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Options for insulation for cathedral ceiling

We are replacing a roof structure on a portion of our 1832 timber frame home with regular framing (17' x 20'; 20' is horizontal roof span). Due to the 4' kneewall and 5 pitch roof, the ceiling height in the peak of the room is only 8', so we want to retain the cathedral ceiling in this large room. We are using a laminated 14" ridge beam and 2 x 10" rafters (16" OC), with no rafter tails (maintaining the original structure design with the timber frame). We are climate zone 5, Marine (SE Michigan, between the big lakes).

We were originally going with a vented roof assembly, using under shingle vents and ridge vents, with air permeable insulation and vapor barrier internally. Max R-value we would get in this situation, using Roxul Comfortbatts (R-30 @ 7.25" depth) + Roxul Comfortboard (R-6 @ 1" depth), would be R-36 (under what we would need for this area; R-40 is okay; R-60 is best, but we know that we'll be under this with a cathedral ceiling and 2 x 10 rafters).

We have considered an unvented roof assembly, using 1" foam over the roof deck @ R-5 (this is all we can manage b/c the gable roof peak of this section is abutting the gutter for another portion of the house), 4" of rigid foam under the roof deck @ R-20 total, then Roxul Comfortbatts (R-23 @ 5.5 depth...a 1/4" too much technically) for a total R-value of 48 (without the R for the roof decking, etc. added in). We have also considered scabbing on a 2" furring strip internally, adding the option to use an R-30 Roxul batt @ 7.25" depth, for a total then of R-55.

I would love to do the second unvented option, but I worry about condensation using rigid foam vs. closed cell spray foam (we are opting for rigid to keep the expense down), due to possible air infiltration into any unforeseen gaps. I considered using spray foam around the edges of the 2" rigid foam, to reduce this, but it still has me worried.

Any thoughts would be great. I dislike the idea of using so much spray foam (environmentally speaking), but I also wanted to get the R-value up as much as possible.

Asked by Emily Rusnak
Posted Sat, 07/12/2014 - 20:18

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Emily,
It sounds like you understand your options. It also sounds as if your design and budget are forcing you into compromises. Life is like that sometimes. Since you understand your options, the last step is just to make a decision.

I assume that you have read this article? How to Build an Insulated Cathedral Ceiling.

My only comment: it's hard for me to visualize the situation your describe in this phrase: "...using 1 inch of foam over the roof deck ... is all we can manage because the gable roof peak of this section is abutting the gutter for another portion of the house." However, if there is some way to re-route the gutter that is interrupted by the gable peak that you describe, perhaps you can find a way to install a thicker layer of rigid foam above the roof deck. A thick layer of rigid foam above the roof sheathing is almost always the best solution from a performance perspective.

Good luck.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 07/13/2014 - 05:59
Edited Sun, 07/13/2014 - 06:00.

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