I'm in the process of getting bids for re-roofing my old 1908 Victorian in St Paul, MN (cold, snowy winters and hot humid summers). The attic was finished by the previous owner, but he didn't do a very good job insulating. I hired pros to add blown cellulose and seal air infiltration at the knee walls and floor joists. However, I doubt the R-Value in the ceiling is very high (fiberglass batts). I'll probably go with a higher end asphalt shingle product and I was wondering if it would be worthwhile to try and add some insulation to the roof deck? The roof is complicated with lots of steep angles and valleys. Any suggestions on the best approach to insulate from above considering my climate? Is it an effective way to add insulation?

It'll be complicated if it has a lot of valleys, but nailbase iso panels can make a HUGE difference. An R20 panel is ~ 3.5-4" thick, and has to be through-screwed into the structural roof deck (or better yet, anchor them to the rafters if you can hit 'em.) For another inch or so you can get up to R25, but 6" screws can be a real PITA to install.

With at least R20 above the structural roof deck the roof deck & rafters will run much warmer during the winter, making it less susceptible to condensation, mold & rot. In an ideal world ALL of the insulation would be air-impermeable (like foam), and live outside the structural wood of timber framed houses, which would all but eliminate humidity related issues. It won't be cheap to install, but if you don't do it now, when?

Thanks for the advice! I've been reading a lot of your older posts on these nailbase iso panels. Would I benefit from the ventilated iso panels or could I stick with the non-ventilated panels? I currently have some icing issues on one corner of the house. I was thinking it would need to add ventilation to the area. That part of the roof is one of the knee-walls which happens to be a cold-room with my A/C air exchanger (high velocity system).

One more question. How common are these nailbase iso panels? I'll be hiring pro installers and obviously want to find somebody who has experience with them.