Cathedral ceiling with exterior rigid foam?
Hello,
We are redoing our home, and also redoing the roof. The roof currently is an 8:12 slope hipped roof on a 30 x 30 square house. Currently there is a 6×10 single pane tinted glass pyramid skylight at the peak of the roof. The house gets southern exposure and is in sun the better part of the day, here in northern CA. The roof is currently (from the interior) 6×10 beams with 2″ T&G a layer of 2″ polyiso, furring strips, and then plywood, then shingles. It gets really hot in the house in the summer.
So. when we redo this roof, the plan is to remove the skylight altogether and add 2 layers of rigid on the exterior. Looking at an article here, there was a drawing that indicated 2 layers of 2″ foam was R30, 3 layers R45, but the highest R value I see for foam is Polyiso at 13 for 2″ panel so R26 for two? Sounds like we would have to sheath the 2″ T&G” then layer foam, then sheath plywood again on top? making a hot roof, or use furing strips?
The other option is to insulate with more traditional methods on the inside of the beams, and then add a new T&G ceiling over that with fake beams.
One other piece of the puzzle is we will have to run fire sprinklers due to the size of the remodel.
Any thoughts appreciated. I feel like I have some knowledge, just lack the experience. I do have an architect and contracter, I am just wanting to do this right and be informed.
thanks!
Replies
How I did the same
I just saw your post as I finished a lengthy description of what I did for a new post topic. Rather than put that all here as a reply I am going to go ahead with the new post but wanted to be sure you saw mine. Good luck with your project. Fyi, I am located near Ukiah, Ca.
How I did my similar roof
Hi rusty52,
You've probably moved on with your project by now, but here are my 2c.
First, my roof sounds similar. The actual roof deck is 2x6 shiplap running from ridge to eave, supported by 3 large beams that run the length of the house, and the 2x6s are visible from inside the house. We like the look of the wood so we insulated on the exterior.
The contractor removed all roof material. You could see sunlight through the gaps between the 2x6s. Then he covered the entire roof with Grace Ice Shield. This step is critical. Some contractors refused the job because they had done a cathedral ceiling re-roof and condensation dripped onto the grand piano during the winter. This is due to air moving up through the 2x6s all the way to the roof sheathing or shingles, where it condenses and drips. The assembly must be air sealed at the bottom, close to the conditioned space. That includes foaming the roof vent penetrations.
Another benefit of the Ice Shield is that the air exfiltration through the roof assembly is reduced. Our feet became warmer in the winter. The Ice Shield is black. Don't put anything white or shiny behind the 2x6s as it will be visible through the gaps.
The next step was to build a 2x6 rafter assembly over the entire roof. I was skeptical but the contractor insisted and he was right. The2x6s had enough sag that the new rafters sitting over them had 3" gaps in spots. I still wonder how to best insulate that. He cut rigid foam to 22.5" between the new rafters and filled them. Then another 2" layer went over all, then plywood sheathing and felt and shingles.
The roof performs very well. Snow stays on it much longer than on any of my neighbors' houses, in a nice even blanket. I don't have icicles. (very cold here in the Cascade mountains). We live in a fire-prone area and the sealed hot roof reduces the risk. The bats are also moved out; they were living below the shakes on top of the previous polyiso. We didn't re-use it.
I talked to over 6 contractors and had bids from 3. I was extremely pleased with the guy I chose - he was super smart and knew how to do it. Darn it, in 2009 he moved to Idaho.