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Sealing sill plate in old 1908 house basement insulation retrofit

baxt1412 | Posted in General Discussion on November 25, 2023 10:27am

Old 1908 house.  I’m installing interior insulation to the concrete block foundation.  Going to cut and cobble the rim joists.  Should I just keep it simple and use a Big Stretch or other high quality caulking for the sill plate to foundation wall junction?  Or 1 part pro gun spray foam? Froth pak only? Or can I use a high performance tape?  Do I go top of the line like Siga Fentrim or what about 3M all weather flashing tape 8067?  

I’ve got bulk water managed for the foundation wall.  I’ll be using 2″ high density XPS (bought out of convenience for the good price not necessarily needing the high density).  Covering with 7/16th OSB, will seal the seams.  I thought about just doing spray foam for the wall insulation top face all the way up to rim joists but I should have plenty of leftover rigid foam insulation and actually doubling up on it to 4″ of the rim joists should actually get me right about to the surface of the walls after OSB.  Just wasn’t sure how to address the sill plate (no sill gasket back in 1908) to foundation wall joint.

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  1. User avater
    unclemike42 | Nov 25, 2023 03:01pm | #1

    I like the pro foam. You can cut tubing to extend the reach where required. a zip tie will help it hold on better.

    I found that metal ties used to hold batt insulation up between floor joists will also serve to hold cut foam board in place while the sprayed foam solidifies.

    Since you are using foam board for both the inside of the block wall and to cover the outside joist, you can foam it all in place and not worry about leakage between the top of the wall and the floor above.

    You are relying on drying from the outside to handle any moisture migrating up the block wall before it wicks up into the floor framing and wall above.

    Seal up the rest of it. If you are located where it gets cool outside, you should be able to find and fix any air leaks by use of your fingertips and an IR thermometer. Turn on any exhaust fans in the house and run the dryer.

    Keep in mind you need a fire block between the foam and occupied spaces, and periodically along the wall if you plan for any space between the foam board and the block wall.

  2. jameswbush | Nov 25, 2023 03:11pm | #2

    I'd go with a combo of spray foam for gaps and 3M all weather tape for smaller areas. Doubling up on rigid foam for rim joists is a good call.

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