Cathedral Ceiling With Inadequate Exterior Foam Insulation
I have been driving myself crazy trying to understand and complete a cathedral ceiling assembly in an old, Southern Ontario farmhouse we recently purchased. I learned what I could from the GBA articles about such ceilings but am still unsure how to proceed. Are you able to please offer any help?
The problem is that we have a new roof with too-thin exterior insulation for our climate zone. We have 1.5″, R-7.5 GPS Silverboard over our 2×8 and 2×4 rafter cavities (different parts of the house), which following the 50:50 formula for fibrous insulation to exterior foam would allow only R-15 in our roof assembly.
The good news is, the SB is micro-perforated and air-permeable, according to the manufacturer. There is a board over the insulation over each rafter, creating a ~3/4″ air channel above the insulation and below the sheathing to the soffit. So, maybe we can fill the 2×8 cavities with 7″ of rockwool and 2-4″ of foil-faced foam under the rafter cavities (air barrier), since the cavities will be able to dry to the outside, to the extent the microperfortions are trustworthy in this respect(?) It would yield R-40 to R-50, depending on how much headroom we were willing to give up.
When I asked the Silverboard manufacturer about this, they kindly ran a model to advise me my dewpoint would fall inside the rafter cavity somewhere, which would have 80% RH. But they didn’t spec the vent cavity, and I haven’t heard back from them about whether this would matter. Is an interior dew point still a problem where cavities can breathe?
The bad news is, there are are a pair of hips on our roof creating a few cavities where there is no venting. Also, our 2×4 rafter section has boards over the rafters and under the 1,5″ silverboard, undermining their breathability. For both sections I gather my only good option is filling them with ccSPF, is that right? Perhaps with 2″ of the foil-faced insulation under the 2×4 filled cavities (for R-40 total and to help protect us from off-gassing). Would it matter in this case that the dew point was still interior – presumably not if the dew point fell inside the foam? Finally, the house does have A/C, so we have to manage the possibility of summer condensation, as well.
Thanks in advance for any help you may be able to offer.
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