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Community and Q&A

Basement Insulation

coffeecremer | Posted in Energy Efficiency and Durability on

I am building a new small house in northern PA (climate zone 6A), which will have a full basement constructed of 10″ cast-in-place concrete. We are planning to insulate the interior of the wall with 5″ of mineral wool insulation, and are also debating whether it would be helpful to also insulate the exterior of the basement wall with R-10 XPS from the bottom of the footing up to grade. The walls are dampproofed with Deco-20 sealer and there is a layer of Mel-drain dimpleboard along the outside below grade. Are there any downsides to insulating both sides of a basement wall?  Would it be better to just pick a side and not insulate the exterior below-grade?

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  1. Expert Member


    The problem with using permeable insulation like rockwool on the inside of foundation walls is that moist interior air can move through it, and when it reaches the cold concrete condenses.

    If y0u want to use rockwool, you need to either move it all to the outside, or put on a sufficient amount of exterior insulation that it will keep the interior face of the concrete above the dew point.

    If you do add exterior insulation it needs to be continuous over the whole face of the foundation wall. Terminating it at grade leaves a thermal bridge which largely defeats the efficiency of the exterior batts or foam.

    You may find this blog useful:

    Two small points:

    - In zone 6, don't you need sub-slab insulation?

    - If your interior drain can be located slightly above the exterior one, consider joining the two, rather than rely on a sump.

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