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Community and Q&A

Mfg. home, block skirt insulation

DexterBob | Posted in GBA Pro Help on

I am setting a 25×65 mfg home on a full slab foundation in zone 5+, high desert Cascades. I will be putting in a 3 course cement block “skirt” with mechanical ventilation. I am considering installing 2″ rigid foam panels 24″x48″ attached to the inside of the block skirt. I am looking for heat retention in the winter primarily (-20 on occasion). Will this be worth the effort and expense??

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  1. Expert Member
    Dana Dorsett | | #1

    In your climate and location, why would you ventilate under the skirting? A full ground vapor barrier (EPDM or 10 mil polyethylene) and 1.5-2" of EPS under the slab makes sense, with 2-3" of EPS outside your CMU skirting that extends down to the slab-foam makes sense. Air seal the space as tightly to the house as you can, and put a 5w fan on a duty-cycler to periodically exchange that air with air from inside the house.

    If you put the 2" foam on the interior side of the CMU skirting it needs to have a thermal barrier against igntion or have a fire-rated facer (like some Dow Thermax polyiso). It also means you have to undersize the slab a bit relative to the width of the house to keep the CMU skirting from sticking out too far. You definitely want the skirt-foam to be continuous with the slab foam to minimize the thermal bridging (especially when it's -20F outside!), and maximize the benefits gained from thermal mass of the slab.

    If you put the CMU on the interior side of the skirt foam, you can go wider with the slab, and if it's EPS or XPS you can finish the foam with a fiber reinforced cementicious product such as QuiKrete Foam Coating applied directly to the foam giving it a stucco-like or concrete-like appearance, depending on how artistic you care to get with it. Since it's technically outside of conditioned space, the thermal barrier/fire issues are less stringent too.

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