2021 IECC: External Insulation and 2×8 Walls
Mark_Nagel
| Posted in Building Code Questions on
2021 IECC* says that for my zone, 4c, I have to have external insulation. The list, however, only seems to show 2×4 and 2×6 walls. Does this mean that 2×8 walls also would have to have external insulation?
* Reference (don’t have access to the code, so this is an indirect): https://thehtrc.com/2020/insulation-requirements-2021-iecc
Please note that I have my reasons for considering a 2×8 wall. I don’t need to hash them out here. Just wanting to know what the new codes mean for a 2×8 wall.
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The International Code Council has made the codes viewable online - here is the link to the 2021 IECC Residential Provisions: https://codes.iccsafe.org/content/IECC2021P1/iecc-residential-provisions
For your question, I do commercial work so take it with a grain of salt as I don't know all the residential sections. The way I read the prescriptive table R402.1.3 is that it denotes the minimum continuous insulation and cavity insulation per climate zone and would apply no matter the framing size. (https://codes.iccsafe.org/content/IECC2021P1/chapter-4-re-residential-energy-efficiency#IECC2021P1_RE_Ch04_SecR402.1.3)
Couple things:
Check your climate zone against the IECC map (https://codes.iccsafe.org/content/iecc2018/chapter-3-re-general-requirements#IECC2018_RE_Ch03_SecR301)
If there is a reason or need for not doing exterior insulation in a thicker wall there are likely alternative compliance options that you could pursue.
Thanks for the links.
The IRC offers a UA alternative--if the building's overall heat loss is less than it would be following prescriptive requirements, you're good to go. Unless locally prohibited. https://codes.iccsafe.org/content/IRC2021P1/chapter-11-re-energy-efficiency#IRC2021P1_Pt04_Ch11_SecN1102.1.5
Key, I suppose, is what the intent is for the application of external insulation. Their description of external insulation, "Continuous Insulation," seems to emphasize the primary use is to eliminate (as much as one can "eliminate" such) thermal bridging. I believe I have a means of achieving this in a 2x8 wall w/o actual "continuous" insulation: I'll look to start up a separate/specific thread on this- I was holding off doing so until I had a better understanding of what the IECC is really trying to say.
As always, many thanks to the sharp folks here on this site!
The primary specification is U factors for individual components (like the walls). See Table N1102.1.2. The R value table (with continuous insulation) is an alternative that you can ignore.
In addition to code, I'd follow the recommendations here:
https://www.appliedbuildingtech.com/system/files/abtg_rr_1701-01_moisture_control_guidelines.pdf