The International Energy Conservation Code (IECC) is revised every three years, and in the just-published version for 2021 the amount of attic insulation in climate zones 2 and 3 has been increased from R-38 to R-49. Commenting in a recent Q&A post, James Boris doesn’t get it.
“I can’t imagine it would save much energy,” Boris writes. “What reasons for this increase might I be overlooking? Or, more generally, what other considerations go into setting code insulation levels?”
To bolster his case, Boris offers these rough calculations: With an outdoor temperature of 100°F, an average roof temperature of about 160°F, and the air conditioner set at 78°, the difference between the roof temperature and indoor temperature is 82°F (the Delta T, or ΔT, as engineers and energy hobbyists like to call it).
Under the previous IECC, R-38 of continuous exterior foam insulation yields a ceiling temperature of 79.94°F. Under the 2021 IECC, with insulation increased to R-49, the ceiling temperature becomes 79.51°F.
“Any indoor air stratification would raise the ceiling’s base temperature, reduce Delta T, and thus lead to less difference between R-38 and R-49,” Boris says. “Am I missing something here, or putting too much emphasis on the difference between ceiling temps?” Additionally, Boris wonders how these calculations might change when comparing an unvented cathedral ceiling to a traditional, vented attic.
Boris’s questions about more stringent energy requirements are where we start this Q&A Spotlight.
First, use a simpler calculation
The real point is energy use, replies Charlie Sullivan, and the increase in attic insulation will make a difference.
“The good news is that’s a simpler calculation,” Sullivan writes. With a Delta T of 82°F, as Boris had assumed, the heat transfer is about 500 Btu/hour less with roof insulation of R-49 than it would be…