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Thread: Thermal Barrier at top of XPS wall.

  1. #1
    DIY Junior Member pex006's Avatar
    Join Date
    Jul 2012
    Pittsburgh, Pennsylvania

    Default Thermal Barrier at top of XPS wall.

    Hi I have 2 inch thick XPS panels on my basement wall. The faces are covered with a thermal barrier, but the the 2 inch thick top that is facing the floor joists is exposed. Does this have to covered with a thermal barrier and if so, how is it usually addressed?

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


    It's usual to L-over the top with a strip of rigid foam to the foundation sill, and up the band-joist to the subfloor (sealing the seams & edges with can-foam) with a continuous strip of foam insulation. Foundation sills & band joists are very often the single largest air leak in a house, far exceeding leakage at windows & doors(!), so sealing & insulating there is VERY worthwhile.

    If you do that, installing a minimum of 3" of rock wool batt insulation snugged up to the band joist foam and well fitted to the wood meets spec as a thermal barrier from a fire-code point of view on the foam. Unlike mid-density fiberglass, rock wool batts are sufficiently air retardent to not lose R to convection through the batt itself, but a snug fit at the edges matters. And unlike fiberglass leaving it exposed does not have a friable micro-fiber floating-in-air issue.

    Both of the major box-store chains carry R15 Roxul (3.5") these days. It's easy to trim to shape with a bread-knife.

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