I am a homeowner in downtown Ottawa, Canada, with a typical 110 year old house. Roof slope is 12/12. The asphalt shingle roof is at the end of its life. I am considering a metal roof.

I am trying to figure out if it is worthwhile to add aditional insulation at the same time, and how.

The attic space is a finished 3rd floor, enclosed in 1993. The ceiling construction is
• 1/2″ drywall and a six mil UVBA
• 5/8 unfaced phenolic insulation board laid crosswise to the rafters (R4.5)
• 1-1/8 unfaced phenolic insulation board cut and laid between the rafters, all joints staggered, edges sealed with acoustic caulk or foam it, four layers = R32
•2-1/2″ ventilation gap
•7/8 original pine sheathing
•ply roof sheathing, new in 1993
•ice guard, roof paper, asphalt shingles

If I install a metal roof on strapping and cross strapping, so it has its own ventilation right under the metal, then what is the need for the current ventilation gap under the roof sheathing? Maybe I could have it filled with foam insulation, adding some R value. But how can I fill a gap 2-1/2″ by 16″ by 16′ (assumed rafter length).

Then I wondered about adding another couple of inches of rigid insulation board on top of the existing sheathing (once the asphalt shingles are gone), then installing the metal roof above an air gap, for even more insulation.

And finally, there is the issue of what sort of sealing membrane to put in, since layering on the insulation isn’t much good if it is air permeable.

I am researching metal roof installations but haven’t seen any material on line that addresses a situation even similar to this.
I’d like to have better knowledge BEFORE I am talking to the roofing sales reps.

All advice appreciated !