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My roof is dried-in right now with 3/4" roof deck and good synthetic underlayment. 2 story 850 sq. ft./level, nearly all cathedral ceiling below the roof deck. I am looking for advice on the best way to assemble the metal roof above in conjunction with insulating below. Located in central TX.
Currently, I am looking at a standing seam roof from McElroy, my plan for install is this: 3/4" polyiso foam (R5) over the roof deck, metal fastened to deck. Then closed cell spray foam to about 4 inches (R26) inside to fully seal and insulate the rafter bays to R31 total. I'm in climate zone 3 so I believe R30 is my requirement. This is rural property so not subject to any requirements, but I am building to code. I have not installed any insulation yet.
Any insights and recommendations on this assembly? Trying to employ best practice as much as possible but be cost effective as well. There might be a better way to do this that I have not thought of yet. The one non-negotiable is probably a minimum of 2 in. of closed cell sealing the unvented cathedral against moisture issues.
If you put a bit thicker foam on the roof deck side, you can reduce the total insulation required on the inside.
I know the CC SPF is what is usually specified for roof assemblies, but I have never had an issue with OC SPF.
If the ceiling assembly is tight and well sealed, OC SPF if fine.
If you are feeling inventive, you can create an above deck vent to help with the convective cooling of the metal.
Do you think there would be an issue with installing the metal over the foam if I go thicker than 3/4, the polyiso board does not compress easily but would I need to introduce purlins/furring instead of scewing through the foam to the deck? One thing I forgot to mention in the original post if it's of any consequence is that this roof is 7/12 gable with a 4/12 section.
As for OC SPF vs CC from the research I have done, I'm under the impression that OC in a cathedral ceiling can have the potential to basically act as a kind of sponge for vapor in an unvented assembly, instead of sealing against the vapor and making sure it dries to the inside. But then again, I guess it might only be in extreme cases? Perhaps I could do OC for the main r-value and the seal with a layer of 1-2" of closed cell - that might not be a good idea though.
What type of above deck vent system are you thinking of?
I have used OC quite well and the important thing is that your ceiling plane is tight and use the ADA (Airtight Drywall Approach).
If you go much thicker than 3/4", you will need some purlins and some wood.
I would look at incorporating a layer of foil faced foam (facing the backside of the roof) with a bit of an above deck roof vent approach.
Obviously, in Texas, the larger issue is going to be the radiant heat. You don't have much of a winter there so a bit more inventive approach to the above deck insulation will really translate to some additional efficiency in the home.
Thanks for the input Eric. I have discussed the whole assembly over with some insulation specialists and come to a plan I think will work pretty well.
I'm planning on OC foam in the rafter cavities and R5 (3/4") rigid foil faced polyiso on the roof deck exterior, with 1x4 battens on top of the foam fastened to the rafters to create an above deck venting space. I also switched to planning on an r-panel roof due mostly to cost factors and the roof being a bit tricky to safely get not-so-rigid standing seam panels up there during installation without damage. With the corrugations in the r-panel adding height to the above deck venting space, I think it should perform pretty well.
Any other thoughts are welcome. Thanks
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