I understand that insulating over a roof deck with rigid foam is very effective, but also expensive.

I’ve also seen advice commonly given that when it comes time to re-roof, certain hard-to-insulate/seal structures such as capes should take advantage of that time to add exterior foam.

The latter is the situation I am dealing with. After thinking through all the details, I am beginning to wonder if EVEN WITH the fact that the roof is already being replaced, is this a cost effective approach? (Perhaps a loose definition of cost effective, covering for a scenario where energy becomes quite expensive and environmental impact matters—is that obscure enough?)

Zone 6a —Maine

The details that appear troublesome:

—The structure has two dormers, so dormer siding (vinyl) will also need to come off and get integrated with the roof air-seal layer.
—The roof sheathing is boards, so a peel-and stick would be needed for air-sealing. These are very expensive. (alternatives?)
—Foam cost (considering reclaimed)
—All new fascia and soffit
—The need to pull existing insulation out of top attic triangle and kneewalls
—The need to add new insulation under rafters, unless all exterior insulation is used, which seems even costlier. The issues with access that come with this.
—Finding contractors to do the work correctly

On the other side, I see the challenges as:
—Adequately air sealing the attic spaces.
—Ultimately, I view this as likely to yield poorer results but at significantly reduced costs. How much poorer of results and how much cheaper is the crux of the payback issue.

I realize specifics beyond what I provided will matter, but does anyone have a good ballpark sense of the practicality of this approach?

Any way to cut costs with it (the adhered membrane issue?)