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Thread: Basement Insulation Options

  1. #16
    DIY Junior Member Yankee1423's Avatar
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    I was putting up drywall in the basement this weekend and took a quick look behind the insulation on the exterior wall before putting the gypsum up. In the above grade area of the basement wall, I put in 1.5" XPS between stud bays and then spray foamed the edges. I then put in unfaced batts to fill in the remaining stud space. Removing the studs here was not an option. The temps have dropped to the single digits and when I pulled the unfaced batts back, I noticed some condensation on the interior side of the XPS. In one spot it was frosting on the XPS. I used a ton of the Great Stuff to seal all the edges completely and then some. Is this going to be an issue once I put the drywall on?
    Last edited by Yankee1423; 12-06-2010 at 08:48 AM.

  2. #17
    In the trades Dana's Avatar
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    Quote Originally Posted by Yankee1423 View Post
    I was putting up drywall in the basement this weekend and took a quick look behind the insulation on the exterior wall before putting the gypsum up. In the above grade area of the basement wall, I put in 1.5" XPS between stud bays and then spray foamed the edges. I then put in unfaced batts to fill in the remaining stud space. Removing the studs here was not an option. The temps have dropped to the single digits and when I pulled the unfaced batts back, I noticed some condensation on the interior side of the XPS. In one spot it was frosting on the XPS. I used a ton of the Great Stuff to seal all the edges completely and then some. Is this going to be an issue once I put the drywall on?
    Air leakage transports 500-1000x more moisture than vapor-permeability through unpainted wallboard. With unfaced batts and no interior-side air barrier a LOT of room air will convect through the fiberglass and condense on the cool XPS. Without an air-barrier copious condensation on the XPS would be expected during single-digit outdoor coolth.

    Once you have the drywall up the amount of convecting room air reaching the XPS will go down by 90% or more. If you seal & caulk every seam, edge, and plumbing/electrical penetration it'll be something like a 99%+ reduction. The better the air-seal, the less condensation potential there is- and it becomes only a matter of the water-vapor permeability of the wallboard & paint.

    Keeping the wallboard at least semi vapor-permeable will allow any condensation or ground moisture to dry toward the interior when temperature & humidity are favorable. Standard latex paint will reduce the vapor permeabilty of the wallboard from about 50 perms to about 2-5 perms- which is plenty of drying capacity, but allows less than 0.01% of the amount moisture to reach the XPS that your unfaced-no-air-barrier fiberglass lets in.

    Make it air tight on the wallboard, and it'll always be dry.

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