Air leaks account for the greatest loss of heating dollars from Canadian homes. These are easily solved by sealing. The greatest conductive heat losses occur through the basement. Canadian basements typically lose more heat than ceilings because attics usually have a little insulation (even Grandpa put wood shavings into the attic) and basements usually have none -- that first little bit of insulation is the most important. The attic, however, should be next on your list for insulation.
An attic is defined as a space between the ceiling of the house and the roof that is large enough for you to move around in -- regardless of the shape of the roof. If you can't get around up there, it is just a "roof space" or sometimes a "compact roof". (search keyword "roof" for the title "ROOF INSULATION")
Attics are the easiest places to insulate. As a result, homeowners and fly-by-night contractors alike have stuffed attics full of all kinds of things and in all kinds of ways -- in too many cases ignoring the air barrier and attic ventilation. Back in the early 1980's, only after thousands of houses had been treated in this manner did the CHIP program (which was responsible for financing most of these nightmares) require that contractors doing the job be certified. Even today there are many untrained and unlicensed insulation installers stuffing all kinds of things in all kinds of manners into unsuspecting attics.
If your attic has been reinsulated at some point in history, and you are not sure how well the job was done, you really should study the overview found on the Nuts & Bolts page of this site to know what you are looking for and then inspect the attic at least twice in the dead of winter: (Keyword Attic: article ATICS) Not all serious problems will necessarily show up on the bedroom ceiling. Dry rot in the roof beams could remain hidden until the roof itself assumes the sunken shape of an old swayed-back horse -- and by then all you can do is shoot it. If you don't find any moisture problems, sleep well. If you do -- get to work!
If you are planning to insulate the attic and you already have moisture problems, additional insulation without additional moisture control will cause even greater problems. Creating a good air barrier is more important for your house and hour heating bill than insulating -- and don't overlook ventilation.