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Community and Q&A

Sealing where sill plate meets concrete block wall.

ddomin4360 | Posted in Energy Efficiency and Durability on

Hey folks. I’m working on planning out the insulation of my basement and something occurs to me. I was going to lay a bead of spray foam where the sill plate and block wall meet.

But I don’t have a sill seal, so my sill sits on my wall directly. I suddenly worried if spraying, even a thin bead might case the plate to retain water and rot or am I overreacting?

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  1. Expert Member
    BILL WICHERS | | #1

    The more you seal and insulate that sill plate, the more risk you have in terms of moisture. Do you have a capillary break on top of the masonry wall, under the sill plate? If you do, you're in pretty good shape. If you don't, you should consider adding one. I like to use thin 1/32" HDPE sheet for retrofit capillary breaks, but you have to jack the house up a little to install it (which isn't as bad as it sounds).

    If your sill plate is open to the air on the exterior, then insulating on the interior is much less risky too. Note that thinner insulation tends to be more vapor open, even for closed cell spray foam, so if you're worried, don't go overboard with insulation. In my own home, I insulated my rim joist with EPS panels held in place with canned foam, and I left the edge of the asphaltic fiberboard "gasket" between the sill and masonry to allow for rapid drying. I did seal all the air leaks though.


    1. ddomin4360 | | #2

      Thanks Bill. I have no capillary break and a double stacked untreated 2x6 sill plate. Presumably jacking up the house is not a DIY job right? lol

      1. Expert Member
        BILL WICHERS | | #3

        Actually, it IS a DIY-able job. You need a moveable header (I use a sistered pair of 2x6s about 6 feet long), and a pair of bottle jacks. You work in stages to get down the length of each wall. It's tedious and slow going, but not particularly difficult to do. Since you only need to open a gap of around 1/8" or so to slide in a thin strip of HDPE as a capillary break, you don't really have much risk of damaging the structure.


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