We have a late 70’s steep pitched a-frame house/cabin.  Off-grid, but fairly substantial capabilities.  The roof (which is pretty much all the wall as well) is 2×6 T&G on 4×8 on 4’oc.  On top of the T&G is asphalt paper and then very worn asphalt shingles.  There were many 3×5′ plexi windows, which were hardly windows, just plexi screwed to the openings with edges lapped with shingles.  Replaced most of these with 2×6 curbs and fixed Velux skylights.  These were rated for 60deg pitch and ours is 62deg.  Used velux flashing kits, which allow for 2×6 (or 2×4, if you snap off some of the flashing) curbs.  We used 2×6 to allow for adding 2″ of insulation in the future.  Note that there is no insulation currently in the roof/ceiling.  The end-walls have some, but that that I have exposed, is pretty trashed from mice.  The curbs are actually rough lumber, so closer to 6″ tall than 5.5″.

The climate is very mild coastal California, banana belt that doesn’t freeze. Officially zone 3 but the on-site weather records have it fit in zone 2.  Wood heating has been more than adequate in winter, but some of these recent summer spells were fairly miserable.  33seer 12kbtu minisplit only takes the edge off until about 2pm.  Then we use a large in line duct blower to exhaust near the ridge and open up doors and windows.  The roof is in bad shape, other than the re-shingled areas around the newer skylights.

The plan is to use 2” of rigid foam and re-flash the curbs with the existing kits, but snapping off 2″.  The should be just enough clearance for 2″ foam and 1/2″ osb or 1x purlins (if metal roof).  Was hoping to use 26ga exposed-fastener metal roofing.  Flashing the curbs for this type of roofing has been a real head-scratcher, with velux only offering kits for shingles.  Their metal roof flash kits are only for deck-mount skylights.  Anyways, the main questions here are:

1. Polyiso or Comfortboard?  I read in the “Insulating on the outside of a cathedral ceiling”  article and comments, and at least in 2019/2020, it was mentioned that the rigid mineral wool was hard to come by in smaller quantities.  Is this still the case?  Home depot has it on their site, but I’ve never seen it in stores.  Wanted to go rockwool board for the fire aspect.

2. How do the air barrier/vapor barrier/underlayment concerns in those articles change when the rigid insulation is vapor permeable?  We are not looking to insulate under the T&G as the place is already small, and wanted to retain the exposed look, and not encroach on interior space.  Cannot go with more than 2″ outside, as the skylight curbs will not allow it.

3. Does this make sense?:  T&G, air barrier (taped underlayment?), 2″ rigid mineral wool, standard synthetic underlayment, 1×4 purlins, ribbed roofing panels.  Seems like ice and water/peel and stick is totally overkill for this climate.  If using purlins, what about using osb for several inches around the curbs, in order to have something rigid for the peel and stick flashing tape to stick to?

4. Is my want for mineral wool over polyiso unfounded?  I see it used on flat commercial roofs and certainly on residential walls.

I know 2″ isn’t much, but it’s a LOT better than what is there now.