Upgrade over old exterior 2×4 wall – GreenBuildingAdvisor

I am planning  to “fix up” the situation over my 70’s kit homes exterior walls.

I’m in Colorado climate zone 6 around 7,000’ft. The roof is a 4/12 pitch.

I  want my “improvements” to make things better, not worse. 

The section uncovered in the photos is on the north side over a bathroom with no fan. The 2×4 trusses are sheathed in 1×12 pine covered by 15lb felt, asphalt shingles, and  pro-panel metal roofing screwed on top.

The 2x blocking is solid from the top plate to the underside of the decking, caulked and painted. There is NO ridge vent. Just a vent at one gable end near the peak.

Despite the lack of roof venting there’s no sign of moisture problems. My guess is due to a polyethylene vapor barrier installed 35 years ago before new ⅝” sheet rock on the inside ceiling, the solid wood deck, and the dryness of the climate here.

My plan is to  remove all the shingles, pull the lower 4′ feet of pine, exposing the top of the wall, create a 1″ vent space with an insulation baffle, cut a vent at the ridge, and replace the removed deck with 3/4″ ply sheathing.

My thought is to use 2″ foil faced polyiso for the baffle. Sitting it on the top plate and extending past the exterior OSB sheathing 3″ to accommodate  two layers of 1.5” Roxule Comfort board exterior insulation in the future.

After air sealing everything, 2″of polyiso and spray foam over the top plate would offer more insulation than just blown cellulose.

The new ¾” ply would let me box in the open truss tails with non combustible soffit for more fire resistance.

Does this plan seem reasonable?

My concerns are;

1. replacing 1” of insulation depth out of 3.5” over the top plate might make the ceiling at the corner colder, not warmer. 

2. ¾” ply with an impermeable roof membrane over it might be more susceptible to condensation on the underside with so little insulation over the wall plate. Even with a 1”vent space.

Any input or thoughts are welcome.