I am looking for suggestions on adding roof insulation to a 1960’s era 1600 sqft daylight basement residential hm located in south central Alaska.

The twist is the the original roof, that is still intact, is a flat roof with built up hot tar.  A sloped shingle roof  with site built trusses was added directly on the top of the flat roof. This has worked well for well over 30 plus years. It’s time to remove and replace the shingles, add ice water shield, update venting and add insulation. The original 1960 flat roof has 2×10 or 0r 12 joists with a maybe 4” of foil faced fiberglass insulation. It is soffit vented via the both ends of the joist bays. The shingle roof on top of the flat roof is soffited and small gable vents but has no added insulation or ridge venting.

I am considering two options:
1) Add blown or batt fiberglass/rock wool on top of the built up roof. Sealing the soffit venting on the flat roof. Cutting vent slots through the built up with a circular saw. Adding ridge vent. Adding soffit vent baffles.

2) Spray foam bottom of shingle roof truss sheathing and making the attic warm. Put a hat on top of the building. Seal both flat roof and truss soffit vents. Cut venting slots through built up with circular saw. 

Suggestions, comments or considerations?

RezonateFreq