Roofing Underlayment Options
E247
| Posted in General Questions on
I’m building a new house that will have a conditioned attic with open cell spray foam applied directly to the underside of the roof deck. The roof will be asphalt shingles. What is the best underlayment for this application? The climate zone is 2; hot/humid.
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First, per code, you need to install air and moisture impermeable insulation against the roof decking. 2" of spray close cell foam follow by 7" of open cell foam, for an R38 minimum insulation. With CC Foam under the roof decking, you should install a permeable roof underlayment, like #30 roofing felt. This is the simplest way.
Another option is to install 1" rigid foam on top of the roof decking, followed by a second nailing sheathing, permeable underlayment and shingles. Some folks like to install 1x4 purlins between the rigid foam and the second sheathing for a ventilated roofing assembly; using this system, you can install synthetic roofing underlayment above the second roof sheathing.
I sandwiched 4 inches of rigid foam between two layers of zip roof. I know I don’t technically need an underlayment before I shingle but it makes me nervous. What would make sense. Seattle WA climate.
Why is an air and moisture impermeable insulation against the roof decking required? I thought that it was fairly common practice to have open cell spray foam directly to the roof decking.
E247, for one thing, because the building code requires it--see table R806.5 here: https://codes.iccsafe.org/public/document/IRC2015/chapter-8-roof-ceiling-construction. The table lists values of impermeable insulation (closed cell foam) to permeable insulation (open cell foam) to prevent moisture accumulation. For zone 2, the table says a minimum of R-5 is required, which would be just one inch of closed cell foam, but that's hard to get accurate so 1 1/2" or 2" is safer. Or 1" of polyiso rigid foam, or 1 1/2" of EPS or XPS insulation. Some foam installers think it's safe to use open cell foam alone, but read this for the full story: https://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing.
I don't think roof underlayment below asphalt shingles needs to be permeable, because the shingles are not permeable, but there is no harm in using a permeable underlayment. The 30# felt Armando mentioned is a good, traditional choice, and there are many others to choose from.
Sorry, I am planning on using Icyene Classic Max which is air impermeable, but vapor permeable. If this is the case, then I don't need to use closed cell foam first nor do I need to install rigid insulation over the roof decking; is that correct?
E247, if it is air permeable then it will meet code, but it's still a generally risky approach. Here's an excerpt from Martin's article that I linked to above:
"Rules of thumb for builders
Here are my recommendations for builders who use spray foam to create a sealed, conditioned attic:
Recognize that this type of roof assembly carries more risk than a vented, unconditioned attic. Keeping ducts within the conditioned space of a building (not in the attic) is preferable to installing spray foam on the underside of roof sheathing.
f you want to lower the risk of damp OSB, choose closed-cell spray foam, not open-cell spray foam, to insulate the underside of the roof sheathing.
To further limit your risk, consider installing ventilation channels above or directly below your roof sheathing.
If you choose to install open-cell spray foam against the underside of roof sheathing in Climate Zone 5 or colder zones, make sure that you include an interior vapor retarder.
If you choose to install open-cell spray foam against the underside of roof sheathing in a humid climate, your HVAC system be designed to condition the attic air and lower humidity levels in the attic."
The plan is to have a AC supply and return in the attic to keep the humidity levels in the attic space low and allow for the sheathing to dry to the interior.
For this application, would it make more sense to have the roofing underlayment be vapor impermeable?
Joe Lstiburek seems to think that as long as you have AC supply and return in the attic, open-cell foam should be okay in your climate zone. But of course, a layer of close-cell is a more conservative solution.
Here's an article describing the mechanism by which open-cell foam can cause moisture problems in a roof, and how conditioning the attic can help alleviate the problem: https://buildingscience.com/documents/building-science-insights-newsletters/bsi-016-ping-pong-water-and-chemical-engineer
E247, I don't see a reason why the vapor permeance of the roofing underlayment matters in this case, since the roof can't dry to the exterior anyway. There is no problem having two vapor impermeable layers (roofing and underlayment) if they are in contact with each other.
For this application (open cell spray foam with asphalt shingles) in climate zone 2, would it make sense to install a vapor diffusion vent as shown in the link below? This would be in addition to adding a supply/return from the AC system in the conditioned attic.
https://buildingscience.com/documents/insights/bsi-088-venting-vapor