This post first appeared on the Green Building Advisor Website
I recently wrote a two-part article covering the benefits, challenges and code requirements for continuous insulation. (You can read the articles here and here.) In the first article, I referenced table R402.1.3 which is found in Chapter 11, Energy Efficiency. The table shows the R-value and U-factor requirements for several building components and assemblies. The table data was taken from the first edition printing of the 2021 IRC. In the article, I stated, “Climate zones 1, 2, and 3 are unique in that they have the option of cavity-only insulation. Climate zones 4 through 8 all are required to have some amount of continuous insulation.” A reader on the Green Building Advisor Website, jimmybpsu, pointed out in the comments section of the article that he had a different version of table R402.1.3 which did allow for cavity only insulation in climate zones 4-8. As it turned out, he was right.
Why did my hard copy code book show one option and the reader say his edition indicates something different? You can view the digital version of the 2021 energy code for free here:
In the upper right-hand corner of the web page from the link, there is a tab which toggles you between two additions of the same code. The first version contains table R402.1.3 where I pulled my information for the article from, the second version shows an updated table allowing for cavity only insulation. Why was there an update in code between normal code cycles? I reached out to Glen Mathewson, who writes a regular column in Fine HomeBuilding Magazine covering code topics, for the answer. Glen stated:
“The short answer is “errata”. The staff at the ICC has one heck of a tough job combining all the approved proposals, including changes made to the proposals during the hearing process. Many proposals overlap the same code sections. So human error always happens. The first printing of a code is always the worst one. If you can wait to purchase, wait until the second printing where mistakes are fixed. ICC provides all of the errata at a website. You can look up errata for all editions from 2000. More importantly, you can submit errata when you find it. I submit errata all the time. At least once a month I find something.
Here is the link to errata central. https://www.iccsafe.org/errata-central/”
Now I know the different versions of the table were not a code change between code cycles but simply correcting human error.
As long as we are on the topic, let’s discuss the R-30 cavity only option. Here’s the updated R402.1.3 table:
How do we achieve the R-30 cavity only option? If you are building with 2×6 framing, the only way to achieve R-30 using common insulation strategies is by choosing closed cell spray foam. Moving to a 2×8 wall, you can use several of the high-density batt options like Rockwool’s ComfortBatt which achieves an R-30 in a 7.25” batt. Several manufacturers of high-density fiberglass batts can also fit in the 7.25” cavity. Standard density mineral wools and fiberglass have lower R-values per inch, using these products, you will need to move to a 2×10 wall. Dense packed cellulose has R-values ranging from R-3.6 to R-3.8. A 2×10 wall cavity will be required when using dense packed cellulose as well. How about open cell spray foam? With R-values ranging from R-3.7 to R-4, this will also require a 2×10 wall.
Technically, to meet the R-30 cavity only option requirements in climate zones 4-8, the insulation chosen will need to have an R-value of R-4.14 per inch if used in a 2×8 wall cavity. Otherwise, you’ll be using a 2×10 wall or you will need to use a combination of closed cell spray foam and fibrous insulation, a flash and batt strategy. In my opinion, the industry would have been better served by lowering the cavity only option slightly to allow common, less dense insulation options to satisfy the code requirement. Then again, maybe the overall goal is to encourage continuous insulation use, in which case the R-30 decision looks brilliant. Just my opinion.