I have an uninsulated, gambrel-roof cabin (kinda like an A-frame but more like a barn with a flat-roof section on top) in a cold CA zone (lows in the 20's). I want to preserve the look of the interior's exposed rafters. I considered insulating from the inside with fiberglass and sheetrock between the rafters, but the 4x6 rafters don't afford much depth for insulation plus ventilation space.
Also how would I ventilate each cavity? The roof style doesn't allow for a continuous vent like a ridge vent.
Maybe it would be better to insulate from the outside with foam panels and replace the aging steep roof at the same time. Or both?
Any help is greatly appreciated

Google "Homasote" for their web site, and look for their composite panel with Homasote mayerial bonded to isocyanurate foam. You can apply it on the outside, it's nailable, and you can get around R-20 with a reasonable thickness.