Unvented roof question
First post here. Building a new home in zone 4a in central NC, Guilford county tk be exact, and wanting to follow many of the guidelines found here for a practical but efficient home. One section of our home will use attic trusses for both storage and as a place to put air handler for that section of house. As a result, I will need an unvented setup.
I have gone back and forth between foam on top with rockwool below or closed cell(2”) below in combination with rockwool(r30). Based on rough cost, I am leaning towards the closed cell foam and rockwool. While the ridgid foam option is cheaper from a material standpoint, the labor building out that roof will swing the cost benefit back to foam/batt combo. I was just reading an article from Building Science center that made a statement that seems to contradict what I have read to this point. It suggest closed cell foam is not need below Zone 5. This is not what my understanding has been. Am I wrong here. I will post a link to that article
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
here is a picture of the “section” of house that I am discussing. As you can see, it also has a 2/12 shed roof coming off that back that will require an unvented setup. Any advice and comments welcome.
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Bump on this. Further reading has caused me to ask myself if using polyiso over the 2/12 roof is a better idea. Any advice appreciated.
We're building this year as well. After 3 years of research and design we are going EPS outboard and rockwool inside with a smart vapour barrier. We are zone 5 with 4300 HDD and need 41% of R to be outboard so inside is R 30 and out is R 20. If we can source recycled polyiso and EPS we will layer the EPS over the polyiso to conteract it's R value losses at low temperatures. The pandemic has reduced the availability of recycled product so it may be all new EPS when we get to that.
Is the 2:12 metal or membrane roof? With metal, the best option is the SPF as it avoids the 2nd roof deck. With a membrane roof, there is very little extra cost to install the exterior rigid, so it should be cheaper.
In your climate you can go all open cell with a conditioned attic. The key is the attic must be conditioned per code. Closed cell SPF+mineral wool is a more robust assembly and would save you the cost of intumescent coating.
Thank you for the replay. We were going to go metal over membrane, for the sake of looks, as bad as that may sound.
Do you happen to have any other ideas in terms of insulating the low pitch (2/12) mono trusses other than what we have discussed? I don’t think I have missed anything, but my knowledge is limited.
Once again, thank you
You can get low slope metal that will work on 2/12, no need for any membrane under it. Standard synthetic underlayment is good enough. Some of the mechanical lock metal can profiles with clips can mount onto bearing plates right on the rigid insulation which would avoid the need for a 2nd roof deck if you go exterior foam route.
Since you are going unvented, the flash and batt you propose or the rigid above, fluffy bellow all work great.
The all open cell SPF will also work but you'll need conditioning in the attic area. Running ducts in there with small supply (check code, but it is usually 50cfm/1000sqft) and return would be sufficient. I would try to put the return near the peak of the roof as that is where moisture typically accumulates.
Is there a way to go unvented. Everything I have read here seems to suggest it’s very difficult to pull off when considering the low slope roof is overbuilt onto the main roof. My assumption is that there is now way to create an air path for venting as the one side of the main roof dead ends into the moon trusses of the low slope roof. Even if I could vent, not sure that it would really save anything or present a scenario that is better, but as I said, I may be missing something. Can’t thank you enough for your time and advice on the subject.