Hot roof with standing seam metal roof – Is this a problem?
I’m in the process of building a house in a high fire zone (California SF Bay Area). In order to achieve this the goal is to remove all soffit vents and use a metal roof.
Since I lack soffit vents, I was planning on doing a hot roof, with 2″ of spray foam, followed by non-faced batt insulation. The spray foam was to avoid condensation build-up, and the non-faced insulation (probably rockwool) was to help with both noise and to ensure we had some interior drying potential.
The roof material is PacClad with Pac-Clad HT underlayment. Roof is already on, and about to do insulation.
The Pac-Clad HT is not vapor permeable (0.02).
Architect also wants to spray-foam and enclose all eves.
Given all of this, am I setting myself up for issue in the future, in the case any water gets between the roofing and the sprayfoam? If this is an issue what are my options to mitigate?
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Replies
schmoot,
There is some risk if the sheathing has no way to dry, but that is mitigated by both your mild climate and that you are using a metal roof, which is much less likely to ever see problems than most alternatives.
To further mitigate the risk, use closed-cell spray foam, and making sure you use a sufficiently thick layer. See assembly #5 in the first article I linked below.
These discuss the assembly you are proposing in more depth:
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
https://www.greenbuildingadvisor.com/article/sandwiching-roof-sheathing-two-impermeable-layers
If this is a new build, I say avoid the spray foam it is the most expensive way to buy an R of insulation and the least green type of insulation.
Note I see large amounts of spray foam in new construction as lazy design work form someone with zero regard for your budget. With some thought and planning a better lower cost solution is always possible. Yes, it will likely be thicker because Rs per inch is the thing spray foam does best.
Walta
Do you have a suggestion on how to create a hot roof without the spray foam? I'm looking at the hybrid approach, 2" of spray foam, rest of it standard batt insulation, this improves the cost substantially.
If your concern is fire this is not a class a roof assembly
I should amend this and say it certainly doesn’t sound like a class a assembly. I would check and make sure it is ul listed. The underlayment usually needs to be specific.
Can you elaborate? Pac-Clad HT is class A fire rated. I assumed metal roof + class A under layment would meet the requirements?
Is something else needed. Talking to my fire inspector, he mentioned the most important thing is to ensure no structures or trees are touching the building and to stop embers from entering the building. This has been the primary focus of doing the hot roof, to stop embers from entering into cavities and causing fire.
That is not necessarily true, it has to pass as a whole assembly because metal is so conductive and needs a noncombustible deck to meet class an unless it is a listed assembly.
This might help. https://www.nevadacountyca.gov/DocumentCenter/View/36995/Metal-Roofing-Handout-PDF
2" of closed-cell foam is still somewhat vapor permeable. I don't know about fire code but I wouldn't be worried from a hygrothermal standpoint in your climate zone.
It depends on your insulation and climate. A standing seam metal roof on a hot roof (unvented attic) can work fine if you have proper insulation, like closed-cell spray foam, to prevent excessive heat transfer. However, metal roofs can get very hot in the sun, which might increase attic temperatures if insulation isn’t sufficient. Also, be mindful of potential condensation issues—without ventilation, moisture buildup could be a concern. If you’re in a colder climate, make sure there’s an adequate air and vapor barrier to prevent any problems.
Consider something like this photo.
You might find this article interesting the photo is from.
https://buildingscience.com/documents/building-science-insights/bsi-129-wildfire
Walta
Thanks for all the replies, confirmed that our construction is Class A rated, although in retrospect having a layer of densdeck probably would have been a good idea, since portions of the roofing are already installed, this ship has sailed.
Circling back on insulation. We have landed on Batt + Flash for above the living spaces. However, had a question on under the eaves. My architect is suggesting sprayfoam as well there, claiming filling it will help with fire. Contractor is argueing that fiberglass is fine. I'm seeing in the building science documents most of them just show as empty as long as the spray foam is capped.
Any advantage for fire in filling the enclosed eaves with insulation and if so does it matter which type?