If you don't have much room to work with, and need the best insulation possible, I don't think you can do better than foil-backed foam insulation. It's rated at R-6.5 per inch, so a 2" thick board is R-13. Furthermore, the foil coating gives you a low-E surface that retards radiant heat transmission across air gaps, and the stuff is an excellent vapor and air barrier when sealed in with canned foam. The only downsides I can think of are price (currently about twice the cost of fiberglass batts) and fire issues. It doesn't quite burn on it's own, but I'm sure it burns fine in a wood-framed structure fire, and it gives off some nasty fumes in the process.
Our upstairs ceilings are only framed with 2x4 rafters, so after leaving a gap for ventilation under the sheathing, I've got to do what I can in about 3" of space. I fit sheets of 2" foam insulation board into each bay, fitting it to the slopes and outlines of the framing, and leaving about 1/2" gap all around. The stuff can be cut fairly easily with a big serrated kitchen knife, but you have to be careful to keep the cut from wandering.
 |
| Cut all the angles for a reasonably tight fit |
I hold them in place temporarily with some little wood battens screwed to the framing, and then spray expanding foam all around to seal the cavity totally air tight.
 |
| 2" foam panels installed in the ceiling |
 |
| Wood battens hold sheeting while foam cures |
Then I to go around and trim off any foam that expanded past the face of the framing, which is pretty easy with a thin knife.
 |
| Trim off the excess with a long flexible knife |
 |
Be careful not to overfill with expanding foam - see how it pushed the electrical box down.
I had to carve out the excess and re-seal this one. |
I then ripped a bunch of 1" furring strips, and screwed those on perpendicular to the framing at 16" o.c. This way I don't have to use thicker than 1/2" drywall to span the 24" ceiling joists, and I can add another layer of 1" foam board across the framing, increasing the overall insulation value. Running the second layer perpendicular to the first also increases the overall insulation factor of the roof structure by covering the 2x4 framing (which is only about R-3) with a layer of R-6.5 foam, preventing thermal bridging from the framing to the interior.
 |
| Perpendicular furring strips allow foam insulation to cover framing |
In the end I've got a 4-1/2" thin ceiling / roof assembly that is well ventilated under the roof deck, but still about R-20 and totally sealed against air leakage. Loose insulation materials like fiberglass and cellulose might rate well in an otherwise sealed cavity, but if there's infiltration into the stud bay or of course in a vented attic, I think foam has some serious advantages that aren't apparent when all you look at is the R-numbers.
Tips: Definitely always wear some kind of eye protection, thin nitrile gloves, and a hat when spraying canned foam. It occasionally pops and you don't want this stuff in your eye. It also is a real pain to get off your hands, and I guarantee you'll get it in your hair if you're working on a ceiling.
