Getting building official approval of strawbale construction in Minneapolis is tricky due to a failed strawbale build over a decade ago (this build is legend, but no one can actually point to who the builder was or where it was located other than somewhere in South Minneapolis). Knowing this, our design team is arming themselves with all of the facts about strawbale construction that will hopefully convince building officials to approve our build.
Luckily, the International Construction Code (ICC) now has a strawbale chapter (Appendix S Section 105.6). Although MN has not officially adopted this chapter, it nonetheless gives us a strong argument that this alternative construction style has been vetted by experts in the construction industry. We anticipate one of the building officials' biggest concerns is going to be moisture, and the ICC strawbale chapter does a lot to address that. Of particular concern initially for our design team was the call out of stricter perm ratings (meaning - increase vapor retardation) with class II vapor barriers needed for the bale walls in climate zones 5-8 to avoid potential moisture issues in the bales. Being in zone 6, we're included in that. The design team was contemplating the need for an additional sealer on the interior plaster to stop vapor flow. Luckily, Andrew, our strawbale expert, was able to provide evidence that our lime based plaster will more than sufficient at minimizing vapor permeability with perm ratings less than 1 making it qualify as a class II vapor barrier (source of lime plaster: https://limes.us/resources/research-papers/ and table here: http://www.stastier.co.uk/nhl/testres/test24m.htm). So yes, lime-based plaster is a sufficient vapor retarder for strawbale building in Minneapolis, MN. Below is the vapor retarder section of Appendix S of the ICC code, which covers strawbale construction.
2 Comments
9/1/2023 11:36:52 pm
Great post! Your insights on vapor barriers are invaluable, shedding light on a topic often overlooked but crucial for maintaining a healthy home. Thanks for sharing this informative piece
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Katie Jones
10/7/2023 08:10:00 pm
Thank you! Glad it's helpful. We figure more that early adopters like us can share their experiences, the more we hope to catalyze more sustainable builds.
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