Basement XPS insulation gaps

Discussion in 'Remodel Forum & Blog' started by drx006, Jun 7, 2012.

  1. drx006

    drx006 New Member

    Messages:
    12
    Location:
    Pennsylvania
    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: Jun 14, 2012
  2. Dana

    Dana In the trades

    Messages:
    2,810
    Location:
    01609
    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?
  3. drx006

    drx006 New Member

    Messages:
    12
    Location:
    Pennsylvania
    Thanks again. Sure glad I posted here! This has been very educational and extremely helpful!
  4. pex006

    pex006 New Member

    Messages:
    27
    Location:
    Pittsburgh, Pennsylvania
    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/residential.aspx?sill-sealer.sill-sealer
    http://www.conservationtechnology.com/building_gaskets.html BG44.gif
  5. Dana

    Dana In the trades

    Messages:
    2,810
    Location:
    01609
    It's fine to sandwich those materials in, if you like- they're quite chemically compatible.
  6. pex006

    pex006 New Member

    Messages:
    27
    Location:
    Pittsburgh, Pennsylvania
    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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