Cathedral ceiling in octagon-shaped home
I live in an octagon shaped house with a cathedral ceiling over the second floor. 3/4 of the interior space on the second floor is open and the ceiling is a little over 1,000 square feet. Four sides of the roof/ceiling are rectangular, while the other four sides come to points. The house has a small third story room at the top with four walls where the four pointed corners come together. This room has a flat ceiling but the roof for it is pointed so I assume it is an insulated attic but it has no access or ventilation. The house has a metal roof (about 10 years old) with exposed fasteners on top of a layer of shingles on top of a deck of plywood. The cathedral roof assembly on the second floor is failing. I bought the house three years ago thinking a metal roof was great.
The rafters are 2 x 8 so that is only 7 1/4″ of space for insulation. The insulation is fiberglass, most likely from when the home was built in 1980 and has evidently seen years of condensation under the deck. The ceiling had cheap plywood planks that we ripped down, and it was hiding drywall with cracks in the joints for warm moist air to do its damage. We noticed discolored leaks coming through the ceiling one winter but found out later it wasn’t a roof leak or ice dam but rather condensation from a new humidifier we installed. The soffit is vented but the roof assembly has no air channel to bring the air up and out. I believe the roof geometry and lack of space is too complicated to vent. The second floor bathroom vented directly into a crawl space under another portion of the cathedral ceiling, with leaky drywall installation. Having opened up some of the ceiling to take a peek, the deck is literally crumbling from years of condensation. Much of the roof is spongy if you walk around on it.
Even though the roof doesn’t seem to be leaking from rain or ice, I know the decking is rotten and I don’t believe I can fix the insulation, air, and drywall problems without ripping off the whole roof.
After reading a lot about insulating an unvented assembly, my plan is to put a white colored standing seam metal roof on, with ice and water recommended by the contractor for the whole roof. I’m assuming all or most of the decking will need to be replaced and am planning on using 1/2″ plywood. I don’t have enough room to put R-20 rigid foam on top due to some low windows on the third-floor room so I need to insulate underneath. I also do not want to lower the ceiling to increase the area for insulation. The vented soffit will need to be replaced or closed off in some manner. Thus my original plan was to install closed cell spray foam. I live in Zone 5, so I need at least R20 of foam on the underside of the deck and the rest could be more spray foam or insulation batt to get to R38. Because my ceiling is fairly large, I don’t think I can follow the R30 rules for cathedral ceilings. The building inspector said this was a good plan and that I would need to cover it with a thermal barrier. I’m now questioning this approach due to the lack of drying direction as well as needing drywall instead of tongue and groove boards.
If the metal on top and the underlayment doesn’t allow water that might eventually get on the plywood to dry outward, and the closed cell spray foam acts as an air barrier and possibly a vapor retarder, am I not dooming this deck to rot since it doesn’t seem like it could dry to the interior either? You might be able to argue that the rafters could allow for some drying direction just as you have thermal bridging issues with the rafters, but one likely has nothing to do with the other.
I know that open cell foam is considered risky for this situation, so I haven’t considered it. After reading about “smart” air barriers with vapor control, such as Intello and Majrex, it seems like it would be safer to install something like Rockwool batt with a smart air barrier that lets moisture escape inward. I know this wouldn’t allow me to bring the insulation up to code (R-30 Rockwool is 7.25″ thick) but having enough insulation seems pointless if the deck traps moisture and could rot out again.
All that said, would it be best to use all closed cell spray foam, mixture of closed cell + air-permeable insulation + drywall, or air-permeable insulation + smart air barrier with vapor control + drywall? If drywall is used, do I still need a vapor retarder paint with the option suggested?
The building inspector may not require spray foam due to the high cost but regardless, I don’t want to cause the same problems for me or someone else down the road. I also realize that there is the additional cost of putting up all new drywall, if using tongue and groove planks do not exempt a thermal barrier over closed cell spray foam.
There seems to be a lot of confusion about whether or not solid tongue and groove lumber is allowed. Michigan code R316.5.2 Roofing says a thermal barrier is not required, over foam plastic, if it is covered by tongue and groove planks, but the wording is muddy and makes me think the tongue and groove planks need to be engineered with glue, like plywood or engineered flooring, which doesn’t make sense.
If I only use closed cell spray foam to get to R38, which acts as an air barrier, can I leave out the gypsum board and just hang solid tongue and groove boards? I’ve read that tongue and groove ceilings are leaky, but if the foam is the air barrier, does it matter?
Lastly, would it be helpful, if I could afford it, to add a radiant foil layer on top of the underlayment on the deck, with 1x purlins opening up an air space under the metal roof? The roofer said he has done this for others to help cool the roof, but I’m not sure it would be worth it, or help with drying outward. The second and third floors are hard to keep cool in the summer, so that may be a good reason to do it unless the spray foam solves that problem.
I just don’t want to make a huge 25K plus mistake and would appreciate any help the community could provide. Thanks for your time.
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