Sanity check on unvented cathedral ceiling assembly
Climate Zone 6b – New Build
I’ve read the how to build an insulated cathedral ceiling found here:
I’m looking for a sanity check on my design and to possibly explore a lower cost option suggested by an insulation contractor.
Using the information in the link above I set up my roof with 14” TJI rafters. The plan was to apply 5.5” of ccSPF to the underside of the roof deck in the rafter cavities. Then I was planning to fill the remained of the cavity with BIBS blown in fiberglass. No vapor barrier on the inside.
This method gives me about an R~67 in the cavity with R~34 coming from the ccSPF and R~33 from the BIBS. This gives me a ratio of almost 51% of vapor impermeable insulation to the whole cavity value.
Using inside design temp of 68F and an outside design temp 22F (average of the 3 coldest months) and 35% humidity I get the inside of my ccSPF coming in at a temperature of 45F well above the dew point of the air.
I’m very confident this will work well, however ,the ccSPF is expensive and an insulation contractor suggested using 2” of ccSPF under the roof deck then using ocSPF to get the R value I’m looking for. If I was to then spray 12” of ocSPF I’d have about R~13 from ccSPF and R~43 from ocSPF making a total R~55. This will be cheaper than my original proposal. However because the ocSPF is vapor permeable and the ccSPF is vapor impermeable I’m confused about where my first condensing surface will be?
Will my first condensing surface be the ocSPF because it is an air barrier? If so, I think this will be a workable solution. My calculations show the inside surface temperature of the ccSPF will be 33F which is below the dewpoint of the air, so if that’s my first condensing surface it wont work.
What are your thoughts on the second method and are there any other suggestions that might help me make a smarter choice in the roof insulation strategy?
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