Roof deck over Garage – To insulate the underside or not… – GreenBuildingAdvisor

I’m in the middle of repairing a pocket deck (5×12’) that sits 50% over our garage, 50% over walled in and insulated space that creates an inset for our garage door.  The deck subfloor has failed due to a mix of improper EPDM detailing at the edges (fishmouthing) and essentially no tyvek behind cedar bevel siding.  The tyvek that did exist was reverse-lapped behind the EPDM…. Wind driven rain went into the wall, and then ran down behind and under the roofing for 40 years… into the structure.  I’ve been dealing with the results for the last two weeks.

With the rot repaired I am preparing for re-roofing.  The original roof had no insulation on top and 8” of soggy wet fiberglass below.  The new roof will have a roof drain instead of a scupper and a 6” layer of foam on top with cricket for drainage. 

I believe the previous damage was exacerbated due to the vapor barrier on the ceiling below that held the leaked water tight to the wood framing members and prevented escape until the water had pooled significantly enough to push its way through 2 layers of Sheetrock.  Serious rot, over decades.

I’m wondering if I should change my approach in any way to plan for future leaks and make them more obvious if/when they happen…

The garage is relatively dry and in the winter at the ceiling it is often 80+F while at the floor (16 ft ceiling) it is 45F.  We are in Alaska, Zone 7.

I am planning to:

1 – Re-roof as described above, which will add 6” of rigid that wasn’t in the assembly before

2 – re-insulate below with Rockwool 8” 

3 – use Certainteed  MemBrain as a smart vapor barrier to allow drying

4 – re- Sheetrock

Is this the best approach?

Additional question – should I leave the area around the new drain pipe exposed because that is my new most likely point of failure?  I could frame a 12×12” section of floor around the pipe and leave the drywall off of that cavity.  The pipe /drain would stay warmer and frost free in winter which seems good. 

Thanks in advance!