I have searched and read various threads, but unfortunately it seems every situation is just slightly unique.  So I am really hoping I can get some guidance from experts or people that have just learned from their mistakes.

I recently purchased a home built in 1977 in Northern Michigan near Traverse City.  Part of the house has a rafter built cathedral ceiling utilizing 2×8 construction on 12″ centers.  The rafters act both as the ceiling in the great room and also as the roofing deck – so there is about 7 1/2″ of available insulation cavity.  The roof was shot when I purchased the house, so we just finished a new metal roof and the entire flooring deck got covered in Grace Ice and Water.  There were no visible signs of rot that would have been caused by condensation.  A couple of rotten spots, but a clear result of a leaking/neglected shingles.  The roof has a 2.5/12 pitch.

Inside the great room is a beautiful popcorn ceiling that will be removed as soon as my wife locates my scaffolding.  It is a second property, so although it is heated in the winter, my utilities have been quite low as we just turn the heat up on the Nest prior to arriving and lower it upon leaving.  The reason I mention that is, depending what I choose to do with insulation, I am never going to recoup any insulation investment while I own the property.  At this point, I have not removed any of the great room ceiling drywall, so I really don’t know if there are any baffles under the roofing deck into the eaves, although I kind of doubt it (building science probably wasn’t discussed much in 1977).  So here is where I need the advice…

With the installation of the Grace Ice and Water, I presume this 2×8″ cavity will not breathe in anyway to the exterior.  With the Northern Michigan climate, do I need to be concerned about condensation and future moisture issues on the bottom side of the roofing deck?  If so, what is the best low cost approach?  My initial thought is approx 4″ spray foam and then R13/15 under the spray foam.  This isn’t a question of code, as if I choose to do anything with the insulation its pretty safe to say it will be significantly better than what is there now.  Also, strapping and lowering the ceiling is not an option as the ceiling is already lower that I would prefer.  If I do remove the drywall and re-instulate the cavity — the age old question is do I need to put a vapor barrier on the warm side?  My personal preference is no.

Finally… if I was to consider putting an engineered wood flooring product on the ceiling… would it be as simple as the following working downward.  Roofing deck, following by spray foam, following by R13 batt followed by wood floor/ceiling (may or may not use a piece of plywood over the T&G wood ceiling.  

Any available wisdom would be greatly appreciated.  Thanks in advance — Brian