|Not a controller myself either, but to echo what ahramin said, you must be IFR at 18,000ft and above (18,000ft @ current altimeter setting) - This is class A airspace. The only exception is in Northern Domestic Airspace (on the US border this only occurs north of Alaska over the ocean) - the Class A airspace starts at FL270 there. The Designated Airspace Handbook from Nav Canada will tell you the exact boundaries, look at section 1.3 and the maps in the appendix: http://www.navcanada.ca/EN/products-and ... ent_EN.pdf
Below 18,000ft, this page provides a good summary: http://www.ivao.ca/pilot/airspace,classes
Above 12,500ft and up to Class A(defined above - usually 18,000ft) you will either be in Class B (both VFR and IFR permitted, but both require a clearance), or class G (uncontrolled). Class B will be designated if there's any controlled airspace(C, D or E) anywhere directly below, otherwise usually Class G. Class B will be common on most of the southern border, with class G being common on the northern parts of the border (parts of Alaska/Yukon border).
A VFR aircraft entering Canadian Class B (implies 12,500ft+) would be required to get an ATC clearance prior to entry. Doesn't matter whether they enter laterally or vertically.