Air Freezing Index (IRC vs NOAA)
Hello, we’re in the early planning stages for building our residence in Freeport, Maine this year. It’ll be about 2000 sq ft with detached garage (unheated, with living space above). We hope to build on a Frost Protected Shallow Foundation, utilizing a foam form system such as WarmFörm or similar.
IRC 2015 specifies that our Air Freezing Index is 2500 (Cumberland County, Maine), but using comparable locations (Portland, Lewiston, Augusta) NOAA says we’re in the 1400s (100 yr average here https://www.ncdc.noaa.gov/sites/default/files/attachments/Air-Freezing-Index-Return-Periods-and-Associated-Probabilities.pdf)
Does anyone know why there is such a large discrepancy between NOAA and code? Has anyone worked with their local code enforcement to build to a different AFI than specified in IRC? The discrepancy represents a significant difference in the requirements for skirt insulation width and thickness, and footing depth for both foundations. I’d imagine we’ll end up just building to code, but it does make me curious.
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Replies
Hi Christian -
Great question; sorry I don't have an answer. There is a strong building science community in your general area associated with Performance Building Supply; you should check in with PBS on both this question and their hosting of local high performance building professionals.
Peter
What's important is the worst case, not the average, and the 100-year number in that pdf is in fact the worst expected in 100 years.
But given this year's record-breaking cold in some places, I'm not sure I'd want to count on the statistics that were used to generate that staying valid for the next 100 years. So erring on the side of caution seems worthwhile to me.
Christian, the Revised Builder's Guide to FPSFs (https://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf) , which is a safer guide than the IRC's limited options, states, "The AFI values are conservative because they are not adjusted for the insulating benefit of a normal snow cover on the ground. A lower return period value may be used for less important structures or those that are resilient to infrequent ground freezing, such as detached garages." In addition to that, the values are averaged from 1951 to 1980, and with more extreme weather these days (hello Polar Vortex) and accounting for various micro-climates, it's better to err on the side of safety. That said, when I design projects with FPSFs in the greater Portland area, as long as the site is well-drained, I (and engineers I have worked with, when required) generally use a value lower than 2500 degree days.
Another thing to watch out for is the the IRC's simplified guide assumes you are using code-minimum R-value under the slab. If you are using more than code-minimum (as you should, in a heated space), the ground is getting less heat from the structure so you should do the more complex analysis in the Builder's Guide. Or go right to the source document, the ASCE 32-01 design manual, though you have to pay for it.