Low-Sloped Roof: Open- or Closed-Cell Spray Foam / Rigid Foam on Top of Decking
Hi,
Homeowner here. Our plan for a new construction residence calls for a low-slope roof (1.5:12). We are currently attaching 3/4″ CD plywood to the rafters as the roof decking, and then a layer of Grace underlayment, and then 24ga standing seam metal on top of that (dark color). Underneath the decking, the specs call for 5.5″ of open cell spray foam. The bid indicates that this will result in R20 for the roof, and should qualify for the “performance” method of compliance when coupled with the correct HVAC system.
A couple of questions:
1. From my reading of various materials on this site, seems like there is advice to not use open cell spray foam, and instead use closed cell. [https://www.greenbuildingadvisor.com/article/open-cell-spray-foam-and-damp-roof-sheathing – ‘During his presentation, Miller warned against the use of open-cell spray foam to create a conditioned attic. “The roof sheathing is humid when open-cell spray foam is used,” he said.’]
2. The readings seem to indicate that an unvented approach with rigid polyiso on the outside of the decking is a better approach, with open cell on the inside of the decking to get to code level R-value. [https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing%5D. My confusion between the articles is that this one says open cell is fine underneath the decking.
3. It sounds there is no benefit to venting between the decking and the metal roof given our slope is not steep enough for the stack effect to apply.
Any comments/advice on whether the initial spec will work for this roof, especially the use of open cell, and whether exterior insulation will actually make this roof perform better?
Thanks
-sunil
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Replies
Sunil, what is below the rafters and open cell foam? Is this in effect a low slope cathedral ceiling, with drywall attached to the underside of the rafters? Or, ... ?
@Darren Adams
It is in effect a low slope cathedral ceiling. The drywall is attached to the underside of the rafters, so any open space is simply a coincidental byproduct of the open cell spray foam not packed to span the 2x6.
> no benefit to venting ... given our slope is not steep enough
Wind is the primary driver of airflow through roof vents and it continues to work to some extent even with no slope. You can make up for the lack of convective drive with a larger vent.
I'd go with Joe L wrt open cell being fine as long as the interior side is conditioned. But you still need to comply with code (eg, IRC 806.x).
@Jon R, thanks for the article reference, makes me feel more comfortable with open cell if we properly design for the moisture concern
Hi Sunil,
Martin Holladay answers your question in this article: How to Install Rigid Foam On Top of Roof Sheathing. Here is some of what he says:
A roof over a vented, unconditioned attic does not need to include any insulation. However, most cathedral ceilings and low-slope (flat) roofs are insulated roof assemblies: with this kind of roof, the insulation follows the slope of the roof.
Insulated roof assemblies can be vented or unvented. There are lots of different ways to insulate this type of roof, but one of the best methods calls for the installation of rigid foam insulation above the roof sheathing.
Note that if you follow this path, the minimum R-values for the rigid foam layer don’t satisfy the full insulation requirement of the building code. You’ll still have to install some type of insulation under (and in direct contact with) the roof sheathing to make sure that the R-value of the assembly meets the R-30 requirement (in zone 1), the R-38 requirement (in zones 2 and 3), or the R-49 requirement (in zones 4 through 8).
A wide variety of insulation materials can be installed under the roof sheathing. Among the possibilities: fiberglass batts, mineral wool, cellulose, or open-cell spray foam. (Closed-cell spray foam is not recommended for this purpose, since closed-cell spray foam prevents the roof sheathing from drying toward the interior if it ever gets damp. For the same reason, this type of roof assembly should never include an interior polyethylene vapor barrier.)
@Kiley thank you. I thought that Martin's article was pretty clear, but when I read the other one I referenced I got worried about open cell because it seemed to take a strong stand against using open cell in any climate. I may have misinterpreted that?
The Joe L article that Jon R shared was also really helpful, and I have enjoyed reading other articles by Joe L - seems like everyone references him.
Thanks everyone for the responses, my takeway is that an unvented approach with open cell is fine, so now I have a follow up question on airflow into that space.
We are looking to have two separate distribution systems related to HVAC - one for conditioned air, and a separate one for dehumidification. Can I just run the supply/return for the dehumidifier into this space, or does it need to be conditioned air? Seems like the former would work fine if we are just worried about moisture control, but I'm not clear on how stringent we have to be when we say the attic should be 'conditioned'.