I’m working on a future article for Green Building Advisor whose working title is “Problems with Attic Trusses.” While researching for the piece, I found myself reviewing the ratio rules for hybrid roof insulation strategies. GBA has two good articles on the topic:
Combining Exterior Rigid Foam with Fluffy Insulation by Martin Holiday
The Ratio Rule for Hybrid Roof Insulation by Allison Bailes
The intent for my article is to update the information in these two articles based on the 2021 IRC codes, specifically the increase in the ceiling insulation R-values. Here is the table I created for the article:
When I compared this table to the information in the two GBA articles, I saw the ratios were wrong. With the change in table R402.1.3 in the 2021 energy code, there should have been an adjustment made to table R806.5 as well; but the table is the same as in the 2018 IRC.
An expert’s take
To be sure the discovery was indeed a problem, I reached out to Kohta Ueno at Building Science Corporation. Kohta emailed me back with some interesting information:
“Quick back story: Joe Lstiburek originally wanted to write the unvented roof code using ratios but the code officials/code writers said, “Nope, it has to be a table.” (The dumb is kind of painful sometimes.) And so, we have the table of air-impermeable insulation R-values provided in R806.5.
Those R-values were sufficient to result in “safe” insulation ratios for the energy code–mandated roof R-values at the time, and are seen starting with the 2007 IRC Supplement, and officially included in the 2009 I codes (originally in R806.4, now R806.5).
There is nobody paying for ‘ongoing maintenance’ of code requirements, so this table has remained unchanged even though the I Code roof R-values have changed. Also, the R806.5 table never addressed the possibility of people ‘superinsulating’ roofs while still relying on the code-mandated air-impermeable insulation—spray foam or exterior insulation—R-values.
The ratios presented in BSI-100 are what I would trust to keep out of trouble.
Note that there is a ‘math-based’ method for calculating the ratios—take the average of the three coldest month temperatures for outside conditions and use 68°F for inside. Then calculate whether the ‘interface’ temperature between the two insulation materials remains above 45°F (safe) or drops below 45°F (unsafe). This is covered in R806.5.1.4, but I am guessing that nobody has ever done that calculation. It is an old-school method dating to the1960s, and it is relatively conservative, compared to a WUFI simulation, for instance.”
If you are using a combination of air-impermeable insulation above the roof sheathing, with air-permeable insulation below the roof sheathing, follow Joe Lstiburek’s ratio advice. This updated table shows the needed R-values based on climate zone:
I’ve heard that the R-value requirements in the 2024 energy code are going to relax back to the 2018 values. There are still several states, including my home state of Minnesota, that are looking to, or have already adopted the 2021 energy code, making the above table relevant for the next several years. I’m unsure how best to get information like this to the architects, designers, contractors, and homeowners that need it. Luckily, we have sites like Green Building Advisor to help share the knowledge.
Randy Williams is a builder and energy rater based in Grand Rapids, Minnesota.
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