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Air-Sealing Between Unconditioned Basement and First Floor

woobagoobaa | Posted in General Questions on

Zone 5 Eastern MA.  First floor sub-floor is a collection of Advantech patched with old gapped board planks.  Air leak city.  Basement rat-slab and rubble walls are essentially not insulated.  Plan is to leave them be for now.

Town will require insulation in the basement ceiling.  I’m thinking an air barrier at the sub-floor will further help with heat loss to the basement?  Overkill?  Thanks

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  1. Expert Member
    NICK KEENAN | | #1

    Do you have plumbing or heating in the basement? If so you really should make the basement part of the conditioned part of the house.

    If you have rubble walls I'm thinking it's an old house. When those old houses were built they didn't have a good notion of insulation, and spaces like basements, attics and crawl spaces were kind of inside the house, kind of not. The modern concept is that the building envelope is a sharp line, everything is either inside it or outside it. As Yoda would say, "Do or not do, there is no try."

    To your actual question, if you want to make the floor the boundary of the building envelope, it should be a true exterior assembly, with air barrier, vapor barrier and insulation.

    For rubble foundations it's often best to use closed cell spray foam on the walls and rim joists. I had that done on a house not far from you for about $2700.

  2. woobagoobaa | | #2

    I'm inclined to treat the first floor / basement ceiling as the bottom of my envelope. Lay Membrain atop the subfloor and finish floor over that (for air/vapor barrier). R19 batts under side of the subfloor (thermal layer). No need for water layer here.

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