Seeking insulation recommendations for converted attic
I’m renovating a converted attic and found that the previous owner has non-standard stud spacing (ranging from 19″ to 24″ between the studs in the walls… I have yet to open the ceiling, but I’m assuming these studs match spacing of the ceiling joists). The current insulation is batts that were installed horizontally instead of vertically (because of the non-standard studs), but they doesn’t seem to be doing the job (they’re probably 40 years old). I’m interested in the best option to insulate the room (walls and ceiling). The walls are only about 3.5′ high, and since it’s a converted attic the ceiling joists support the plywood/shingles for the roof.
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Is this a 1.5 story cape? Where are you located?
It was originally a single story Craftsman bungalow, located in Eastern PA. The attic has been finished into a second story (but since the roof wasn't raised the tallest point in any room upstairs is about 5'8').
What I would do, and what I've done in the past, is to put polyiso across the attic side of those studwalls, and detail it as an air barrier. I hang the sheets sideways drywall style. This gives you extra R value in the form of continuous insulation, so it helps with thermal bridging, and it also gives you an attic side air barrier which the insulation in the wall really needs to perform to spec. I would use at least 1" thick polyiso here (R6), mainly for strength. In my own home, I used 2.5" polyiso (R16). I would then use mineral wool batts in the walls, cutting the batts and installing them sideways as and where necassary to deal with oddball stud spacing.
I would extend the polyiso up past the top plate and tie it into 1/2" polyiso used as an air barrier going up over the top plate but spaced off the roof sheathing 1.5" or so. I would then use loose fill (blown) cellulose over the ceiling of that attic room. Putting the polyiso in as described makes it act as an insulation dam for the cellulose, which makes it easier to install the cellulose so that it covers the top plate, which is important for best performance. You'll want to air seal any penetrations in those top plates as you go too, and canned foam (Greatstuff, etc.) is best for that.
Pay particular attention to the joists in the attic side space too. It's common for those to not be blocked off at the bottom of the wall, which makes them huge air leaks for the home. There are many ways to seal them off, and it often ends up similar to insulating and air sealing a rim joist. I would seal in panels of either rigid foam or cheap plywood/OSB using canned foam here.
It's important to consider air sealing as well as insulating in these types of homes. It's really important to think about air sealing in ALL types of homes, but these "added some living space in the attic" homes are famous for air leaks, so it's worth paying some extra attention when doing energy upgrades.