Here is my response to a similar question on one of the other boards. I reasearched this a lot, and though I'm not in a cold climate, most of the recommendations would hold for your situation: I had a similar situation. Some genius had put 3 inches of blown fiberglass in the attic (over the knob and tube wiring!) and that's it. The room was hot in the summer and cold in the winter. Here is what I did: - r13 on the floor between the soffits and the back of the kneewall - this was over conditioned space down below (I didn't cover it, but I could have if needed for storage)- rafter vents at the soffit-end of the insulation keep the soffits open - r13 kraft faced insulation on the back of the kneewalls (2x4 wall construction) - 2" isocyanate foam insulation (r14)with reflective surface on the angled portion of the ceiling, leaving 1.5" air space up to the unfinished part of the attic. My rafters were true 2"x6", so I didn't need the fancy channeled insulation. This insulation is R7 per inch, so I got R14 on the slanted portion. - R38 faced insulation in ceiling of the finished space, with eyebrow roof vents to keep air moving through there. - seal up all penetrations (electrical boxes, pipes, etc) with caulk or spray foam) The principles you need to follow are that you should insulate any of the unconditioned space where air from the outside is flowing. Any vapor barrier (e.g. the kraft facing)should face the INTERIOR - don't put insulation then vapor barrier, because then any moisture from the interior will reach the cooler air of the unconditioned space and condense, leading to mold. If you use the foam insulation, you have to cover it with a fire rated material (e.g. sheetrock). You could also consider using the new reflective sheets on the bottom of the roof sheathing - I didn't do this, but apparently it has a huge impact on heat gain. The impact - the upstairs is comfortable all year. It has no dedicated heat or AC, but never gets below 65 during the winter or above 72-73 in the summer. If it gets stuffy on the hottest days, I open a couple of windows to get a cross breeze. Hope that helps.

polyiso is easy to cut, it is the other foam board with less R-value that is so difficult to cut. as it is comprised of tiny styro balls formed to make sheathing & topped on one side with somewhat reflective plastic that quickly dulls when left exposed. these sheathing boards are difficult to cut. little rigidity, and even with sharp razor knife the particles fall apart as cuts are made, having only one backing nothing holds it together. IMO...real pita! in my mind the high R-value(R-6-7 per inch) is closed cell,the low (R-3 per inch) is open cell. as mfg sheets do not define it this way...I hesitate to do so online. but essentially tha is what it is high density & low density foam. just wondering OP, which type and how much per sheathing? here 1" polystyrene is about $20 each. no 2" unless special order. best of luck op.

You need to check with your housing authority - they may have a minimum - R10 or R15. I'm in Minneapolis - zone 4 - I went with a total of R15. Installed top/bottom plate. Bottom plate pressure treated, with tar paper on basement floor - no direct contact with cement. I went with 1.5" x 48" wide XPS (pink stuff) against the wall - no studs yet (this makes a thermal break). Used PL300 to glue it to the wall - don't need a lot - one tube does 3 - 4 - 48" sheets. Then put 2x4's flat, 16" OC (you can get XPS scored 16", use a serrated bread knife to finish the cut), with 1.5" XPS between them - for a total of 3" XPS - R15. Take your time to make sure that the studs will fall in the center of the drywall joints. You may have to add additional 1x2's or 2x4's for the dry wall butt joints. Basement Insulation Basics Just an aside: my 1950's home had the old style basement windows. Replaced them with glass block - a lot more light once you get rid of the window frame and storm window. I found the glass block windows come pre-made. I ripped the windows out, including the frame. Installed the glass block windows with shims. Used a little foam to hold it in place. Next day, removed the shims, and foamed the rest of the opening. I framed around the window, before insulating, so I could do sheet rock 'returns'. Also what gives more light, is if you angle the bottom of the window 3" or 4" rather then make a right angle window sill.

I'll add this then: Durock? It's an approved substrate for any of the surface applied membranes. Sure it generates dust when cut with a power saw or angle grinder. But if you score and snap with a blade? It's pretty clean. It is heavy when compared to foam...but you'll be installing a half-dozen sheets max. Foam board? Sure, it's even lighter in weight. Here's all I recommend. Find the foam you want to use. Choose a membrane, example, Hydroban sheet membrane. Then call Laticrete's tech department and ask what thinset/adhesive they recommend for adhering their membrane to the EPS/XPS/foam of your choice. If you get a solid answer, by all means move onward with the project! If not, repeat for each foam/membrane/adhesive combo you can think of. Honestly, and again...I'm a guy who thoroughly appreciates the DIYer mentality...I built my own houses, boats, kayaks, canoes, airplane, swimming pool, I've drilled my own wells for water, I've done a couple of frame off car resto's...all solo. Trust me, I embrace the DIYer mentality. But showers? Here, on this forum, I do worry about compromised showers. Because houses are the most valuable investment most people on these forums will ever make, and water damage from a shower can just muck things up in tremendous fashion. Plus, it's near impossible to properly repair a shower short of tearing it out and rebuilding it. If your primary concern is weight and dust? Then I'll wave the bugger flag. And I'm waving it in an attempt to help you out. Just buy USG foam board and be done with it. Hydroban sheet is maybe $1.25 a foot? Kerdi about $2. Foam board? Maybe .40 cents. Adhesive? Who knows. Best case, you're saving yourself $1 a sqft by trying to make your own versus buying USG's product. For a shower of your size, that's maybe $75 you're saving. On an expenditure of how much for the entire shower? It's a fairly small percentage. Again...I THOROUGHLY embrace the DIY spirit. But with you being motivated by weight and dust? I'd spend the extra $75 to get the USG foam board. Lightweight. No dust. And you KNOW it's going to work. Plus you'll avoid the pain of having to lug a heavy and dusty bag of thinset home and the pain of trying to thinset or adhere the membrane of your choice to the foam of your choice with the adhesive of your choice on your own. Just my opinion. Worth the price paid. Good luck!! Seriously. Whichever way to proceed, I'd love to hear how things turn out.