Hi all,

I am planning to finish a ventilated attic in the Raleigh NC area (climate zone 4A, near the border with 3A). My rafters are 2×8 (actual depth around 7″) beneath asphalt shingles on OSB sheathing. Currently no exterior rigid foam and the roof is new so not considering that option. The attic has mostly cathedral ceilings which local code requires to have R38 insulation. 

I like the idea of site-built ventilation baffles out of rigid foam boards following the guidelines here and am hoping to have them double as a radiant barrier via foil facing. 

I am debating between two options right now:

Option A: Add 2×2 furring strips to the underside of the rafters to create 8.5″ of usable depth. This depth will be occupied by the 1.5” air gap, 1.5” faced foam board R 9.3, and 5.5” of Rockwool R23. On the underside of the furring strips, install 1.5-2” rigid foam boards ~R10 across the rafter bays as a thermal break before the drywall. 

Option B: Add 2×4 cross hatched to the underside of the rafters to create separate depths of 7” and 3.5”. The 7” depth will be occupied by the 1.5” air gap, 1.5” faced foam board R 9.3, and 3.5” of Rockwool R15. The 3.5” depth will contain another batt of Rockwool R15 before drywall.

What are your thoughts on these 2 options? 

For Option A should I do a faced or unfaced board for the underside of the furring strips? 

Do any of you have experience attaching drywall through rigid foam boards? DIY Dave on YouTube has a video about this but that’s about all I’ve found.

For both options, I’ve seen mixed feedback on this site regarding faced boards inside the rafter cavities similar to my proposed baffles. How concerned should I be about moisture on the interior face of the baffle’s foam board? I am aiming for an airtight drywall installation but would like to have backup for if/when moisture finds its way into the insulated cavity. Anyone have examples to share of introducing a vapor permeable membrane into a setup like this?

Thanks