Three years ago I wrote an article about a problematic recommendation in the 2012 building codes — namely, the “R-20+5” recommendation for walls in Climate Zones 6 through 8. This recommendation — actually, a prescriptive minimum R-value requirement for walls — gives code approval to walls with R-20 fiberglass batts and R-5 exterior rigid foam. (Image #2, below, shows the relevant code table.)
In my article, I noted that 2×6 walls with exterior rigid foam in Zones 6 through 8 need thicker foam than the R-5 foam listed in the prescriptive table. In Zone 6, rigid foam on the exterior of 2×6 walls needs to be rated at R-11.25 or more, while in Zones 7 and 8, the foam needs to be rated at R-15 or more. (For an explanation of why thin foam can be risky, see Calculating the Minimum Thickness of Rigid Foam Sheathing.) In short, the code recommendation for R-20+5 is problematic in cold climates.
When that article was being written, energy experts in Vermont were debating possible amendments to the IRC and IECC as part of the process leading up to the adoption of a 2015 residential energy code for Vermont. After my 2014 article was published, the debated amendments were finalized, and the Vermont Residential Building Energy Standards were published.
Let’s look at Vermont’s solution to the R-20+5 problem.
Changing the prescriptive table
All of Vermont is in a single climate zone (Zone 6). So the first way that Vermont code experts amended the IRC and IECC was to remove the irrelevant climate zones from the prescriptive R-value table.
While the original table in the IRC and IECC notes that walls in Zone 6 need at least “20+5 or 13+10” insulation — in other words, R-20 insulation between the studs of 2×6 walls plus R-5 of continuous insulation on the exterior of the…