There is a great article by Joe Lstiburek in the February, 2009 issue of the ASHRAE Journal called "Building in Extreme Cold". He recommends 5 layers of 2" rigid insulation in the roof, 4 layers in the walls and floor.

Take a look:

http://www.av8n.com/physics/img48/PsychrometricChart-SeaLevel-SI.jpg

(Without correcting for altitude, which won't buy you much...) Assuming you keep the peak winter indoor temp & RH below 22C & 30% RH, the dew point occurs at the layer in the insulation corresponding to +4C. (Find the intersection of the 30% curve and the 20C dry-bulb temp, then move horizontally left to the 100% curve, and read the dry-bulb temp there.) The simple model says that's (45+4)/(45+22)=75% of the R-value has to be XPS. That would be R36.6, or 7.5" of XPS, to R13.5 (3.5") of cellulose.

Will the roofing system or inspector allow using ISO board (polyisocyanurate) instead of XPS? Two layers of 3" iso (6" total) gives you ~R40 for somewhat less money than XPS. (It doesn't have the same compression loading capacity of XPS however.) See figure 7 in this document: http://www.airtightinsulation.com/UserFiles/File/building-science.pdf

You'd be adding the rest from below in cellulose, but if foil-faced iso is used you'll need to forgo the interior vapor barrier, leave it on the roof deck as shown. If fiber-faced iso is used, the poly vapor barrier goes on the warm side of the cellulose. The issue is that XPS & foil-faced iso are very vapor-retardent, and if you then use an interior vapor barrier you'll then be trapping moisture in the cellulose layer. Fiber-faced open-cell iso is very permeable- you may be able to put a 2-3" (R13-R20) layer of FIBER-faced iso above the roof deck and do the rest with blown-cellulose between the rafters from below, in combination with a full interior vapor barrier (and no poly between the roof deck and the iso.) You may need to build in a ventilation gap between the top of the iso and the nailer if the roofing is impermeable or stays saturated (snow loaded) often. Furring strips holding down the iso should could provide the gap.

At $1.60/board-foot for 2lb foam there's room for working with some complexity to avoid that expense. (OUCH! Even if that's $1.60CDN it's still 15-20% more that in costs in my neighborhood.)