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Has anyone used a product like this yourself, and if so, what was your experience? (sh.itjust.works)
submitted 1 week ago by to c/loadbearingwisdom@sh.itjust.works
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cross-posted from: https://sh.itjust.works/post/69077976

It’s $545 for 36 cans on Amazon.

I need to insulate and seal my basement ceiling and rim joist bays.

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[–] 2 points 1 week ago (4 children)

How thick did you go for that? I need to do that before winter.

Cheaper alternatives?

My understanding is that it's best to use spray foam here because it's an outside surface you're insulating from inside, so moisture will condense there unless you seal it.

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  • [–] 3 points 1 week ago (3 children)

    As long as there's something acting as a vapour barrier you'll avoid condensation, it doesn't need to be spray foam. You want vapour barrier in the warm third (by R value, not necessarily thickness) of the wall to avoid condensation. So hot places that mostly run AC will have their vapour barrier to the outside off the wall, where cooler places you tend to heat is the opposite.

    You can do the joist ends with batts and poly, it's just really hard to get a good seal especially with I-joists. I think the foam stuff is R5 per inch? Might be a bit better than that but was targeting R20 so 4" inches thick, maybe a bit thinner if it's R7 per inch. R value is a measure of how well something resists heat transfer btw, higher number is better.

    Check how things are built in your area as it can be very regional depending on climate and building codes.

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  • [–] 1 point 1 week ago (2 children)

    Thanks.

    Yeah we're on the edge of zone 5/6, so fairly cold in the winter, we do more heating than cooling, and a high r value is desirable.

    I did some research after posting my question, and it seems like a cheap way to get sealed and high r value is to do what you said and cut poly to fit, and then seal the edges with spray foam. But then instead of doing more layers of that, just put a few inches of unfaced batting on top to boost the r value. I'll probably go that route, do the whole basement for $50-75 instead of $500.

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  • [–] 2 points 1 week ago (1 child)

    You'll probably want the batts in first before the poly to get the vapour barrier on the warm side, but yeah spray foam isn't cheap so it's best used for spots you can't insulate conventionally.

    There's the added benefit of making future work easier if you don't need to chip out a bunch of foam to run a new wire for example.

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  • [–] 1 point 1 week ago

    Hmmm, I see the logic of warm side cold side and condensation. I just can't get my head around the idea of creating a type of vapor barrier and potentially sealing moisture between the joists and outside plate like that.

    Not saying you're wrong, just thinking out loud. Thank you for your input!

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