R-60 rigid insulation attachment for cathedral ceiling with exposed rafter.
Puffle
| Posted in GBA Pro Help on
Hi, I am designing a home with double studded wall, cathedral ceiling (exposed beams and rafters, decking over the rafters). The roof slope is 3/12, Marine 4 climate zone (roof R-value R60).
My question is:
are the standard insulation panels “stainless steel anchors” per manufacturer’s specifications enough or is there additional support required to attach the panels and support the metal roof assembly?
Thank you!
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Replies
That is a bit overkill in zone4, both in R value and venting.
You are not in snow country, so venting is not really needed. You can skip all the battens and the 3rd roof deck.
R60 built with expensive materials in mild climate has an ROI of pretty much never. Since you have continuous insulation, most codes allow U factor based compliance which might be only 5" of iso. The energy saved going from 5" to 8" is on the order $10/year, so generally not worth it.
As for installing the metal roof, no issues over a solid substrate on the rigid. This is done all the time and works.
Some metal panels with clips can also be installed onto steel bearing plates directly on the rigid which would let you skip the extra roof deck.
The zip over the service cavity is your main air barrier, make sure to connect this to the wall air barrier for proper continuity.
What Akos said.
- Skip all the framing over the roof sheathing.
- A second layer of underlayment unfortunately doesn't buy you anything. If it fails it will be at penetrations, which go though both layers.
- I would also suggest making your electrical chase from 2"x4"s. That will make hitting them though the foam to secure the roof sheathing a lot easier.
Thank you for our fast reply. Yes, the original roof system seemed overkill, -The Zip sheathing connects to the wall zip sheathing - except where the rafters penetrate. 2x4 on the electrical chase makes sense. I have never used U-value to figure out the compliance. I need a 0.024 U-value for the roof assembly to meet Washington state code. I am also stumped to how to address any of this in the energy code compliance worksheet ... Any help is greatly appreciated.
Your new stackup is much simpler to build.
1/Ufactor=Assembly R value. So 0.024 is an R42 assembly. An assembly R value takes into account the contributions of all the elements like the air films, interior/exterior finish along with the insulation.
You can use one of the on-line calculators like:
https://www.ekotrope.com/r-value-calculator/
to build your roof and see how much rigid you need. Make sure to include all the bits including the service cavity and shiplap ceiling as that gets an extra R2 for free.
Thank you for giving me the R-value for the service cavity. I hadn't been able to find a r-value for the air space by myself! Using the U-value for CI roof system reduces the insulation thickness quite a bit - in my case taking in account the OSB R-0.55 (zip sheathing) and the 1/2" plywood R-0.62 over the insulation:
R-2.0 (service cavity) + R-0.55 (zip sheathing) + R-0.62 (sheathing) + R-0.06 (roofing felt) + R-0 (metal roof)= R-3.23
Hard to believe that I will need less than R-39 CI to comply with the energy code! Thanks again!