Research your code and recommended energy code and guidlines. R 20-R28
are substandard and inneficient. Your ceiling is where most your heat
loss is. R 35 in Zone 5 is considered not optimal, R 60 is. I dont know
your Zone but going to R 100 is getting more common in cold areas. You
need an airspace but I dought you need 2", 1" should do, Foam chutes are
cheap and easy to install. Furring out 2" is a good idea. For the best
performance spray on foam, there are types with R7 per " , twice that of
fiberglass. Also Fiberglass looses efectivness at extreme cold, maybe
near 50-60% at below zero temps. A vapor barrier is needed on
fiberglass, not foam, and inside next to the sheeting. 6.5" of closed
cell blown in foam is R 45, it may impress you but its still not an
optimal R value. Research, www.Energystar.com will help. Insulation
manufacturers Dow, Owens Corning etc will recommend maybe R60. I will
bet your Govs green sites might be even higher, R 70++, I dought your
code is under R35 unless you are on a coast with completly moderate
temps, but in canada? What you do now will be it, so if you can do it
go for the maximun as it will have a big payback and increase your
houses value.