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Basement slab details – where does insulation and poly go when you have lally columns?

user-7523812 | Posted in General Questions on

We’re pouring a new foundation under our 1850s farmhouse in Vermont.  Before pouring the slab, we’ll be installing 5″ of EPS and 15 mil poly (Stego wrap).  However, because this is an existing house and we’re not redoing all the support beams in the basement ceiling, we’ll have at least six lally columns in the basement.   We need to build footers for the lally columns.  My GC would like to skip the insulation and poly at the footer locations.  This seems like a lot of area not to have insulation and poly.    What should I do with the insulation and poly at the lally column footers?

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  1. Expert Member
    Malcolm Taylor | | #1

    User ... 812,

    - Pour the concrete pads so the top of them is flush with the surface of the compacted fill you are using under the insulation.
    - Set the metal posts.
    - Run the rigid insulation over everything.
    - Cover with poly.
    - Pour your slab.

  2. Expert Member
    Michael Maines | | #2

    I usually do it the same way as Malcolm; here's a drawing that includes a second layer of foam but the important layer is the one above the footings. On some projects I don't insist on having the foam run over the top of the footings; the heat loss is pretty minor, but if you're planning ahead you might as well save a few Btus. On Passive House and similarly efficient projects the foam needs to run under the footings (in a cold climate).

  3. user-7523812 | | #3

    Michael, are you putting the poly above the footer? One of our challenges is that when they set the house down it can creep a little. They'd rather not set the lally columns in the slab ahead of time, in case it is in the wrong place or it turns out they need more lally columns than were originally planned. They are considering making the footer a strip that will handle multiple lally columns, instead of just one lally column. This would allow them to add more lally columns, as needed, and tweak the location of the lally columns. However, it also means we'd have more slab without insulation and poly.

    1. Expert Member
      Malcolm Taylor | | #4

      User... 812,

      If the poly and foam are above the footings, whether they pads or strips, why do you end up with any areas without poly and insulation?

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