dirt floor up the walls to the sill plate, followed by foam board. Maybe I'm assuming too much, but I would think Martin would call that out as being problematic if it were.

    That said, I understand your initial suspicion that it could be a condensing surface, but I'm not sure that the risk is there. For instance, my crawlspace is at 56F right now, with 46% humidity. If I've done my math correctly, the surface of the liner would have to be about 35F to be a condensing surface. While I can't say for certain, I don't think that the surfaces down there get anywhere near that cold.

    That can be further mitigated with controlling the air that reaches those wall surfaces by detailing your foam through taping or other sealing details, or de-humidification to keep your relative humidity low enough to not be an issue.

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