Trying to understand Code required insulation for condensation control
jollygreenshortguy
| Posted in Building Code Questions on
When 2021 IRC raised insulation levels it did not make a corresponding increase to the condensation control insulation levels. Consequently 2021 has lower ratios compared to 2018.
Does that mean these lower ratios are actually fine from a condensation control perspective? If so, then shouldn’t they work equally well for 2018 insulation levels?
Ex. Zone 7
2018 | Total R 49 | Condensation Control R30
Ratio 30/49 = 61%
2021 | Total R 60 | Condensation Control R30
Ratio 30/60 = 50%
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Replies
The original numbers were based on Building Science Corp research and estimates. They wanted to use ratios, not hard numbers, but the code council thought that would be too complicated.
I agree that the numbers should have changed with the overall R-value update, and I use the original ratios. The way the code change process works, someone needs to propose and defend an amendment.
The ratios are a bit squishy.
For example our code allows for 30% in Zone5. When I run up against it, I ran the assembly through Ubekus calculator. I'm not sure how accurate it all is, but at 30% it was fine with low interior humidity, at 40% (BSC ratio) it was fine with normal house humidity. Adding a smart vapor retarder allowed the lower ratio to work with normal humidty which is what I ended up going with.
Interestingly there was an article here about flat roof failures (unvented fluffy insulation flat roof in multi residential) and adding just R5 above the sheathing avoided failures. That is a lot less than the 40%.
So I don't know. More exterior is definitely the safer option.