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Thread: Finishing Basement - Insulating Walls and Floors

  1. #16
    Test, Don't Guess! cacher_chick's Avatar
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    Jan 2008
    Land of Cheese


    I have no idea what the permeability of that insulation is. If it is wrapped in poly and tight fit, it seems like the wrong thing to do IMO. Even the wrong paint on the wall can act as a vapor barrier, so some research needs be done before you add anything different to your stack of materials.

  2. #17
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    Quote Originally Posted by Crobinson661 View Post
    Cacher_chick, The poly on the wall is out. I totally agree. I guess two of the questions I am trying to answer is:

    A) Will the XPS allow enough ventilation to not direct any moisture that MAY find its way into the wall up to the sill.

    B) I had a link in my previous reply for John Mansville Comfortherm insulation. Its basically rolls of insulation encapsulated in poly. Now, I HAVE seen several recommendations that say to use fiberglass inside of your studwall, but I don't feel entirely comfortable with that. I know poly is a no-no, but as long as the poly around the fiberglass remains intact, it should be fine, right?

    Thanks again to everyone for the support.
    A> If you have 18" of exposed exterior on foundation you have plenty of capacity for ground moisture to dry toward the exterior.

    B> Any insulation on the interior side of the foam needs to be allowed to dry toward the interior. Air-permeable batts wrapped in plastic WILL eventually take on some moisture, but would take forever to re-dry.

    DougB: As a degreed engineer why not do the math? Just the above-grade portion of the foundation in most homes in MN would represent on the order of 20,000 BTU/hr of heat load at the outside design condition (in Albany NY it would be more like 15K) and a large fraction of an otherwise reasonably insulated home's heating fuel use. In a 965' town house in Albany wedged between 2 other units it's probably half that. Say you have only 60' exterior basement perimeter, with 2' of exposed above grade concrete with U-factor of 1.0, and another foot of band joist with a U-factor of 0.5, at Albany's 99% outside design temp of -2F and an inteior temp of 68F that's a 70F delta-T on 120' of U1 concrete for 8400 BTU/hr plus 60' of U0.5 wood for another 2100 BTU/hr, or 10,500 BTU/hr total. If you insulate that to R20 you're in the weeds at well under 1000 BTU/hr- the place stays warmer & drier, both winter and summer.

    Finishing it without insulation would be a mold hazard (and in some places, a code violation). Also, water sealing the wall from the exterior of does not keep the concrete from wicking up from the footing, which is best allowed to dry toward the interior, especially if you have less than a foot of exposed exterior to dry toward. If you use foil-faced iso, leave the bottom 8-12" either uninsulated, or insulated with unfaced EPS.

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