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Thread: Basement XPS insulation gaps

  1. #16
    In the trades Dana's Avatar
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    Sure- most any acoustic sealant type caulk would work, and an low-expansion sealing caulk like Backerseal fills the bill.

    Toxicity of the isocyanates used in 1-part gun-foam are pretty low compared to others in that class, but those few people who have an allergic sensitivity to isocynates probably shouldn't be the persons doing the installation. The total quantity/exposure is pretty low post-installation, given the extremely low surface area involved- nowhere near the same lingering exposure as spraying 2" of 2lb foam as the insulating layer (which is also pretty low.) When used solely for seam sealing, even very modest ventilation rates I'd be a bit surprised if it would be detectable even with a gas spectrometer in an air sample 10 days post-installation.

  2. #17
    DIY Junior Member drx006's Avatar
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    Thanks for the info on the Great Stuff isocyanates. That makes me rethink using it alittle. If I decide to go with the Backerseal, does it matter that it is open cell foam between closed cell XPS foam. I read that open cell foam is more susceptible to moisture.

  3. #18
    In the trades Dana's Avatar
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    Open cell is far more vapor permeable (passes more water as water vapor) than closed cell, but it's still air-impermeable, and is more flexible than closed cell. But in this application it doesn't really matter. Open cell foam is not damaged by liquid water, but it it will take on water when submerged, but it releases that water after flooding events. The OSB nailer face is far more susceptible to water damage than open cell foam. Your primary goal here is air-tightness, not water vapor tightness, since far more moisture can be moved via air-convection than through via vapor diffusion, even the most vapor-permeable of foam.

    And the total amount of cross sectional area of the sealant facing the room is small- no matter what the vapor-drive is, the amount of moisture diffusing through literally ANY sealant would be of no consequence compared to even single square inch of air leakage.

  4. #19
    DIY Junior Member drx006's Avatar
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    I really appreciate you taking the time to answer all my questions? Do you think the benefit/cost of taking down the panels to seal the foams gaps(I have 33 gaps total) is worth any benefit gained or do you think I should just seal the OSB with the fiber mastic you suggested earlier? If it was your basement, what would you do?

  5. #20
    In the trades Dana's Avatar
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    So, with 1/8" wide x 8' tall you have 12 square inches per seam, times 33 seams is 396 square inches, or about 2.75 square feet, maybe half of which is above grade. The entrained air in a sealed gap won't convect much due to the narrowness, so it probably has an effective R value of at least 3 at that point, which is about the same amount of thermal bridging you'd get with 2x4 timbers. Adding all the gaps together is the same thermal briding of about three 2x4s- not a big deal.

    Were it my basement I'd air seal the facers with mastic and plug the top (& bottom, if accessible) with can foam, forcing the tip of nozzle in a half inch or so and squirting until I see it clear the top. I'd also lay a bead of either can foam or acoustic sealant along the seam between the XPS and CMU (where accessible.) Stopping air motion behind the panels and through the panel seams is 100x as important to the thermal and moisture performance as any conducted heat loss or vapor diffusion issues through that ~400 square inches of seam area.

  6. #21
    DIY Junior Member drx006's Avatar
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    Ok thanks. My wife will be glad to hear that. She wants to leave it as it is and seal with Mastic. So the 2 inch deep space doesn't factor in the cubic inches calculation? Just the 1/8 inch gap width x 8 foot height? It's amazing the detail that needs to be addressed rather than just throwing up some walls. Glad I found this site. Thanks again!
    Last edited by drx006; 06-14-2012 at 12:35 PM. Reason: Extra info.

  7. #22
    In the trades Dana's Avatar
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    The volume/cubic inches don't matter. The surface area (square inches) presented to each side of the assembly is what matters most for both heat & moisture transfer. (R-value is 1 divided the U-value, and U-value is denominated BTUs per hour square-foot per degree-F, permeance is denominated in grains per hour per square foot per inch of mercury vapor pressure difference. The volume of the material doesn't matter.)

    If the gaps were wider, lets say an inch or more the model of convection within that mini-cavity gets more complicated and the effective-R would be lower, but in a narrow crack like that it can't convect as much. But if the gaps were that wide they'd be worth filling too, eh?

  8. #23
    DIY Junior Member drx006's Avatar
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    Thanks again. Sure glad I posted here! This has been very educational and extremely helpful!

  9. #24
    DIY Junior Member pex006's Avatar
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    Hi I recently made this post, but was unable to log in under my old user name, but I had another question. I haven't been able to do anything with the gaps yet, but am getting around to it now. I saw a Expanded Polystyrene Sill Sealer at my local hardware store. Could I cut 2 inch strips and place it in the gaps to seal them or would I still need something like a BG44 this at the very front just behind the shiplap? Would it be ok that it is Expanded Polystyrene sandwiched between Extruded Polystyrene? Here are some links to the products.

    http://greenguard.pactiv.com/residen...er.sill-sealer
    http://www.conservationtechnology.co...g_gaskets.html Name:  BG44.gif
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  10. #25
    In the trades Dana's Avatar
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    It's fine to sandwich those materials in, if you like- they're quite chemically compatible.

  11. #26
    DIY Junior Member pex006's Avatar
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    Quote Originally Posted by Dana View Post
    It's fine to sandwich those materials in, if you like- they're quite chemically compatible.
    Thanks. Do you think I should use both or just the sill seal and mastic over the front of the panels?

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