Adding rigid foam to cathedral ceiling

My living room has a cathedral ceiling with 2×8 rafters 16″ on center. It currently has fiberglass batts with Styrofoam baffles for air vents, soffit vents and a ridge vent. We constantly have issues with ice dams from heat loss and it being north facing so it never gets the heat of the sun. We are in zone 5, near zone 6.
My plan is to rip everything out from the interior and replace it with the following:
1) 1″ baffles made of 0.5″ EPS
2) R21 faced fiberglass (5.5″ thickness)
3) 2 layers of 2″ polyiso, staggered, with seams taped, edges spray foamed
4) 1×3 furring strips parallel to the rafters
5) 0.5″ drywall
I estimate this should get me close to R50. Is there any concern about moisture being trapped in the fiberglass batts with them being sandwiched between foam boards? Is there a better way to fix 4″ of polyiso to the rafters besides 5″ screws? Anything I may have overlooked? All advice is appreciated.
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Replies
DIYDaddit,
1/2" EPS is fairly vapour-open, so it won’t be an impermeable sandwich. If you want something higher perm to allow more drying into the vent cavity I would use house-wrap or plywood/fiberboard for the baffles.
I wouldn't bother with the baffles, instead spend your money on high density batts (fiberglass or mineral wool both work). You still need baffles at the eaves, but I would skip the rest of the roof length. Make sure to seal your top plates as well as these tend to be leak sources in old houses.
You can detail the foam layer under the rafters as your main air barrier for a solid cathedral ceiling. I would not do 2 layer of rigid, not much energy to be saved by the 2nd 2" layer. I have pretty close to this assembly in one section of my roof and it has been trouble free.
Akos,
While it seems to make sense up here where codes mandate a 2 1/2" ventilation space, I wonder whether the merit of forgoing baffles may not be as great where the gap is only 1", and is much easier to block or restrict?
My stackup is 2x8 with 2x6 MW batts. Poly with taped seams over the rafters for air barrier (plus code required VB) followed by 2" poyiso and drywall. Baffle at soffit with spray foam to seal top plates.
I guess with a sloppy install it is possible to block the vent gap, but it also doesn't take much care to keep it open.
I guess if you expect the insulation install to be sloppy, some cheap cardboard baffles are the way to go. Cheaper and less fussy work than foam.