Ceiling air barrier and attic insulation for a remodel in climate zone 6
Our house is located in the Minneapolis metro area CZ6, and we are in the midst of a whole house remodel. The home has a low 3/12 pitched roof and new drywall will be installed on the ceiling soon. I have installed site-built attic baffles using 1″ R-5 XPS foam with a 1″ air gap (aside – thanks GBA for the info on that topic, it was very helpful). This setup allows for approximately 2″ of insulation to be placed on the exterior side of the top plate. I figured more insulation in that area was better than having a larger 2″ air gap.
The current plan suggested by insulation contractors calls for 2″ of ccSPF R-14 above the entire ceiling to act as both air barrier and maximize the R-value over the small space above the eaves. Then add blown-in fiberglass on top for a total R-49. An alternative they quoted to the ccSPF was using just sealed 4mil poly and a thicker layer of blown-in fiberglass. After doing further research, I’m questioning those choices, especially after discovering a small leak in the roof prior to finishing the attic baffles.
Would there be an alternative setup that might be less risky regarding potential moisture issues that may arise from the either interior or exterior/attic side? ie smart vapor retarder as the air barrier, using open cell vs closed cell, only using ccSPF over the top plate, blown-in cellulose instead, etc.
Keep in mind we are trying to balance performance needs with the goals of using green and low-VOC materials when possible. Thanks
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Replies
Hi Kris,
I'm a little confused about where you plan to insulate. When I read that you made your own ventilation baffles from rigid foam, I assumed that you were planning to insulate the roof plane. Did you do that for more insulating value above the top plate. The rest of the post makes it sounds like you are planning to insulate the ceiling with a flash and fill method. It does seem like you'll want to do everything you can to get the most R-value in that small space above the top plate, but there are lots of options for air-sealing and insulating the ceiling, if that is what you are planning. And keep in mind that in your situation, where it is not only directly above the plate, but all along the perimeter that there won't be a lot of room for insulation, air sealing is paramount.
You'll find a lot of info on attics here:
https://www.greenbuildingadvisor.com/collection/attics
And an article on slow-sloped roofs here:
https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
I find the details confusing as well. Are you describing a flat ceiling and not a cathedral? If so maximize the R-value and air sealing at the top plates near the eaves, spray polyurethane would be the easiest and would be airtight. You will need to install a windwash barrier of some type to keep the spray foam out of the soffits. Some opt to just spray foam the outside top plates and the interior partitions and not flash the entire ceiling. Polyurethane foam is about $1.00 per board foot (12" x 12" x 1") installed. It is expensive in relation to blown cellulose or fiberglass. Once you have stopped the air leakage blown insulation performs very well in the Twin Cities.
Sorry for the confusion and thank you for the responses. I appreciate the advice.
Brian - Yes, the site built baffles were to help with additional R-value over the top plate and keep attic insulation in place. I’ve attached some photos for better clarity on the setup.
Doug - This is a flat ceiling. I friction fit rigid foam between trusses on the exterior side of the top plate and foamed them in place to baffles which should keep unwanted material out of soffits. The white EPS was used as an extension for the baffles to prevent wind washing.
My concern was that spray foaming the entire ceiling to create a good air barrier would eliminate any vapor permeability. MN requires a vapor barrier if not spraying the entire ceiling. That’s why I was wondering if a smart vapor retarder and blown cellulose would be a better combo then the flash and fill with fiberglass. I understand the importance of air sealing the attic, so should I even worry about vapor permeability in the ceiling?
Thanks for the suggestion to only foam the top plates near the exterior walls. Glad to know that isn’t uncommon.