Question regarding unvented roof assembly
I’ve got a question about insulating the roof of a small mudroom addition. The dimensions are 5′ x 8′ with a 6/12 simple gable roof. Because of the size of the project and the relatively low headroom inside, I decided to go with an unvented roof assembly and a cathedral style ceiling with exposed rafter ties. The fascia boards etc have been installed so venting the roof and changing the design is not an option.
The roof is brand new standing seam (galvalum type product) over 5/8″ Zip System. There is no iso board or any other kind of insulation on the exterior. There are no roof penetrations.
I’m in Climate zone 5. There is no heat source currently in the addition and eventually there will be a wall hung electric heater which will only be used when needed. There is no plumbing of any sort in the addition either.
The original plan was to hire a spray foam contractor and let him or her worry about it. However, all of the spray foam companies in my area have minimum requirements and it would have been too expensive to pay their minimum fee for such a small job.
So then I was planning on doing a cut and cobble installation with xps and canned foam. I was planning on firring the rafters out to achieve the desired R value and then going over the rafters with a continuous layer of 1/2 inch xps to prevent thermal bridging. This sounded like a lot of work. I’m sure I would have taken the time to get the details right but since I have now read a bit more on the subject, it seems that this is too risky of a plan.
A couple of days ago I read an article on this website saying that one way to do it would be to spray a layer of foam against the bottom of the roof sheathing. Then I could fill the rafter bays with insulation bats (the article probably stated the preferred material but I can’t seem to find the article). I would most likely go with mineral wool. Then, even though I’ll most likely be installing t&g boards on the ceiling for looks, I’ll first sheetrock and tape, thereby creating an air seal.
The total square footage of the bottom of the roof sheathing adds up to about 40 sq ft. I was thinking of buying the Touch’n Foam 200 kit. If I’m understanding the specs correctly I should get several inches of depth out of the kit. At that point I will see how far I need to fir the rafters to achieve the desired R value with mineral wool and that should do it.
I’m hoping that I’m remembering this correctly. It seems a little weird that there is nothing in the assembly to prevent thermal bridging but I guess the idea is to seal the bottom of the roof sheathing and allow the rest to dry to the interior.
Does all of this sound right? What am I getting wrong? Thanks in advance.
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