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XPS rigid foam in basement

Vernon Mcconnell | Posted in General Questions on

hi, I live in toronto, ontario. I want to insulate my basement walls that are poured concrete. They did a horrible job when they poured them so they are not plumb. Does it matter if the xps is not 100% against the wall in certain places.??? im planning on using 1 1/2″ foam floor to ceiling with can foam around edges.

thank you.


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  1. Expert Member
    Dana Dorsett | | #1

    It's fine if the foam isn't in contact with the foundation wall everywhere, but it's important to make it air tight to prevent convection around the insulation layer.

    XPS is about the least-green solution here, due to the extremely powerful HFC greenhouse gases used for blowing agents (>1000x CO2). R7.5 (1.5") isn't really much insulation for your climate, and that R7.5 eventually falls to R6.3 over time as the blowing agents diffuse out. To hit IRC code minimums for foundation insulation in your climate (the boundary of US DOE zone 5A/6A definitions) it would need to be continuous R15 insulation or it's thermal equivalent. I'm not sure what the NBC or Ontario code calls out.

    If the foam is trapped to the wall with a fiber insulated 2x4 wall you can get there in a fairly mold-safe manner with 1.5" of polyiso (labeled ~R9) or 2" of EPS (labeled ~R8), which is sufficient for dew point control at the foam/fiber boundary at the above-grade section to omit the interior side vapor retarder, allowing the assembly to dry toward the interior. EPS and polyisocyanurate are both blown with pentane, a far more benign blowing agent.

    Alternatively, building the studwall 1.5" away from the fouindation and installing ~1.5" of closed cell spray polyurethane works. Most closed cell foam is still blown with HFC245fa,( about 1000x CO2), but there are now versions blown with HFO1234ze, with very low envirnomental impacts (and a higher R/inch, not that you'd notice). Icynene also has a water blown 2lb foam (MD-R-210) that would work, delivering ~R7.5 @ 1.5".

  2. Vernon Mcconnell | | #2

    Thanks Dana. Couple of questions, 1. If I was going to use the EPS how would I hold it to the wall? Would I use can foam around the edges and tape seams?? 2. I was going to use r14 Roxul in 2x4 wall. Do I need to use a vapour barrier (6ml poly) before drywall. 3. The second method: 1 .5” away from wall (studs), would they just spray foam the wall behind the cavities then I would fill with insulation or they would fill the whole cavity with foam?

  3. Expert Member
    Dana Dorsett | | #3

    Walnut sized blobs of foam board construction adhesive work for securing any foam board to the wall. Can-foaming the top & bottom gaps and using housewrap tape on the seams also works. Due to historical shrinkage issues with polystryrene foams some folks put a thin layer of fiber reinforced duct mastic over the tape for good measure, but I'm not convinced that's really necessary (or more effective than tape alone.) With foil faced polyiso it's better to use high quality foil tapes for the seams.

    Do NOT install 6mil poly vapor barriers behind the drywall, since that creates a moisture trap around the moisture susceptible studs.

    With the spray foam solution they only need to install enough foam so that it fills in completely behind the stud edges, and enough to make the R14 batts a compression fit. Batts are manufactured with a natural loft slightly thicker than the nominal 3.5" stud depth, which is necessary to prevent convection paths around the batts. . A full cavity fill of closed cell foam is both very expensive (both in Looies and environmentally) and a waste of good foam, adding less than R2 to the "whole-wall-R" compared to R14 batts due to the thermal bridging of the studs. Closed cell foam from the slab all the way up and over the top of the foundation & foundation sill, on up the band joist to the subfloor makes it easier to assure a good air seal, since it's essentially seamless.

  4. GBA Editor
    Martin Holladay | | #4

    Dana is giving you good advice. Here is a link to an article with more information to guide you:
    How to Insulate a Basement Wall.

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