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Floor insulation above crawl space

alawishy | Posted in Energy Efficiency and Durability on


I have cut n cobbled 3.25 inches of foil faced polyiso between floor joists that sit above an earthen crawl space. The foam is protected from the bottom with PT plywood. The foam is cut loose fit and sealed with great stuff. The existing wood framed floor also needed to be raised to the level of the slab floor on the other half of the room. I accomplished this by sort of building a mooney wall on the floor. That is, I ran 2×4 “joists’ perpendicular to the old floor. I then insulated between these with roxul. I have not yet installed the floor sheathing. I have started to worry about condensation/rot within this new assembly. I am in seacoast NH. Zone 5 close to zone 4. The crawlspace has a poly vapor barrier on the ground but this is not sealed with tape, goo, etc. I am thinking that the success/failure of this depends on how well the foam layer keeps the cold away from the fiber, especially the great stuff air seal being sufficient and lasting. Would it make any sense to add a smart vapor retarder above the roxul? Any other suggestions? Also, the room is not air conditioned directly, in terms of any summer time problems you might be considering.

Thanks for you feedback.

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  1. GBA Editor
    Martin Holladay | | #1

    The success of this assembly depends on airtightness. Ideally, you would have included a continuous layer of rigid foam under the floor joists before installing the PT plywood. A continuous layer of rigid foam is far, far better than cut-and-cobbled foam.

    That said, I wouldn't worry -- as long as the plywood was installed with attention to airtightness. If you forgot to seal the plywood seams, buy a high quality tape like Siga Wigluv and tape the plywood seams.

    You don't need a smart vapor retarder above your Roxul, because your subfloor (OSB or plywood) is already a smart vapor retarder. Don't forget to use construction adhesive when installing your subfloor.

    I know it's a little late for this advice, but I can't resist telling you that in your climate (New Hampshire), sealed conditioned crawl spaces make a lot more sense that vented crawl spaces (especially if you have any plumbing down there). In other words, it might have been better to insulate the crawl space walls rather than the crawl space ceiling. For more information, see Building an Unvented Crawl Space.

  2. Expert Member
    Dana Dorsett | | #2

    It would have performed better if the R15 rock wool were between the old joists, and the R20-ish polyiso was a continuous layer underneath the joists, which would have been less work. But with the 2x4s perpendicular to the joists the thermal bridging is still dramatically reduced, and it'll perform well enough- better than code-min, anyway.

    It's not too late to insulate & seal the crawlspace, and it would reduce the mold risk at the cold edge of the floor joists a bit, but it's not clear if it's really worth it.

  3. alawishy | | #3

    Thank you Martin and Dana. I do not have plumbing or utilities in this space. I would have done as you suggested with the foam under the joists, but it seemed at the time that there was no practical way for me to install it, considering that the original floor joist bottoms sit 12 inches above the ground at a maximum, maybe even less. I did have the entire floor stripped down to the joists at one point, so it would have been possible to work from above/inside instead of from below/outside. I probably could have taken the unvented crawl space route at this time, even if I still couldn't have figured out some ninja trick for getting the foam (and protective sheathing of some sort) underneath the joists. Protective sheathing, because the 'critters' did significantly get to the original insulation that was installed in the floor. One reason for taking on this project to begin with.

    Thanks Again.

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