Interpreting 2021 IRC Energy Codes
Looking for advice/feedback on the 2021 Energy codes.
If I am understing correctly, the 2021 codes for climate zone 5 are as follows:
R20 + 5ci,, R13+ 10ci, or 0+ 20ci. Based on the example of R20 +5ci, this would entail a 2×6 exterior wall with a min cavity insulation of R20 PLUS a min of R5 of continuous insulation.
This seems to conflict with what I have been taught, or at least understood. I am under the impression that you would need at least an R7.5 of continuous insulation over a 2×6 exterior wall, to avoid in condensation within the wall cavity for this climate zone.
If my interpretation of the new codes is correct, then would it be correct the following construction would be safe: 2×6 exterior wall, mineral wool for wall cavity ie Rockwool R23 (to prevent any possible deterioration of the insulation if condensation were to build up in the wall cavity), Zip R6 sheathing, and a smart vapor retarder (ie:Membrain)?
I realize there are other products/solution that could meet the R5 ci, so looking for advice on what to avoid/best practices.
Sincerely,
Jason
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Replies
Hi Jason,
I checked and your understanding of the 2021 IRC wall insulation requirements for Zone 5 seems correct per table N1102.1.3 (R402.1.3).
I'm not sure where you're getting the "7.5 ci to avoid condensation" rule of thumb... the actual likelihood of condensation depends on vapor barriers, humidity, operating conditions, etc. In principle the code presents a coherent set of requirements for insulation & airtightness, but 1) there's no way for it to account for every variation [zones are big and variable, people have variable operating preferences], and 2) the code development process ultimately specifies "the worst building you are legally allowed to build" as arrived at by a consensus process, meaning that you might have any number of good reasons for going beyond code.
If you use a good envelope system to control air movement while allowing vapor permeability, with proper overhangs/drainage/rainscreens etc., you will generally be fine.
I think the ICC code development process is quite impressive as contentious regulatory development goes. That doesn't make it perfect. One weakness is that more complicated issues that cross several regulatory boundaries can be hard to handle with consistency and scientific rigor (when that's even available).
Warmly,
Keith
Keith,
The R7.5 comes from Martin's article: https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing I think based on the 2018 IRC.
It’s not just Table 1102.1.3 you must comply with, but in climate zones 4-8, you are also required to use vapor retarder in your wall assemblies. Depending which vapor retarder material and class you choose, you maybe required to use R7.5 over a 2x6 wall. See 2021 IRC R702.7.
In addition, in many Western and mountainous counties, you could have more than one climate zone depending on altitude. For that reason, one must know how to perform a hygrothermal analysis to be sure.
Put simply, the code specifies prescriptive options for thermal performance and it also specifies prescriptive options for vapor performance. Sometimes the way you handle one locks you into how you handle the other.
Creative Rehab,
The code problem you discuss (the "R-20+5" problem) has been around for years. I wrote about the problem in 2014: "The 2012 Code Encourages Risky Wall Strategies."
Worries arising from this code glitch may be overblown, however. You should also read another article -- one I wrote in 2017: "Rethinking the Rules on Minimum Foam Thickness."
Thank you all for your replies. It appears the new updates are requiring exterior insulation on all new builds in Zone 5, and am trying to navigate the process to avoid potential issues. There seems to be some ambiguity on the best practice on how to achieve this, without creating potential risky design. Hopefully the industry as a whole can come to a better consensus, which ultimately has a better outcome.
> It appears the new updates are requiring exterior insulation on all new builds in Zone 5
That's one option. Higher R-value walls with conventional insulation are also a possibility.