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Foundation Insulation and Sill Overhang

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  • #16
    Re: Foundation Insulation and Sill Overhang

    I'm very much a newbie and should probably keep my mouth shut, but that's never stopped me before. . .

    Why not upgrade the sill to 3X material? Increasing the thickness 1" would allow you to cantilever the plate an additional 1/2", no?


    • #17
      Re: Foundation Insulation and Sill Overhang

      Originally posted by rautyasu View Post
      Our inspectors (local and state) are holding us to a 3/4" to 1" maximum overhang for the sill over the foam.
      Why overhang the mudsill?

      Keep the sill tight to the foundation edge, cantilever the perpendicular joists 1.5"-2" to cover the foam, keep the parallel joists aligned with the sill and fur out top and bottom chords the foam thickness, install foam to cover mud sill (helps seal the sill-foundation joint from air, and offers nailing for foam at sill).

      I finish exterior foundation foam with surface-bonding cement mixed with some acrylic mortar fortifier.

      Question about your cross-section: why, in Wiscosin's climate, don't you use energy trusses to get full insulation depth to exterior wall line?
      Robert Riversong
      Master HouseWright


      • #18
        Re: Foundation Insulation and Sill Overhang

        Sorry I haven't been back to this thread. Do I need to tweek my preferences to get an e-mail when there are replies out here. I hate to post and run.

        Anyway, I thought about the water table style. I ended up using a hidden 'water table' or 'termite table' style with the foundation foam and the wall foam aligned at the surface but and aluminum flashing inbetween and an MCA sill.

        Some debate over how fast this is going to rot, but as long as the chinese are gobbling up copper faster than it gets turned out and we won't permit any new mines (e.g the Crown Butte mine the environmentalists stopped because it was 'near' Yellowstone that everyone called a gold mine, was actually a copper mining project with a gold credit -- but I digress) I figured I should test some less expensive systems on some minor projects and check how it fares over the next decade.

        Various problems including condensation on the colder aluminum will certainly challenge it. But the termimesh recommended in other threads isn't available for purchase as such and so I can't really compare the costs or toalk to other contractors installing it.

        I did consider stainless steel sheet but decided to try brown aluminum and see how badly it goes.

        I suppose maybe the best alternative is just make a "termite/outsulation table" of a wider PT sill and if you're a real glutton for punishment use PT ply for the bottom piece that runs past the bottom of the sill and is exposed for a short distance to the outside of the foundation insulation.

        So that leaves the 1 and 1/2 inches of sill with no outsulation, but maybe that is a small price to pay and a 2x8 isn't that bad of a heat conductor. Still have to come up with a detail for covering the foundation insulation.

        I kind of like the Brown Aluminum, it evokes stained wood and it's only 10" to grade so I was just going to keep it a single piece.

        I think some folks have a parging system kind of like Dryvit. For us down and dirty guys, what about staining Hardibacker or Durock?

        RR - do you use any mesh when you parge foam. Does it stick. Is there a tradename of this surface bonding product.

        Funny, I hadn't thought about overhanging the studs and joists, but that again the question is, what prevents termites getting from the foundation insulation into this overhanging material. This is why I proposed overhanging the mudsill of PT.

        I suppose you could just put a cap of metal on the foam. With your suggested detail you couldn't use a "Z" flash, but your siding or outsulation on the wall would shed water outside so the barrier is for termites not to double as a flashing.

        Maybe a piece of hardibacker on the bottom of the extended studf/joist assembly and that would still leave you and inch or so overlap of foundation foam onto the mudsill, although I've usually found that the foam sits in well on its own if you have a good bit of it buried and I haven't ever attach it, but I like the detail. Although the hardibacker will conduct cold to the outside edge of the mudsill but that is only in a 1/2" or maybe even 1/4" profile.

        Drat, I already cut my foam flush with the top of the foundation so I might not be trying your idea on this one.


        In this case I'm pretty close to


        • #19
          Re: Foundation Insulation and Sill Overhang

          Originally posted by riwiseuse View Post
          Do I need to tweek my preferences to get an e-mail when there are replies out here.
          Click "quick links" above and go to "edit options" where you'll need to set your "default thread subscription mode" to "instant email notification".

          RR - do you use any mesh when you parge foam. Does it stick. Is there a tradename of this surface bonding product.
          Since my mudsill is flush with the outside surface of the foundation foam board, I staple 1/2" hardware cloth (galvanized wire mesh) to the sill and extending over the foam to a few inches below grade, securing to the foam if necessary with fender washers and galvanized screws.

          Surface-bonding cement is fiberglass-reinforced modified 4000 psi cement used to parge dry-stacked CMUs. It typically comes in 50 lb bags. Quickcrete makes Quickwall and Sakrete makes Surface Bonding Cement in gray or white. It has high tensile strength and impact resistance, and is resistive to water absorption.

          I suspect it would stick OK to XPS, but I use the belts & suspenders approach of using wire mesh and additional acrylic modifier to make sure it stays on for the life of the building and does not wick moisture.
          Attached Files
          Robert Riversong
          Master HouseWright