Is continuous insulation really required?
user-945392
| Posted in Building Code Questions on
For climate zone 6, the “Insulation and Fenestration Requirements by Component” table of the 2012 IRC (and more recent versions) gives the option of either 20+5 or 13+10 cavity insulation plus continuous insulation (for wood frame walls). So, looking only at that, continuous insulation is required. But looking at the table for “Equivalent U-Factors,” there doesn’t seem to be any such stipulation for continuous insulation. So why am I consistently reading everywhere that continuous insulation is now required by code? Where in the code is continuous insulation a requirement regardless of which approach one takes?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
R20 is a 2x6 with any of the fluffy insulations (fiberglass, cellulose, rock wool, open cell). R13 is a 2x4. Foam board is R5 per inch. So they're saying you can meet code with a 2x6 wall with one inch of foam or a 2x4 wall with 2 inches. Or you can come up with whatever wall assembly you want that, as an assembly, meets the equivalent U-factor requirement.
That's how I understand it (not that 1" of foam on a 2 x 6 wall in climate zone 6 makes any sense from a durability point of view... but that's another issue!). So why does everything I read say that continuous insulation is required by code? That's what I'm struggling with...
> So why does everything I read say that continuous insulation is required by code? That's what I'm struggling with...
I think the answer is really simple. Few people read the actual code. Fewer people read the code and notice anything but the prescriptive fluffy + continuous options. And the fewest number of people read the U-factor alternative section and actually understand it. :-)
Probably because you can’t meet the current energy codes for normal (not fancy stuff like double stud, etc) 2x4 or 2x6 walls without at least some continuous insulation.
Personally, I like exterior rigid foam, but you can meet current energy code with cellulose filled double stud walls WITHOUT continuous insulation, among other methods. You just end up building a wall that is considered unusual (and thus also mysterious and “hard to build”) by most builders. Everyone knows how to hang and detail 1” or less exterior rigid foam so that’s “normal” and “easy to build”.
If you find a good builder that is familiar with building houses with well beyond code levels of insulation, building acceptable structures without the use of continuous insulation isn’t really all that difficult.
Bill
I'm a big fan of continuous exterior insulation, too. I just didn't want to tell a client that it's required if, in fact, it's not, which seems to be the case. Thanks for the responses!
Will,
Looks like you already have your answer but Martin wrote a great article on this topic a couple of years ago that may be of help:
https://www.greenbuildingadvisor.com/article/three-code-approved-tricks-for-reducing-insulation-thickness
Generally speaking states and/or localities are free to adopt and/or modify energy codes however they see fit. This is why there's a disconnect between "current code" and that which is required in your area.
Will,
I think you're getting tripped up on the difference between the prescriptive and performance requirements of the code. Or at least those who say that continuous exterior insulation is "required" are getting tripped up on the difference. Continuous exterior insulation IS required if you are following the prescriptive path.
The R20+R5 continuous requirement is a prescriptive requirement. If you do that you automatically meet code. So, if you're doing standard construction with nothing fancy, the continuous insulation is a requirement. However, the code also allows a performance path, through the total UA calculations. The eventual building performance is the same, you just take different paths to get there. In one path, continuous insulation is required. In the other, it's not.
It's kinda the same as sizing floor joists. You can go to the IRC span tables and find that a 2x10 Douglas Fir floor joist at 16" on center can span 14' or so. So, you could say that for a 14' span, a 2x10 joist is required. And you would be right. But the tables also give you other prescriptive options to get there, as well as performance options. If you've got a structural subfloor that can span 32", you could space your floor joists at 32" on center, but there aren't any tables for that. So, your engineer would use the performance requirements (live loads, dead loads, etc.) to decide what size floor joists are required. The overall building would still meet code, but it would use a different path to get there.
Wow, thanks for all the thoughtful responses!
@Peter Engle, oddly I'd never really thought through "prescriptive" vs. "performance."
Previously on the "to do" list, but no longer, as your joists explanation made it perfectly clear. Either way, though, I wasn't getting tripped on the difference, as I knew there were two "paths," one of which really didn't seem to require continuous insulation. Everyone here seems to have confirmed my suspicion that everything I'd been reading (elsewhere) was maybe written by people that were getting tripped up on the difference.
@John Clark, my question is somewhat academic, as here in Maine, the older 2009 IECC is used. I've been trying to use the current code in support of upgrading a project's building envelope.
@ Rick Evans, I'll check out the article.
Thanks!
>" So why am I consistently reading everywhere that continuous insulation is now required by code?"
You are reading that "....everywhere..."? Show me where! (Maybe I don't utilize enough different sources?)
Compliance via U-factor is good enough. (<< Period, full stop.)
I've recently seen a house (on my usual commute route) being built with 2x8 studs 16" o.c. with ZIP sheathing. It remains to be seen if they will be installing continuous insulation, but I'd be surprised if the did. (Code min there is 2x6/R20). So far the poured concrete foundation is completely uninsulated- not sure how they'll deal with that either.
I have read that in AK 2x8/R30 rock wool a polyethylene vapor barrier on the interior is a fairly common solution since it's easier & cheaper to build than any continuous insulation solutions of similar performance. From a resilience point of view the integrity of the vapor barrier's air seal is critical in Zone 8. Continuous insulation on the exterior adds quite a bit of resilience in less severe climates.
Another house being built on my commute has 2x6 w/ZIP, with 2" of rigid rock wool strapped in place 1x4 vertical furring, with cedar shakes mounted on 1x4 girts mounted to the furring. In a zone 5 or 6 climate that assembly is extremely resilient no matter what they put between the studs, no matter what the interior side stack up is.
Maybe I should have said "in many places" instead of "everywhere"...
I haven't necessarily read all of these in their entirety, but when they say "continuous insulation is required by code" (or similar) I typically didn't expect to find a contradiction to that later in the same source.
https://www.buildings.com/buzz/buildings-buzz/entryid/517/making-sense-of-continuous-insulation
https://www.constructionspecifier.com/energy-codes-and-continuous-insulation/ (Reading this again, I see that ultimately it does not describe continuous insulation as the only option, and I may have originally missed that its stipulation was for steel-framed buildings)
https://www.echotape.com/blog/continuous-insulation-care/
https://www.bautexsystems.com/blog/best-practices-for-continuous-insulation-in-exterior-walls
https://www.insulfoam.com/continuous-insulation/
I just stumbled across this: http://ajandris.com/continuous-insulation-not-a-code-requirement-2/ which suggests I'm not the only one to have been mislead by a lot of what I'd been reading...
You can also use a computer software (RES Check) in many states. RES Check provides a simple all-house analysis. You might be able to have less wall insulation if you have better windows, etc. Very easy to use. Or you can use any program that provides compliance with applicable ASHRAE code.