said by Msradell:"Hot Deck" insulation systems are becoming quite normal these days especially when using spray foam insulation.
Interesting, our house is timber framed. Shell is stressed skin panels with 3.5" of Isocyanurate foam insulation (R-30).
The house faces due South we have always had ice dam problems in the North facing roof. When we re-shingled a couple of years ago added a ventilated roof deck on the North face. That has dramatically reduced ice buildup.
Even with high level of insulation melting snow will tend to freeze when it hits the cold drip edge at night.
If the OP decides to add rigid insulation to the roof deck he is going to need to add a deck over the insulation. Adding some strapping to raise the new deck off the insulation is cheap.
/tom