BK109 Thermal Performance — 2025 California Residential Code

Cob walls shall be classified as mass walls in accordance with Section N1102.2.6 and shall meet the R -value requirements for mass walls in Table N1102.1.3.

The unit R -value for cob walls with a density of 110 pounds per cubic foot (1762 kg/m [3] ) shall be R-0.22 (RSI 0.0387) per inch of cob thickness. The unit R -value for cob walls with a density of 75 pounds per cubic foot (1201 kg/m [3] ) shall be R-0.54 (RSI 0.095) per inch of cob thickness. Linear interpolation is permitted. Extrapolation is not permitted. Walls that vary in thickness along their height or length shall use the average thickness of the wall to determine its R -value. The thermal resistance values of air films and finish materials or additional insulation shall be added to the cob wall’s thermal resistance value to determine the Rvalue of the wall assembly. Cob density shall be measured at equilibrium moisture content.

Where insulating materials are added to the face of a cob wall, the combination of additional insulation and any associated connecting, weather-resisting or protective materials shall comply with Section BK104.1.2, Items 1 through 4.