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Insulating an unfinished attic under the roofline: fiberglass batt + foamboard?

joe_westersund | Posted in General Questions on

I have a 100 year old Cape Cod style house in Portland, Oregon (climate zone 4c). Rafters are 2×4, 30″ on center (weird!). The kneewall spaces are conditioned (unventilated), with no ventilation channels, and with R-11 kraft-faced fiberglass batts stapled into the rafter bays.

As part of a re-roof, I’m removing the fiberglass batts from the kneewall attic rafter bays, getting edge vents and ridge vents installed, and putting a continuous channel of SmartBaffles going up each rafter bay from edge vent to ridge vent. (The roofers will install baffles in the cathedral ceiling from outside, pushed into the fiberglass batts. In the kneewall attic, I’ll install the baffles from the inside). 

Here’s my question: once the baffles are in place, what’s a good way to add insulation?

Options I’m considering (layers described from roof down):
Option 1: roof sheathing -> baffle -> fiberglass batt compressed in rafter bay, with kraft paper down -> polyiso with foil down, attached across rafters. May need furring strips?
Option 2: roof sheathing -> baffle -> polyiso cut and cobbled into rafter bay -> fiberglass batt with kraft paper up (so it’s not exposed), somehow attached across rafters, outside the rafter bays.
Option 3: a better idea?

Basically, I’m wondering if I can make use of the kraft-faced fiberglass batt I already have. And, after reading that I’m not supposed to leave kraft paper exposed, I’m wondering what materials are OK to leave as the exposed (bottom) layer. Is exposed polyiso OK?

Thank you for any advice you can provide to this building newbie.

-Joe

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