First thing I would research: do I want a cold attic or a warm attic where my duct-work is located. I do not believe the foam board would be the best idea because of your weather/ climate.
Please consider close-cell spray foam or consult a local insulation company.
Consider what you need? Insulation Thermal Performance Factors!
- A reduction in thermal bridging.
- High or Low density and type of insulation. (recommend high-density polyurethane foam)
- Reduce air leakage and gaps. (foam board will not stop this)
- Area/ mass of the insulation assembly is restricted. (low roof pitch)
- Duct work in un-conditioned space. (not a good idea and will add to increase energy cost's)
- Convection-conduction-radiation-heat transfer should all be address when you are considering the best way to insulate your home.
- No ice bridging/ damning on roofs.
I recommend: spray foam high-density which has an R-value R-6.0 to R-7.0 per inch. Great for warm or cold climates and limited space. In my opinion, this is one of the best ways to insulate the roof. It solves the infiltration and moisture problems. It also provides a warm attic, which is useful for storage and suitable for HVAC equipment and ductwork.
As with wall systems, it costs more, but it works.
Reference: Energy Star.gov, and Residential Building Insulation Systems for Cold Climates by: Joseph T. Kohler, Ph.D., P.E.
For more information:
Read "What is the safest and most effective insulation for our attic crawl space?" a Q&A answered by Ian MacLeod.