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IECC 2021 Updated U-Value Table

maine_tyler | Posted in Building Code Questions on

Ok I’m finally trying to understand where the ‘you need continuous insulation according to the 2021 IECC’ is coming from.

When I go here:

and look at table 402.1.2, all I see are U-factor requirements. R-value requirements calling out specific cavity and CI R-values appears to be an ‘alternative’ approach rather than prescriptive.

This isn’t to be confused with the ‘total UA alternative,’ which still appears to be there as 402.1.5

Is this an update to when it first came out?

Has GBA already done a round-up of the 2021 IECC updates?

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  1. andy719 | | #1

    I'm not a residential code enforcer or builder, but here is my opinion after taking a look:

    Yes, the 2021 edition reversed sections from the 2018. The 2018 listed the R-value chart as the primary, with the U-factor chart as the alternative. The 2021 made the U-factor primary, with the R-value as the alternative. My guess is that builders in most jurisdictions are going to stick with the R-value chart anyway just because it takes the headache out of the doing the math to find u-factor, assuming you actually take the framing into account along with the sheathing to find the total wall u-factor.

    Also, your question about continuous insulation depends on your climate zone. Most of the concern on this forum is from the heating climates, zone 4 and higher. The required u-factor for those zones is 0.045 which is an R-value of 22.2. The 2018 had the u-factor at 0.047, or an R-value of 21.3. For the 2021 edition, some officials might look at that 22.2 u-factor and assume R21 cavity plus 1/2" drywall and osb meet it, but I'm guessing they will have some doubts when they see the R-value chart lists R30 cavity as the equivalent without using continuous insulation.

    Most people on here look at the u-factor as the assembly value which takes into account the framing. So, R21 cavity only gives you an R18 assembly with 2x6 wood framing. You aren't going to get the necessary additional R4 with just osb. So you can either add continuous insulation, or you can bump up to 2x8 framing. With the availability of products like zip-r sheathing, I'm guessing most builders won't choose to go to 2x8 studs.

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