More spray foam: maximum rafter cavity fill vs underfill and covering the rafters
I live in Nashville, TN, CZ 4. We built an unvented unconditioned attic with complex geometry and three cathedral ceilings! So we are stuck with foam. The quotes we have gotten are just ridiculous. Not just in terms of expense, but what they try to pass off as compliant with performance code. (Thanks to Armondo, Bill, Michael, and others who helped me understand the performance code sham. But now that I know how ridiculous they are being, it’s even more enraging) Anyway, we have 2×10 rafters. All three companies we received quotes only put 5.5 inches of open cell on sloped attic roof assembly. Code here is R49 btw. So far, one company is proposing 2 inches of CCSPF and 8 inches of OCSPF. At a similar price, they have suggested they can also do 2 inches of CCSPF and 5 inches of OCSPF, but they will completely cover the rafters with foam. I am not sure which solution is better. Should I just max the cavity and not worry about the thermal bridging? Or should put in less cavity fill and stop the thermal bridging? As always, thanks for looking. I really appreciate you guys and all the expertise you put out for the world at large
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The option for 2" CCSPF and 8" of open-cell, including thermal bridging (and resistance) of the rafter, yields about R-19 for the assembly. That's using R-6.5/in for closed cell, R-4.0/in for open-cell and R-1.2/in for framing lumber.
The option to use 2" of closed cell and 5" of open-cell will likely be more like 3" of open-cell around the rafter. In that case, the assembly will perform somewhere around R-25. If they actually use 5" of open-cell on all sides of the rafter, the assembly will perform higher than R-25, of course.
Michael,
Forgive my ignorance. Just a question about the math. I know the raw values of the closed cell, open cell, rafters. But how do you arrive at the final R19?
I am not sure how the foam guys are planning to accomplish these goals. I would assume they will put the minimum possible and still comply with the quote. So I would imagine they will put 5 inches in the cavity. But as for 3 inches along all sides of the rafters (as opposed to 1 or 2 inches), it’s hard to know.
Atleast if they put 2 + 8 inches I can check and know, yah, they put in 10 inches, as it should be brimming with rafter edges. Gets harder to critique their work if we opt to foam the rafters for thermal bridging.
Bottom line: you feel the total rvalue of the assembly is better served by covering the rafters rather than filling the bays.
That is very helpful. Thank you!
Many people think that you simply average the R-values but that's not how it works. You calculate the U-factor for each "slice" through the assembly, multiply that by the relative area, then convert back to R-value and add those together. It is often surprising how large an effect thermal bridging has on performance.
I'll run through the calculations quickly:
((1/45)(22.5/12))+((1/11)(1.5/12)) = 0.042 + 0.011 = 0.053. That's the combined U-factor; the inverse is R-19. There is always a chance that I have a math error but with R-11 rafters and R-45 cavity insulation, R-19 seems about right for the assembly.
Units for U-factor are Btu/h•ft²•F, so in the calculations I divide the inches of width (22.5 or 1.5) by 12 to get their equivalent in feet.
I think your math is off. You're "multiplying it by the relative area" when you should be taking the area-weighted average.
((U-1/45)(22.5in) + (U-1/11)(1.5in)) / 24in = U-0.0265 = R-37.7
Because you're taking the weighted average of the two U-values, there is no need to convert from inches to feet, because when you divide by 24 in, the units all cancel out.
We're both assuming the rafters are at 24" O.C.
If they were 16" O.C., the formula would be:
((U-1/45)(14.5in) + (U-1/11)(1.5in)) / 16in = U-0.0287 = R-34.9
You're right; that's what I get for not double checking my approach!
Thanks for the math. Forced to get my calculator!
I am sorry, I’m not sure why my question comes in as a big block of text. I promise I used paragraphs!
As far as R value, there are plenty of insulation options that are about the same or better as OC. High Density Fiberglass batts are 4.3 per inch. Foam Boards go from 4 on up to 6.
Thermal bridging will always be there, regardless of what kind of insulation you use. The only way to really deal with that would be to "bury" the rafters in foam, or have the foam trimmed flushed with the inside edges of the rafters and put some rigid foam up over everything.
If you completely fill the assembly, and assume R6/inch for closed cell and R4/inch for open cell, you have about 9.25" worth of foam sandwich in between the 2x10 rafters. I'll assume R1/inch for the rafters. If you have rafters 24" on center, a simple average gives a whole assembly R value of about R39. Actual performance will be slightly less than a simple average in this case, but it's close. If you underfill the cavities significantly, leaving lots of exposed rafter face, overall performance will be more significantly less than a simple average would show due to the "fins" of the uninsulated rafters acting a bit like heatsinks.
Are you going to have some kind of finished ceiling surface directly beneath the rafters? If you will, then you can't really overfill the cavities and will have to trim things flush. If you then put 1.5" polyiso under the rafters, you'd be at R48 for the assembly, but you'd be at R50 for code purposes (since the thermal bridging doesn't factor in for the "R49" code requirement).
Note that I think only 2" of closed cell is a bit light here, I would try for 3-4". The reason I give a range is that you really need at least about 3" for a truely safe assembly (R43 total for the sandwich, R18 of that from closed cell for a bit over 40% of the total mix), and it's difficult for the installers to ensure the closed cell layer is really AT LEAST what you want EVERYWHERE. You need a minimum thickness EVERYWHERE, average isn't good enough for moisture control. Asking for more than the minimum requires helps to ensure that there are no thin spots that are less than required for moisture control.
Bill
Bill, when you have a thermal bridge--aka greatly different R/in values--simply averaging does not get you particularly close. You need to calculate the UA for each component, add those components and then convert back to R-value.
I think Bill has either correctly done an area-weighted average of the U-values to arrive at R-39, or this is one of those goofy cases where you can average the wrong things and still get (approximately) the right answer!
For what it's worth, and this is definitely not to say that code minimum is the right choice, many regions have different roof insulation requirements for sloped ceilings vs flat ceilings.
For example, in Oregon (where I work), a flat ceiling needs to be R-49, but a sloped ceiling (such as spray foam between roof joists) is only required to be R-38. That's just a code thing that applies to all types of insulation, and isn't related to the dodgy math spray foam companies sometimes try to get away with.
That is very interesting. I did not know foam had a special place in the codes, which they could allow them to hit a lower Rvalues in flat versus sloped assemblies. Thanks for pointing this out
It's not foam specific in the Oregon Residential Specialty Code.
Sloped ceiling (any insulation type): R-38
Flat ceiling/attic: R-49
There are a few other caveats and exceptions, but those are the values in the majority of situations.
Worth checking your local code just to understand what the rules are, even if you end up shooting for R-49 anyways!
Bill,
One of the cheaper quotes we got was for 3 inches of CC only. No OC at all.
Could I just use that 3 CC and then just tack say 2 or 3 inches of polyiso on the rafters? This would leave an air gap between the CC and polyiso which makes uncomfortable. Especially given that i really wouldn’t be able to verify how well they taped off the seams between the rigid foam. For this thought exercise please assume the subs do a average normal/below average job of air sealing the can lights etc. (basically allowing for moisture seeping into the attic)
Thanks for this suggesting this idea.
3" of closed-cell foam, + filling the rest of the cavity with fiberglass batts is a straightforward and relatively common assembly (often referred to as "flash and batt").
Something to consider if that ends up being cheaper than closed-cell + open-cell. That's what mdhomeowner was getting at in their comment.
Your comment about the can lights makes me think that it is unclear where your air barrier is, which you definitely want to be confident about. Typically when you do spray foam in the rafter bays, the spray foam ends up being an air barrier, and the attic floor is no longer an air barrier, so it doesn't matter if the can lights leak air, because the attic is now interior conditioned space. It's a room just like any other in your house.
Looking back at your initial post, you said "unvented unconditioned" attic. That's probably not what you want. The two good options for attics are:
"vented, unconditioned"
or
"unvented, conditioned"