This article first appeared in the January/February 2024 issue of the Journal of Light Construction. www.jlconline.com
The International Residential Building Code is supposed to provide clarity into how builders construct homes. In the IRC’s own words, “The IRC was created to serve as a complete, comprehensive code regulating the construction of single-family houses, two-family houses (duplexes) and buildings consisting of three or more townhouse units.”Usually, the codes do a decent job giving us options on how we should build homes with the occupant’s safety and health as our number one priority. Codes also address energy efficiency and building durability. Sometimes though, codes can be quite confusing. How we handle vapor retarders when using continuous insulation is one of those areas.
I recently wrote a two-part article covering the benefits, challenges and code requirements for continuous insulation. (You can read the articles here and here.) In the first article, I referenced table R402.1.3 which is found in Chapter 11, Energy Efficiency. The table shows the R-value and U-factor requirements for several building components and assemblies. The table data was taken from the first edition printing of the 2021 IRC. In the article, I stated, “Climate zones 1, 2, and 3 are unique in that they have the option of cavity-only insulation. Climate zones 4 through 8 all are required to have some amount of continuous insulation.” A reader on the Green Building Advisor Website, jimmybpsu, pointed out in the comments section of the article that he had a different version of table R402.1.3 which did allow for cavity only insulation in climate zones 4-8. As it turned out, he was right.
Code compliant blower door testing of a small home can be very frustrating for both the person performing the test and the homeowner or contractor who are required to have the test. A few months ago, I tested a 952 square foot, newly constructed home with a volume of 7616 cubic feet, one of the smallest I’ve tested. The house had a leakage rate of 416 cfm which resulted in a 3.28 ACH50 number. A failed blower door test (my climate requires 3 ACH50 or less). If we take that same CFM rate but increase the ceiling height to 9 feet, which changes the volume to 8568 cubic feet, 2.91 ACH50, a passing test. Either way, 416 CFM of air moving through the blower door isn’t much, some kitchen exhaust hoods can move more air than that. Seems unfair to punish smaller homes when blower door testing, especially when the blower door test is testing the surface of a structure, not it’s volume.