More Q&A Spotlight
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…
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6 Comments
Great commentary, Michael. Regarding your comment, "It does not make sense to go overboard with insulation that has a high level of embodied carbon," I'll add one thing that I think you will agree with: in most cases, it doesn't make sense to use insulation that has a high level of embodied carbon in the first place, when they are alternatives that have so much lower embodied carbon, especially in new construction.
Charlie, thank you, and yes I agree with you. Typical of my writing approach, I had another thousand words that I cut to keep my response to a reasonable length. Among my cuts were recommendations for how to deal with vaulted ceilings using low-carbon insulation, which I do regularly. Using low-carbon insulation does take some forethought, or else you might end up in a situation like Boris, but I haven't found a situation yet on new construction where foam was necessary.
Looking forward to your book with more of those thousands of words.
A paragraph I cut from my response but I'm curious if others have data they could share about the prevalence of building codes:
"According to Pew research (https://www.pewresearch.org/social-trends/2018/05/22/demographic-and-economic-trends-in-urban-suburban-and-rural-communities/), as of 2016, 31% of US residents live in urban areas, 55% in suburban areas and 14% in rural areas. I don’t have data but I would estimate that the vast majority of urban municipalities follow building codes, most suburban areas do, and some rural areas do--perhaps 60-80% of new construction in the US is required to follow building codes."
Something to think about in discussions about ever tightening energy codes and pushback when they impact architectural design considerations- perhaps (and I say this as a designer) it means that your design needs to sometimes have energy efficiency outweigh the aesthetics of the home. Frankly a good architect/designer is more than capable of designing a beautiful home that is also highly efficient. Its a different way of looking at home design, and sometimes the way you did it yesterday just doesn't fit the times anymore and its good to learn a new skillset and adapt to our changing environment.
Bill,
I agree. The questions I find the most frustrating on GBA concern new builds where energy efficiency wasn't considered at the design stage, and the posters find themselves boxed into a corner. Roofs seems to be the most common problem.
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