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Community and Q&A

Foundation red flags?

thrifttrust | Posted in Green Building Techniques on

I’m planning a high performance retirement home in Detroit (Zone 5). The house will be a 26′ X 40′ single story rectangle with a basement. I have extensive remodeling experience, but I have never built a foundation.

After water control and air sealing, thermal bridging seems the most important consideration. Typically, the concrete guys come in three stages: footing, walls, then slab. I rarely find insulated footings described. Besides tradition, an impediment seems to be the necessity of installing a level and compacted gravel base under the footing insulation and another under the slab. Considering the extra steps, does it make sense to install an insulated monolithic slab/footing?

One concern may be the weight of concrete walls loading the perimeter? Which brings me to another crazy idea, permanent wood foundation walls. Online I see a lot of dismissive comments, but few accounts of actual failure. My feeling is that the care and detailing of a high performance house can mitigate potential problems.

Peel and stick waterproofing membrane would be applied to the PWF sheathing extending to the base of the slab insulation, on the way connecting to the poly under slab membrane. Two inches of Roxul from the top plate down would provide continuous insulation, drainage and relief from hydrostatic pressure. This would be bolstered by two inches of XPS from the top plate to 6″ below grade to support synthetic stucco foundation cladding and line up with four inches of above grade rigid foam. With cavity insulation, the wall assembly would be economical and have an R value north of 25 below grade and 35 above.

Does this make sense?

Doug

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Doug,
    Builders have decades of experience building Permanent Wood Foundations (PWFs), and they work. Your instinct about insulation placement -- that continuous exterior insulation is preferable to fibrous insulation between the pressure-treated studs -- is a good instinct.

    Since this is a green building web site, I am duty-bound to tell you that green builders try to avoid the use of XPS, because XPS is manufactured with a blowing agent that has a high global warming potential. For more information, see Choosing Rigid Foam.

    I don't think that the prejudice against wood foundations is going to disappear any time soon, so homeowners concerned about resale value will probably continue to specify concrete foundations. Resale value may not matter to you, however.

  2. Malcolm Taylor | | #2

    Doug,

    Choosing a PWF may make sense, but I don't think the reasons you give for leaning that way inevitably lead to using one. Concrete foundations are commonly used in high performance houses and detailing them so that the lack of insulation under the footings isn't problem is pretty routine.

    My own feeling is that being uncommon PWFs represent another layer of complication, when building a high performance house is complicated enough.

  3. thrifttrust | | #3

    Thank you for your thoughtful responses. It seems, like everything, the devil is in the details. I intend to insulate the cavities as well as the exterior. Is this a problem Martin? Won't the R8 external insulation eliminate danger of condensation in the wall? I've considered fibrous cavity insulation but also layered EPS. Although labor intensive, it's less than $1/sq. ft. to fill a 5 1/2 inch cavity to R22. Thoughts? BTW, I only intend to use XPS for the exposed foundation wall. I used it on my current home. I sculpted bullnose corners and covered it with Styro Tuff II foundation coating. 6 years now and it's holding up and has aged well. I haven't seen an EPS product that would carve easily.

    Malcolm, I'm thinking that PWF walls simplify construction, save time and reduce costs. One pour into a form that my crew and I prepare and we're done with concrete. Then we just build a two story frame house. If the footing isn't insulated the foundation insulation has to be inside: increasing complexity, eating floor area and you still end up with a thermal bridge at the top of the foundation wall.

    Indeed, resale will be my children's problem, but I hope the high performance features and extensive documentation of the build process will calm foundation anxiety. In researching PWFs I came upon charming videos of an owner/builder who employed PWF walls in a high performance build. Waterproofing our Permanent Wood Foundation

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Doug,
    Q. "I intend to insulate the cavities as well as the exterior. Is this a problem, Martin?"

    A. No.

    Q. "Won't the R-8 external insulation eliminate danger of condensation in the wall?"

    A. Yes.

  5. thrifttrust | | #5

    Thank you.

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