Addendum to Continuous Insulation Series – GWC Mag

I recently wrote a two-part article covering the pros/cons and code requirements for continuous insulation. In the first article, I referenced table R402.1.3, which is found in Chapter 11, Energy Efficiency, of the 2021 International Residential Code (IRC). The table shows the R-value and U-factor requirements for several building components and assemblies. The data I referenced was taken from the first edition 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 are required to have some amount of continuous insulation.”

GBA reader “jimmybpsu” pointed out in the comments section of the article that he had a different version of table R402.1.3, which allows for cavity-only insulation in Climate Zones 4 through 8. As it turns out, he is right.

Why does my hard-copy code book show one option while the reader’s edition says something else? It’s the difference between print- and web-sourced information. The digital version of 2021 IRC’s chapter on Energy Efficiency allows you to toggle between two editions of the same code. The first version includes table R402.1.3 and is the same as my print edition; the second version shows an updated table allowing for cavity-only insulation.

Why are there two versions of the code?

This begs the question: Why was there an update in code between normal code cycles? I reached out to Glen Mathewson, who writes a regular column, Know the Code, in Fine Homebuilding magazine. He responded:

“The short answer is errata. The staff at the International Code Council (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 errata for all editions from 2000 here. More importantly, you can submit errata when you find it. I submit all the time. At least once a month I find something.”

I’m glad jimmybpsu brought this to my attention. Now I know that the different versions of the table were not a code change between code cycles but rather the result of correcting human error.

How is R-30 achieved with cavity-only insulation?

So long as we are on the topic, let’s discuss the R-30 cavity-only option. Here’s the updated R402.1.3 table:

With 2×6 framing, the only way to get R-30 using common insulation strategies is with closed-cell spray foam. Moving to a 2×8 wall allows for high-density batt options like Rockwool’s ComfortBatt, which achieves R-30 in a 7-1/4-in. batt.

Several manufacturers of high-density fiberglass batts can also fit in the 7-1/4-in. cavity. Standard-density mineral wool products and fiberglass have lower R-values per inch; using these products requires a 2×10 wall. Dense-packed cellulose has R-values ranging from 3.6 to 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 3.7 to 4, this will require a 2×10 wall too.

Technically, to meet the R-30 cavity-only option in climate zones 4 through 8, the insulation material will need to have an R-value of 4.14 per inch to be used in a 2×8 wall cavity. Otherwise, either a 2×10 wall will be needed or a combination of closed-cell spray foam and fibrous insulation, aka the flash and batt method.

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 the use of continuous insulation, in which case the R-30 decision is brilliant.

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Randy Williams is a builder and energy rater based in Grand Rapids, Minnesota. Images courtesy of the author.