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Foundation insulation for Vermont

john_campbell | Posted in General Questions on

I am designing my own home in north central Vermont.  I planning on insulating my stem walls, down to frost line and upto 18″ above grade, with EPS foam board to R20, from Performance building supply in Portland, Maine. The exposed foam will be covered by aluminum flashing and back filled to around 14″ exposed.  I’d like to have a finished slab for the first floor of the house. My question is do I need to insulate under the slab as well? house is wood heat w/ propane back up.

There will be full french drain surrounding the foundation walls.

i have worked as a carpenter for a number of years but haven’t had to deal with designing foundations. I’m open to other suggestions but I am trying to keep costs down and details straightforward to do as much as possible by myself.



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  1. Expert Member
    Dana Dorsett | | #1

    For a slab-on-grade in climate zone 6 the IRC calls out a minimum of R10 down to a minimum depth of 4' below grade, independent of the local frost depth. The insulation can be on either side of the stem wall (or split interior/exterior)

    The IRC calls out a minimum of R15 continuous insulation all the way down to the slab, not the frost line. That too can be either interior, exterior, or both.


    Assuming slab on grade pouring the stem walls in a 2.5" + 2.5" EPS insulated concrete form may be easier to deal with than 4.5-5" of foam only on the exterior. If there is going to be continuous insulating sheathing on the above-grade framed walls it's pretty easy to adjust the placement of the bottom plates to keep the foundation foam and wall foam co-planar, with EPDM (not the much more conductive aluminum) Z-flashing at the transition.

    The slab can simply be floated, using the stemwall EPS as the expansion joint.

    Regarding sub-slab foam, take a look at Table-2, p10 of this document:

    The zone 6 row indicates that as much as R10 of sub-slab foam would usually be cost effective on a lifecycle basis. Note that the other R-values are "whole-assembly R", not center-cavity R, factoring in the thermal bridging of the framing, and the R-values of the sheathing & siding, wallboard, interior & exterior air films, etc. Eg, the suggested zone 6 R35 "whole-wall" wall could be roughly a 2x6/R23 24" o.c. rock wool studwall plus 3" of continuous foil faced exterior polyiso insulating sheathing (which would be ~R40 at center-cavity) but there are other wall assemblies that can hit that mark.

    Going with USED EPS or XPS under the slab can be a lot cheaper (and a lot greener) than virgin stock EPS. There are a few vendors of used & factory seconds foam in VT, some of which advertise here:

    Used EPS & XPS is typically about 1/4 - 1/3 the cost of virgin stock goods. While XPS loses some performance over time as it's blowing agents diffuse out to wreak havoc on the climate, even at full depletion it delivers R4.2/inch, the same as EPS of similar density. EPS performance is stable over time. A lot of used EPS is 1.25lbs density "Type-VIII" EPS, which has a somewhat lower compression rating, but it's still WAY more load capacity that would be needed under a residential slab (but not usually enough to be under the footing of a stemwall, which would have to be specified by an engineer.)

    While polyisocyanurate foam (used or new) is fine for above-grade insulation, don't use it in contact with the ground. Polyiso can potentially wick and retain ground moisture, whereas EPS & XPS will eventually dry out when the tide goes out, even if temporarily submerged.

  2. Expert Member
    Peter Engle | | #2

    +1 on the suggestion for using insulated concrete forms (ICF). They stack like Lego blocks and make your stemwalls much more DIY friendly.

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