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Method · M-08 Code path exists Materials & Methods

Cob

Hand-built monolithic walls of clay subsoil, sand and straw. Has a model-code appendix in the 2021 IRC that towns can choose to adopt.

Code path
IRC 2021, cob construction appendix (Appendix AU; numbered U during approval). Applies only where a jurisdiction adopts it.
Climates
temperate, hot-dry
US permit route
IRC appendix, if your town adopts it

Risks

  • Low thermal resistance; energy codes treat it as a mass wall
  • No fire-resistance rating in the appendix
  • Many insulation products fail its vapour-permeability rules

Needs a professional

  • Engineered design outside Seismic Design Categories A, B and C
  • Local adoption of the appendix, or an alternative-materials approval

Sources

  1. Cob code appendix approved for the 2021 IRC. ICC Building Safety Journal, August 3, 2020. · accessed November 4, 2026
  2. Adoptable appendices in the International Residential Code for cob construction and tiny houses. ICC Building Safety Journal. · accessed November 4, 2026

Fact-checked November 4, 2026. Spotted an error? Tell us.

In the Journal

Field Notes

Is it legal? Natural building and the code

A plain guide to model-code appendices for cob, straw bale and tiny houses, and why an appendix existing is not the same as your town allowing it.