This post first appeared on the Green Building Advisor Website.
I’ve been working on a future article for Green Building Advisor that has a working title of “Problems with Attic Trusses”. During research for the piece, I found myself reviewing the ratio rules for hybrid roof insulation strategies. GBA has two really good articles on the topic:
Combining Exterior Rigid Foam With Fluffy Insulation – GreenBuildingAdvisor by Martin Holiday
The Ratio Rule for Hybrid Roof Insulation – GreenBuildingAdvisor by Allison Bailes
My intent for my piece was to update the information of 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.
Extracted from the 2021 IRC Table R806.5 Insulation for Condensation Control and Table R402.1.3 Insulation Minimum R-values and Fenestration Requirements by Component.
As I began comparing the table to the two articles linked above, I realized that the ratios were incorrect. With the change in table R402.1.3 in the 2021 energy code, there should have been an adjustment to table R806.5 as well. That table is the same as in the 2018 IRC. 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 kinda painful sometimes. Thus, 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, so starting at the 2007 IRC Supplement, and officially included in the 2009 I codes (originally in R806.4, now shifted to 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.
Takeaway is that 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, 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 statistically, nobody has ever done that calculation. For reference, this is an old-school method going back to 1960’s Canadians… it is also relatively conservative, compared to a WUFI simulation, for instance.”
The takeaway, if you are using a combination of air impermeable insulation above the roof sheathing, with fluffy insulation (including open cell spray foam) below the roof sheathing, follow Joe Lstiburek’s ratio advice in BSI-100: Hybrid Assemblies | buildingscience.com. This updated table shows the needed R-values based on climate zone.
I have 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 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.