Sheathing repair on unvented cathedral ceiling with closed cell spray foam – GreenBuildingAdvisor

Minnesota, zone 6.

I have a house with a 17 year old roof that needs replacement. The roof is an unvented cathedral ceiling with closed cell spray foam applied directly to the sheathing.

I’m concerned about how to properly address and replace compromised sheathing. Since the insulation is attached to the sheathing, it seems to be that removing compromised sheathing also removes insulation. I’ll be up front in that I don’t know IF any sheathing has been damaged, but based on my experience it’s almost a certainty- water always wins.

I have spoken to one roofer who says that with spray foamed roofs they do not replace sheathing, they just throw another layer on- so if one board is bad, the entire roof is getting overdecked.

Is there a well documented way on how to address sheathing replacement or repair on spray foamed roofs? It seems like overdecking only kicks the can down the road by leaving rotten decking in place, and does not solve the “moisture sandwich” problem of sandwiching sheathing between two moisture barriers. 

Right now I’m thinking that instead of doing a direct overdecking, having the roofers install 1×3 furring strips along the rafters and then adding a second layer of sheathing to that, all capped with a ridge vent. My thinking is that this would allow the sheathing to better dry outward when leaks occur. 

Ideally I would like to avoid overdecking at all and do piecemeal replacements of sheathing, but I’m nervous about entrusting that to generic roofers. 

Look forward to others’ insight and experience with this.