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all! For some reason Great Stuff offers all of their cans in Pro style, but not the Big Gap Filler. I have 3"-ish gap between a base plate and a floor joist in the basement that has very little insulation behind it. The reason is I think that the gap between that joist and the foundation wall is basically half an inch. I am guessing it was nearly impossible for the builders to wedge regular fiberglass insulation in there, or they were simply lazy. I think this would work really well insulated with foam. In fact, Big Gap Filler, with its advertised 3" expansion might be the ticket. Alas, it is not made in Pro gun style, which seems quite useful for controlling angle and thickness. Could I simply run multiple layers of the regular stuff, even if it's advertised to only expand to 1"? Am I overthinking it, and I should just liberally ugga dugga the gap filler in there?

Cheers

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  • Are you aware of the high flammability of Great Stuff? Worse, the fumes it generates when burning are the sort to incapacitate people trying to escape the fire. See Sunshine Mine disaster. Might want to do a flame test. Outdoors, not like the guys at Browns Ferry nuclear plant. Or watch post10's flame test, including the surprise at 13:14. youtube.com/watch?v=CdItsso3ur0
    – Harper - Reinstate Monica
    Commented Oct 23, 2023 at 19:12
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    It's possible for spray foam to be better behaved, but not possible in the "comes in a can" stuff. It requires a 2-part epoxy-like mix, which requires either manufacture at a factory, or pro installation like at the front of the video. (and if the mix is botched with too much B-part, you get a toxic hazard problem.)
    – Harper - Reinstate Monica
    Commented Oct 23, 2023 at 19:20
  • Thanks for the warning, Harper! Did not actually know that. I have seen the 2-part stuff at a local Home Depot. By the looks, I'd need a professional to do it right.
    – Zoinks
    Commented Oct 24, 2023 at 14:00
  • Just watched that whole video. Ouch. I might have to rethink my plans. I was going to use rigid board, Sika Rmax Pro, it's a polyiso board, to insulate the base plate of the house accessible from the basement. Might need a new plan. Also going to think about what I want to do with this one cavity though. I do have tons of Rockwool, and it would be neat to figure out how to get it in there. I was thinking of foam as a quick and dirty solution. It seems that it's not meant for that purpose though!
    – Zoinks
    Commented Oct 24, 2023 at 14:13
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    Yes, 2-part can have better fire characteristics, so there's a chance for a pre-manufactured board. As far as 2-part application in-situ, the problem is the B-part will make you very sick. People have had to tear down houses because of a botched mix with too much B-part. Hard to make a 2-part consumer product because you'd need to break an internal vial, somehow assure it is mixed properly, and have to use the whole can in 10 minutes. Customers would screw it up and poison themselves.
    – Harper - Reinstate Monica
    Commented Oct 25, 2023 at 19:55

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It works.

Source: I have liberally applied gap filler inside of gaps larger then it is advertised to fill.

The biggest problem I found was that it doesn't stay put if the gap is larger then it's meant to fill. 1" filler inside of a 3" hole has a tendency to roll down inside of the gap. But if you are just filling an empty cavity with the stuff it works...

That being said, great stuff doesn't have the best R-Value, and cellulose, or a close cell foam insulation would provide better insulation.

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  • All this - personally I'd cut some close cell foam for the bulk of the fill and then use the Great Stuff to finish sealing the gap. They make close cell in half inch.
    – KMJ
    Commented Oct 23, 2023 at 16:47
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    My problem is that it might be tricky getting anything solid in the cavity. The cavity is roughly 7.25"x3" x 8 feet. However, access from the long side is only maybe 1/2" wide. I did discover that I can access it from the back, which could potentially allow putting long strips of foam board in, but there's a number of NM-B lines in front of it! I might have to try to figure something out using Rockwool, but I am not 100% sure how to shimmy it all the way down (I think original builders ran into the exact problem).
    – Zoinks
    Commented Oct 24, 2023 at 14:18
  • I like the sound of this Sika Rmax Pro stuff. It's R-6.5 per inch! However, it seems that perhaps it's not the best in terms of flammability or longevity, because it's open-cell polyiso. What would be your go-to for close cell?
    – Zoinks
    Commented Oct 24, 2023 at 14:22
  • @zoinks I don't have one. sorry... I think sprayman's looks good, but it is pricy for that small of an area (at 200 board feet the cost drops to ~1$ a board foot). But the price for a single can is high.
    – Questor
    Commented Oct 24, 2023 at 14:36
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    I think my idea is pretty bunk at this point. I am realizing that the cavity is, in the end, a pretty sizeable volume, and also that foam isn't a magic bullet. Plus I found that maybe I can get stuff in there. I'm going to test trying to fit Rockwool in there, I think.
    – Zoinks
    Commented Oct 24, 2023 at 14:50

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