Q: Will 2 inches of foam be enough to insulate a cabin roof? My place is in the eastern townships of Quebec, Canada so we get serious winter with temperatures down below -30ºC (-22ºF). I want to leave the underside of the roof with visible and exposed rafters and roof boards inside, so my plan is to put the foam on top of the roof.
A: Two inches of rigid foam will deliver a surprising amount of real-world insulation, but numerically it’s far lower than the R value required by code. I have a cabin on my property (that’s the roof being built above) and it has 2 inches of extruded poly foam on top of 1 1/2” thick roof boards. I built it this way for the same reason you have in mind, so I could see the rafters and roof boards from below. The cabin is almost as small as yours, it’s lived in year-round and in practice it doesn’t matter that the insulation is not up to modern standards. We burn a bit more wood, but it’s not significant because the roof is so small. In your case an A frame has a very large roof area because it’s so steep. I’d definitely go with more insulation than just 2 inches in your case, but there’s something to keep in mind. The only problem with putting more insulation on top of the roof is that it can make the roof look quite thick from the edges. I’d recommend experimenting with different thicknesses of foam in a small space near the eaves. See how things look before you make up your mind.
Watch a video tutorial I made a while back explaining how I install insulation on top of roofs when I want to retain view of the roof structure underneath.