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Community and Q&A

Close Foam Insulation in Basement with No Sill Gasket

Nate_Houston | Posted in General Questions on

I am in the process of planning out finishing my basement. I am in zone 3A and my basement walls are hollow concrete blocks and the sill plate is non pressure treated wood that sits on top of the concrete block with no capillary break. The sill plate is in good condition but sits below grade on at least half of the basement and completely covers the cavities. The home sits on a sizeable hill with good drainage but it does have efflorescence towards the bottom of the wall all the way around the under ground portion of the wall but has never had a leak that caused any water.

I planned on cleaning the efflorescence off and using a sealer on the block walls then framing the walls 1 inch off of the block walls and then spray foaming the walls with closed cell foam. My concern is this will cause the moisture to move up and rot the sill plate. I do not want to jack up the home being that it is bolted down to add a capillary break. 

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  1. Expert Member


    Unfortunately there is no way someone could provide absolute reassurance that insulating the wall, sill, and rim joist won't cause moisture problems and possible rot. Neither could they say it definitely will. All we know with any certainty is that the insulated assembly is more prone to potential problems than the one left alone.

    So what would I suggest having a low threshold for risk?

    - Insulate the interior of the walls with a layer of foam with a high permeance so it can dry to the interior.
    - Leave the sill exposed. You take a small energy hit, but know it will stay dryer.
    - Protect the areas where the wall framing is below grade on the exterior by covering them with a waterproof membrane.

    I'm interested to see what other posters think.

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