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Crawlspace foundation with wing insulation as well as vertical exterior insulation?

brownwoodwork | Posted in Energy Efficiency and Durability on

Hi. In northern New York state, climate zone 6. Preparing to build a 20 x 30 second home for a client that will be insulated but not heated for the foreseeable future. Some day it might be used year round.

Crawlspace foundation with footings to frost depth. We are planning to insulate the stem walls on the exterior with 2″ XPS. A slab is planned at level of footing. We are contemplating skipping the sub-slab insulation of 2″ XPS and adding wing horizontal wing insulation instead–in addition to the vertical stem wall insulation.

The thinking is this will assist with drainage away from foundation (footing drains are planned too) and also couple the slab with the relatively warmer ground temperature during the depths of winter when the building will be unheated. The cooler slab might be a benefit in the summer time as well.

Does this make sense? Will there be a big energy penalty if the house becomes used year round at some point?


  1. charlie_sullivan | | #1

    Your design intent is to protect the footings from frost with depth, rather than following the frost-protected shallow foundation insulation requirements, but without the building heated, you could have air temperatures below freezing inside the crawlspace, which would cool the stem walls, slab, and footings, and could potentially freeze the ground below the footings, as well as the ground below the slab, cracking the slab. The earth on the outside of the stem wall might actually be warmer than the air in the crawl, at which point the insulation on the exterior of the stem wall would help allow freezing of the footing rather than helping to prevent it.

    It's probably not as bad as I am describing there, because the interior of the insulated building won't be all the way down at the exterior air temperature, particularly if you have some solar gain and have excellent wall and ceiling insulation, but it would be hard to guarantee that cold air in the crawl wouldn't cause trouble, particularly if you have some stack effect infiltration, pulling cold air into the crawl. So I'd recommend insulating under the slab, and insulating the stem walls inside and out. I don't see an need for wing insulation, unless you think you are cutting it too close with the footer at frost depth rather than well below.

    And there are other reasons to insulate--as you mention for the benefit of future year-round residents, and for the summer, even though the uninsulated slab would give you free cooling, it could also lead to humidity and mold problems in the crawlspace, because the incoming air gets cooled without dehumidification.

    I'd recommend EPS instead of XPS because:
    1) It's cheaper, not only per inch, but also per R-value.
    2) It's much more environmentally friendly. XPS has its bubbles blown with a gas that has more the 1000X higher global warming effect than CO2. EPS is more than 200X less harmful in that way.

    If you want specific guidelines, a good conservative choice would be to follow the frost protected shallow foundation guidelines, making sure you follow the more stringent requirements for unheated buildings. But I would think you could omit the wing insulation called for in them, given your soil depth on the exterior, and just follow the slab and stem wall insulation requirements there.

  2. Expert Member
    Dana Dorsett | | #2

    What he said, and more:

    The cooler slab in summer is often below the dew point of the outdoor air, creating a potential mold issue down there. Put at least an inch of EPS under the slab, continuous with 1.5-2" of interior wall-foam to control that.

    IRC 2012 code-min crawlspace wall insulation in zone 6 is R15 continuous:

    The 2" of exterior XPS is only R10- it doesn't even meet code on day-1, and in 50 years at it leaks it's climate damaging HFC134a into the environment it will have dropped to about R8.5. It's easier and often cheaper to just use minimalist 2" + 2" EPS insulating concrete forms for the stem walls, which comes in at about R16.5-R17. That's only slightly ahead of code min, and a pretty easy DIY for a stemwall height pour.

    If the footings are to the frost depth, there should be very little risk of frost heaving of the foundation even without the wing insulation. With insulation on both sides of the stemwalls and R4 under the slab, the likelihood of a frost heave under the slab is also pretty small, as long as the crawlspace is reasonably air-tight to the exterior, especially if it has an insulated unheated house above it.

    There's a long term rationale for 3" of EPS under the slab, and that would be enough to reasonably design for Net Zero Energy, if the rest of the building envelope was also brought sufficiently above current code-min. But it only takes about 1" to mitigate summertime mold issues in the crawl space.

  3. brownwoodwork | | #3

    Thanks for your thoughts, Charlie and Dana. I understand the concerns you bring up.

    Hope to air seal well with mechanical ventilation and insulate to above code standards in general. We are on 2010 IRC in NYS, so think we are good with the local code to use the R10 the architect specified. Even though code isn't high water mark for insulation. We are trying to balance the current use of the camp with the possible future usage. It may always be a summer place and we have to consider how to spread the budget available today, but it will never be easier to bump up the insulation than when we are building it.

    I don't think the building would be colder than outside at any point during the winter, due to insulation and solar gain. But want to be able to sleep at night having it snowed in on the side of a mountain with the wind gusting and thermometer at -30.

    The interior frost concern during the winter and the condensation/mold issue the rest of the year probably isn't worth trying to save on not insulating the slab. And am also sensitive to the environmental issues with XPS so we will look at EPS--harder to source in our area.

    Thanks again for the thoughtful responses!

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