Sprayfoam Shortage on Unvented Roof Assembly (Zone 5a)
I intending to place 3.5″ of CCSPF in my unvented 2×12 cathedral ceiling then R19 batts plus open void to meet our required R38. Instead we have a rookie variation between 2″-4″ of spray foam. I have 3 days to remedy, live in a remote market in northern Idaho and can access batt insulation, xps, eps, & Intello plus tapes in that timeframe. I will not have time to order and receive (or drive for) more foam before sheetrock.
How should I salvage the situation?
Data:
Original 2.5 story Craftsman house is 1934 2×4 ballon frame construction, lathe & plaster (zero insulation in the walls) and R19 cathedral ceilings upstairs.
This addition/project brings 50% of main floor up to 2×6 walls with R19 batt and taped Tyvek under siding, and modern double-pane windows.
Despite leaking air like a sieve, excellent solar orientation (and window ACs) keeps the house habitable year-round.
Gas central-air heat retrofitted on an octopus duct system.
The area in question will have asphalt shingles, over fully adhered Grace Ice/Water, 5/8″ CDX w/ V-match on eaves & rakes, 2×12 rafters, & shorted CCSPF,
The plan to complete the assembly was: batt, air gap, 5/8″ gyp, latex paint.
There are 6 2’x4′ skylights and at least a dozen shallow articulating 4″ LED cans in the assembly.
How should I now complete the assembly with the materials I have in order to moisture issues down the road?
Pics attached.
Thanks in advance for your help!
Cheers,
-Nethaniel
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Replies
Nethaniel, what is your climate zone?
Thanks for asking, Michael. I hid that in my title line - Zone 5a.
Ah, hidden in plain sight! The IRC requires R-49 for your zone, but if you are extending the insulation across the full width of the wall's top plate you can reduce it to R-38. Or your local code enforcement may only require R-38.
The IRC provides guidance for condensation control in roofs here: https://codes.iccsafe.org/content/IRC2015/chapter-8-roof-ceiling-construction#IRC2015_Pt03_Ch08_SecR806.5. According to the IRC, you need at least R-20 of closed-cell foam for condensation control. If your code enforcement is not strict about interpreting this requirement, what it's really saying is that the foam needs to be at least 40% of the total R-value. For a total of R-38, that would be R-15.
Closed cell foam ages to about R-5.6/in and this isn't just a game to get past code, it's to protect your house from moisture accumulation, so although I avoid climate-damaging foam whenever possible, in a situation like yours I would not accept less than 40% in the foam. 40% of R-38 is R-15, which leaves you needing R-23 in the batt or fill layer. 6" mineral wool or 8" fiberglass will work for that.
If you decide to have the spray foam contractor come back and they fill the cavities to at least 4" of foam, at R-22 (not the R-28 or higher they will claim), you can add R-19 batts. But with 2x12 rafters it's also a chance to increase the total using 8" batts or fill, for a total of about R-50.
Edit to add: your attached files are in Zip folders. I'm not sure it's important to see your images, but if you could post them directly it would be easier (and safer) to open.
Very helpful. Our insulation does extend the full width of the plate and our jurisdiction has not adopted the most recent versions of the code yet.
It sounds like we generally hit the 40% of R38 threshold with the current foam plus batt fill.
Is the 40% ratio of foam to fill that matters? Condensation issue otherwise?
Sorry about the zip files. Photos from my phone were saving as .HEIC and couldn't be loaded on the site.
Thank you for the feedback. Really helps to hear from a 3rd party.
Cheers,
-N
Yes, it's the ratio that matters most from a moisture accumulation ("condensation") viewpoint. But you should still at least meet the R-38 minimum--that's not very robust for a cold climate. In order to reach R-38 with R-19 batts, you need at least R-19 in the foam layer. 2" to 4" of closed cell foam is about R-11 to R-22. If the average depth is 3", that's R-17, and with R-19 batts you'll only be at R-36.
Any areas of foam that are less than 40% of the total R-value, or about R-15 for an R-38 assembly, are at risk of moisture accumulation. Assuming R-5.6/in for the foam, that means any areas with less than 2 3/4" of foam could develop moisture problems.
This article describes the issues (and BSC was the source for the ratios used in the IRC):
https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies
Here are a couple of articles that Martin wrote that explain the issues:
https://www.greenbuildingadvisor.com/article/flash-and-batt-insulation
https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
And here's one I wrote that explains the moisture accumulation/condensation issue:
https://www.greenbuildingadvisor.com/app/uploads/sites/default/files/Why%20Flash%20and%20Batt%20Makes%20Sense.pdf (I didn't choose the title.)