There’s been a bit of digital ink laid down here by well respected authors describing the process of insulating an old leaky cape cod style roof (1.5 story house) by placing an air-tight membrane over the old sheathing, then adding an adequately thick layer of exterior rigid insulation before installing new roofing.

If memory serves, this method of improving leaky and poorly insulated cape roods is often described as being a good choice, IF it is time for a reroof. And I buy it… from a performance perspective. But the other sell to make must address cost and feasibility. ‘Good choice’ is complicated, and advice based on performance alone is close to useless. 

Martin’s recent article drives a related point home. He even makes reference to 1.5 story roofs, “Does your house have poorly sealed kneewalls? Welcome to the club.”
(https://www.greenbuildingadvisor.com/article/implementing-energy-saving-advice-isnt-always-easy)

Perhaps Martin hasn’t reroofed yet? Or is his advice to foam over a cape not advice we should take? I understand the difficulty in trying to write prescriptive advice for the masses and am not taking a jab; just trying to sort out whether something is worth pursuing from a ‘good choice’ perspective rather than a pure ‘performance’ perspective.

This is a long way to get to a more technical question about how to implement said advice, if it wise at all. This recent FHB article* uses a sort of hybrid approach: https://www.finehomebuilding.com/project-guides/insulation/insulate-a-cape-roof-to-avoid-ice-dams

The catch to a retrofit over-roof is this: “Building scientists recommend, and building codes require, that the insulation installed between the rafters must be in direct contact with the underside of the roof sheathing.” (https://www.greenbuildingadvisor.com/article/installing-rigid-foam-above-roof-sheathing)

So now we have to MOVE existing insulation from an attic flat to its slope! And the knee walls. Work inside and out. If the attic is small, as it often is in capes, there may be no attic access. Its not a small task, though it’s usually tacked on like an asterisks.

My proposal is to do what they do in the above linked FHB article by leaving the attic insulation alone and stopping short the exterior foam. But instead of performing ridiculous air sealing gymnastics around the rafters (seriously!) just let the air barrier run over the peak and exist ABOVE the attic insulation.

There are a few particulars that makes this appealing:
-the attic is very small, so could easily be filled to the brim with cellulose.
-A vapor open air barrier would be used (at least at the peak). Sort of akin to vapor diffusion ports.
-With no foam above, no ratio to worry about.
-Over venting using 2×4’s will probably be used, so extra moisture robustness in afforded.

Is it me, or does this seem simpler and as effective as over roofing the total roof with foam, then somehow magically moving the attic insulation to the sloping portion?

*PS that article misstates that: “When adding exterior insulation to a roof or walls in our climate (zone 6), you should try to get at least one-third of the total R-value (in this case, about R-19) on the exterior for dewpoint control.”