Insulation for condensation control
When combining closed cell + open cell in unvented attics & rafters (with at least 2″ of closed cell direct to underside of rafters):
1.) do the same ratios given in IRC 2015 Table R806.5 apply? The terminology refers to min R-values for the “air-impermeable” insulation, however presumably BOTH are AIR-impermeable (at certain thicknesses…say 2″ for closed cell + 7″ for open in Climate Zone 5). I consulted Building Science Corp’s article, https://buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies#_ftn7, but even Joe Lstiburek describes in Fig. 6 the open cell as the “air permeable insulation,” which confuses me. I understand it to be vapor permeable but believed it to be considered an air barrier at that thickness, is it not?
2.) Related question is would 2″ of closed cell be sufficient in Climate Zone 5 mitigate moisture accumulation within the 7″ of open cell on very cold nights? And if not, is it a concern or does open cell have enough capability to dry itself out before there is a problem? [I would actually rather have a total R49, so would then go to 4″ of closed cell (rafters are 2x12s), but am curious as there is a cost impact.]
3.) Lastly, at 7″, does the vapor permeability of open cell become more like a Class III vapor retarder? I can’t find data other than at 5″, it is about 10 perms, so I am assuming more than 5″ would be somewhat less perms.
(No need please to bring up discussions of venting vs. non-venting, insulating above deck, etc., my question refers to this specific intent).
Thank you!
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Replies
Update in regard to item #2, I decided to go with 4" of closed cell (R26) + 5" Roxul batts (R23) which will give me R49 total as well as an ignition barrier with the Roxul.
But I would still like to know people's thoughts on questions 1 & 3. Thanks!
Julie,
You are correct that open-cell spray foam is not considered air-permeable. It is relatively vapor-permeable, however.
For more information on this issue, see "Air Leakage Through Spray Polyurethane Foam."
While open-cell spray foam may be somewhat more protective of the potential condensing surface than fiberglass or mineral wool, it's still a good conservative approach to use the same ratios for closed-cell to open-cell as the ratios recommended for foam-to-fluffy in roof assemblies that combine rigid foam and fiberglass batts (which are generally the same ratios recommended for flash-and-batt assemblies).
For more information, see "Combining Exterior Rigid Foam With Fluffy Insulation."
If you are in Zone 5, a good conservative approach would be for 41% of the total R-value of the roof assembly to come from the closed-cell spray foam layer. If you are aiming for R-49, that means that you need R-20 to come from the closed-cell spray foam (about 3 inches), with the remaining R-29 coming from the open-cell spray foam (about 8 inches of open-cell foam).
Q. "Would 2 inches of closed cell be sufficient in Climate Zone 5 mitigate moisture accumulation within the 7 inches of open cell on very cold nights?"
A. No. In that assembly, the total R-value is R-38.9, and the closed-cell spray foam layer has an R-value of R-13, which means that the closed-cell foam layer is providing only 33% rather than 41% of the total R-value of the assembly.
Q. "At 7 inches, does the vapor permeability of open cell become more like a Class III vapor retarder?"
A. Different brands of open-cell spray foam have somewhat different permeances, but I would put the vapor permeance of 7 inches of open cell at about 7 perms. That would, indeed, make it a Class III vapor retarder.