I worry too much.. Here's some background. I had to put a new roof system on my 12x24 camp with a 12/12 roof. Wanting to be "green" I used all local RW lumber. It's got 2x6 rafters 1/2 x 6 sheathing. There's tar paper and black steel roofing on top of that and it's south facing so the roof get's hot for sure.

Currently it's a cold roof with fully vented soffets and ridge vent. But yesterday I preped it for 4in of closed cell spray foam to make it a "hot" roof. I did this by making tar paper blockers in every ridge and eave hole.

It's in very wet area in the white mountains of New Hampshire and moisture has been coming and going in the camp for a year now. The RW a bit multi colored

The worry. When I was putting the paper up at the peak, I swear I saw something that looked like dry rot on the very tip of a (one) rafter. It's wasn't white or moldy. I just looked as if mini bugs had eaten a piece of the tip. But I doubt it's bugs. More likely to be fungi.

So the foam is going in three weeks from now and I'm thinking (bad things). Should I rip all of the tar paper back off the peak before it's too late and spray the wood that will not be foamed (rafter peaks) with borate? Or since I am making it a hot roof, will the moisture be minimal?

 

spray open cell

Is the steel roofing up tight against the roofing felt, or is it up on purlins? If it's the former, you're spot-trapping moisture in the roof deck, and will be doubly-bad with 4" of cc foam. Up on purlins it should be ventilated enough to dry toward the exterior.

Is this building heated & conditioned? Cathedral ceiling or attic? If attic, is the attic floor insulated & sealed?

Conditioned space air leaking into the ventilation cavity/attic under the roof deck would likely have condensed on rafters & underside of the roof planking during the winters. Sealed with foam from below this won't/can't happen. With some ventilation space between the steel and roofing felt any leakwater etc. will have somewhere to go other than the wood.

Mind you, 4" of cc foam is a strong class-II vapor retarder, and you're putting in on the COLD side of the the assembly, which means if you also have interior-side insulation (on say, the floor of the attic), you have to be sure to split the total R in such a way that condensation doesn't occur in the attic, since you now can't use an interior-side vapor retarder to keep the more humid conditioned space air from permeating moisture even if when perfectly air-sealed at the attic floor boundary (which is hard to achieve in the first place.)

If you went with open cell foam and put in very good interior-side air barriers & vapor retarders you can heap up attic floor insulation with abandon, and it'd still dry toward the exterior. But without the perfect air barrier and vapor retarder at the attic floor you'd run about the same condensation & mold risk as you would with cc foam.