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Can I re-insulate my cathedral ceiling using the stack-of-pancakes approach with a vent?

Trying harder | Posted in Green Building Techniques on

I have a gable roof with a cathedral ceiling built with 2×8 rafters 24” on center. There is a ridge vent already installed and the roof assembly to the north of the ridge has soffit vents while the assembly to the south ends in the main attic space which is also ridge and soffit vented. I have spent several hours reading through the “musings” and other online resources and decided to move forward with the stack-of-pancakes method. I have not, however, come across an example of a vented stack-of-pancakes and wondered if that had something to do with the permeability problems that kind of an assembly might present. Is there some other reason? Would this plan still have the potential for foam to pull away from the rafters and create condensation problems? Here is my assembly plan from the top down, please critique:
• asphalt shingles (existing)
• 1/2” plywood (existing)
• 1-1/4” vent (adding) [built on-site]
• 4” Polyiso (adding)
• 2” Polyiso (adding)
• 1/2” Polyiso (adding) installed to underside of rafters to minimize thermal bridging
• 1/2” drywall

I live in Jefferson County CO and according to Energycode.pnl.gov I am zone 5 which requires a minimum of R-38. My calculations would put this ceiling at R-40 Should I proceed?

Thanks

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Trying,
    I'm giving you the green light. From a moisture performance perspective, this assembly will work.

    Some points to consider:

    1. By far the most important layer is the continuous layer of rigid foam on the interior side of the rafters. It's a little sad that you can only install 1/2 inch of continuous rigid foam. If you could bump that up to 2 inches, the performance of the entire assembly would be greatly enhanced. Those inches are the most important inches.

    2. Airtightness is key. Each layer of rigid foam should be carefully air sealed.

    3. I wouldn't do it the way you suggest. I would install a much thicker layer of continuous interior rigid foam, and would install mineral wool batts between the site-built ventilation baffle and the continuous rigid foam.

  2. Doug McEvers | | #2

    I second the recommendation by Martin. Air space, fiber insulation, (2) 1 1/2" layers of polyiso with seams staggered and taped to the underside of the rafters. I would then add some pine strapping to the underside of the rafters for the drywall attachment. Much easier than trying to install drywall directly to the foam with long screws.

  3. Trying harder | | #3

    Thank you gentlemen! I’ll see what I can do about achieving an assembly closer to what you recommend. Would you recommend half inch XPS for the on-site built vent if I build it your way? Alternatively, if I were to proceed with my original design I planned to use the top side of the first layer of foam as the vent. With that be OK? And with either method what kind of foam should I use (Foil faced, fiber faced, unfaced…)?

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Trying,
    Here is a link to an article that tells you everything you need to know about site-built ventilation baffles:
    Site-Built Ventilation Baffles for Roofs.

  5. Doug McEvers | | #5

    I attach pine strips to the top side of each rafter (tight to roof decking) for the desired air space depth. To this you can use fiberboard or OSB as your vent material fastened to the underside of the pine strips. We just finished an ice dam solution project and used the same detail, dedicated air space (1 1/4"), reused R-28 fiberglass insulation to fill the rafter space and 3" of polyiso foam (R-20.3, all seams and edges foil taped) attached to the room side of the 2 x 10 rafters.

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