If you were faced with the following cantilevered bay, how would you go about insulating it? I can seal most of the wood air dams between joists with Great Stuff, but as you can see, there are two joists with no air dams - the gap continues all the way back between the first and second story.

  1. Given the existing ducting and plumbing in these areas, is there any way to get an effective seal?

  2. Once the joist gaps are sealed as much as possible, anything else? Should I put batting between the joists against the cover? The cover in this case will be caulked plywood.


problem 1

problem 2

  • 2
    I'll be interested to see responses. To me this feels like a frozen/burst pipe waiting to happen, unless you're in a warm climate. – keshlam May 3 '15 at 15:21

First I would caulk or spray foam (as appropriate) all the penetrations in the rim joists to air seal the area.

Then I would fill the stud cavities with mineral wool batts, cutting holes and channels in them to accommodate all those utilities. This is practically impossible to do correctly with fiberglass but easy with mineral wool due to its density and rigidity; you just cut it with a bread knife.

Then I would cover the bottom up with rigid foam board. 2"+ would be good. Finally I would cover that up with cementboard or another rigid, impervious material.

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  • This is essentially what I did, though I used fiberglass instead of mineral wool. I filled the second through-hole with a couple pieces of rigid foam board with Great Stuff to fill the gaps. The first through-hole I just filled with Great Stuff for Large Openings (black can). Not ideal, but hopefully it will do the trick. I then filled the cavities with fiberglass and covered the whole thing with 3/8" plywood, caulking all seams. – user2446 May 4 '15 at 7:01

I'd first be concerned with freezing pipes. If that's not an issue in your climate I'd pay to have the whole thing spray foamed. That's pricy but a good R value and prevents air leakage. Otherwise I'd use rigid foam and try to fill all the spaces and voids, with Great Stuff to "glue" it in place. That would be cheaper than spray foam but a lot more work as well.

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