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I recently bought a house and am looking at doing improvements and repairs, but unfortunately I don't have a lot of experience or knowledge in such things. We had a home inspector look over the place and he noted the lack of insulation on one side of the skylight shaft and provided the pictures shown here:

skylight shaft with spray insulation

side of skylight shaft with no insulation

I wanted to pass my current plan by some people on here before proceeding...

I was planning on placing batting up against the bare wall (which already seems to have a vapour barrier there against the drywall) and then using vapour barrier sheets to hold it in place by stapling the barrier into the studs visible in the photos. I'd then tape the sheets of vapour barrier together. This is what was suggested to me at the hardware store I bought the materials from. Is it daft, good, acceptable, or something else?

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You do not want a vapor barrier on the cold side of the insulation. Switch your plan to use an air barrier (or an open mesh, since you should have little air movement in the space) and it makes more sense. On the other hand, if you already bought the plastic, just poke it full of holes (but do not poke through to the warm-side vapor barrier.)

But - this will leave you with essentially uninsulated areas at each stud, and compressed insulation near them - particularly galling since they are "flat-ways" (1-1/2" thick.) It would be better to attach ~6" wide boards to the studs (sticking out from the wall), so you have a nearly 6" deep cavity to fill with insulation, and no compression where the plastic or mesh is attached.

Edit post-comments: You could also fill in between the flat studs with 1-1/2" rigid foam cut to fit (and/or sprayfoamed at the edges), and then sheet over the whole thing with 1-1/2 to 2" foam covering the studs (with a bit more sprayfoam sealing any joints or gaps), but your question indicates that you already purchased materials, so I don't want to send you off on a mission to spend more money if it's not really needed. But if you'd rather do that than add boards, it should work fine - especially if you can return the materials you bought.

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  • would it still be considered "uninsulated" at the end of those 6" boards? (though much smaller area than the 4-6" flat side of the boards there)
    – Tim Tisdall
    Commented Dec 16, 2013 at 20:28
  • Using a typical value for wood of 1.1 per inch, it will be about R6.6 for 1/2-3/4 width, rather than the R1.6 for 3-1/2 inches width (those look like "2x4" in the picture) at present. With R19-ish in between, it's "insulated" by most standards. If you'd like to do better than that, you need to move to something that leaves no (or little) exposed wood (the gory mess of foam on the other sides being one example, though hardly a pretty one.) You could run a bead of spray foam (cheap can stuff) over the edge of the board after installing the plastic/mesh.
    – Ecnerwal
    Commented Dec 16, 2013 at 20:47
  • If I installed foam board, would I even need plastic/mesh over top? I got the impression I'd only need to seal the gaps in between the boards with either tape, spray foam, or something similar.
    – Tim Tisdall
    Commented Dec 16, 2013 at 21:16
  • No plastic or mesh needed - foam board you can just nail (or glue) in place. And, as you say, fill any gaps (filling is better than taping, from the insulation point of view, anyway).
    – Ecnerwal
    Commented Dec 16, 2013 at 21:26
  • maybe this is nutty, but what about this: using long nails to penetrate the batting without compressing it and hold it in place. The plastic should stop the batting from just sliding off the nail. Then the batting could go over the studs. Maybe cutting 2" notches in the batting so it can also sit closer to the drywall. (The batting I got is 6")
    – Tim Tisdall
    Commented Dec 16, 2013 at 21:38
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Try filing the gaps between the boards you have with foam board to create an even plane, then cover all at the same thickness with the batt insulation. Tape between the sheets of batt and it should be good.

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