Prescriptive Code vs. Performance Plan
I follow a few builders that occasionally insulate with closed cell foam. One such builder in Climate Zone 5 recently used 4″ in the roof and 3″ in the walls, giving up to R-28 for the roof and R-21 for the walls (but likely less in both cases). Code in Zone 5 wants up to R-49 in the roofs, so that’s not even close.
My understanding is that builders using closed cell will typically meet code by ignoring the “prescriptive code” and instead use something like REScheck to meet the “performance plan”. This does an extensive modeling of the house to show that the resulting energy performance of the house is equivalent to a “reference house” in that zone that is compliant with the prescriptive code. That is, this house will be just as energy-efficient with R-28 roofs as one with R-49 due to how “tight” the rest of the house is.
But how is that even possible unless the prescriptive code is essentially assuming that houses that meet code aren’t going to be built very well? If I build a house in Zone 5 that actually meets all of the prescriptive code and ensures that it’s air sealed to 1 ACH and is sited to favor sun in the winter but shaded in the summer and has an efficient heat pump for HVAC, then wouldn’t that house be far more energy efficient than a similar one that is using less insulation? Doesn’t that further imply that the “reference house” must be relatively poorly made to not be notably better, too?
In the end, I can see three possibilities:
1. The “performance plan” doesn’t actually produce homes as efficient as one matching prescriptive code
2. The prescriptive code is wildly overspec’ed by design, assuming that the house is built relatively poorly overall and the excess makes up for it
3. I am completely missing something in all this
It’s probably option 3, honestly. What am I clearly missing or misunderstanding?
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In my experience, it's #3. Spray foam installers (and manufacturers) lie shamelessly about their products' performance, consistently claiming up to R-8/in and conflating air sealing with insulation. The "total building performance" path is fairly involved (https://codes.iccsafe.org/content/IRC2021P1/chapter-11-re-energy-efficiency#IRC2021P1_Pt04_Ch11_SecN1105). I know many code officials who either don't understand the energy code or don't believe it should be enforced, so if their paperwork says it's compliant, they are satisfied.
To meet prescriptive code, spray foam installers, like every other insulation installer, should be required to meet the full required code value, preferably using the aged R-value (for spray foam, that's R-5.5 to R-6 per inch) but at the very least they should use their official values, tested after 6 months of carefully controlled aging, which is usually around R-7/in--so at bare minimum, they should install 7" of closed cell foam to reach R-49. Anything less is illegal and there should be consequences, but the market (and code officials) are so enamored of spray foam that they get away with installing a lot less than is legally required.
Twelve years after Martin Holladay wrote this article, little has changed on this topic: https://www.greenbuildingadvisor.com/article/its-ok-to-skimp-on-insulation-icynene-says.
+1 for this being "option 3".
I regard spray closed cell spray foam as R6/inch. Claims of R7/inch are with pretty new foam, but that R value number drops over time as the foam ages and the blowing agents diffuse out. It's safer to use R6/inch for design purposes with this material.
The other common claim is something along the lines of "spray foam is so awesome for air sealing that you can use less R value!". That's not really honest, since if you do a proper job of air sealing before using any kind of insulation you're right back to just considering the R value again, and code doesn't allow for that "added benefit" of spray foam anyway.
R28 in the roof didn't even meet code back in the R38 days. New R49 requirements, and potential even newer R60 requirements, would require nearly 9" of closed cell spray foam even using the optimistic R7/inch numbers.
I'm not a fan of using spray foam as a "regular" insulating product -- I think of it as a niche product for things like unvented cathedral ceilings and a few other applications where spray foam really is the best option. A vented attic with loose fill insulation is the the cheapest, and often also the best performing, attic insulation method. For walls, you're really better off using batts or dense pack cellolose in the walls and putting the monetary savings (compared to spray foam) towards continous exterior insulation (usually rigid foam). You'll get a MUCH better performing wall using continuous exterior insulation and batts compared to just filling the stud bays with spray foam. If you really want to use spray foam in a wall, just use open cell -- it's cheaper, and it will get you a full fill since it will be overfilled at install and then trimmed flush.
Bill