Cathedral ceiling with two types of closed cell
Hi All,
I’ve read a good amount about cut and cobble, insulating cathedral ceilings, etc. I’m in CZ 6, so needing to get R25 aka 51% ratio in impermeable insulation (unless I’m mistaken) against the sheathing. Unvented roof.
I would really love to do a stack of pancakes approach, but I see that Martin is against it (no matter how many times someone new asks). So instead, curious about doing the following:
1″ of closed cell spray foam against the sheathing to seal air and vapor, followed by multiple layers of cut and cobble XPS to fill the I-joist cavity.
This is mostly on the table from a budget perspective. I know it will take more time than doing R25 in spray foam, but it will cost much less as I’ll be doing the cut and cobble labor. Since the biggest concern with the “stack of pancakes” approach is air leakage, it seems like this would solve the issue while hitting the right ratio numbers.
Interested to hear if there’s anything here, or if it’s not advised.
Thanks,
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Replies
Hi Sean,
Part of the issue with using CCSF first, then adding rigid insulation under is the spray foam will have highs and lows. Not being even will create air pockets between the foam and CCSF where moisture can condense on the foam. I agree with Martin, roofs are the most critical, and often hardest areas to get the details right. I'd bite the bullet and get the right ratio using CCSF or change the assembly to a vented roof and use fibrous insulation.
Randy