Assessing Spatial Variability of Thermal Conductivity and Volumetric Heat Capacity in Hemp-Fibred Cob Walls through In-Situ Measurements

Abstract

Earth-based construction materials are experiencing renewed interest due to their environmental benefits and thermal performance capabilities. However, the inherent heterogeneity of these materials poses significant challenges for reliable modeling and performance prediction. This study investigates the spatial variability of thermal properties in hemp-fibred cob walls through comprehensive in-situ measurements and advanced statistical analysis. A full-scale cob wall (2.70 m × 1.20 m × 0.10 m) was constructed using sandy-loam soil from Guérande, France, reinforced with hemp shives at 3% by dry mass. Thermal conductivity and volumetric heat capacity were measured at 50 locations of the wall volume, using the Hot Disk Transient Plane Source technique. Statistical analysis revealed moderate spatial variability with thermal conductivity ranging from 0.438 to 0.837 W/m·K (Coefficient of Variation equal to 16.2%) and volumetric heat capacity from 0.45 to 1.19 MJ/m³·K (Coefficient of Variation equal to 21.8%). Geostatistical analysis using empirical variograms revealed spatial correlation ranges of approximately 32 cm for thermal conductivity and 31 cm for volumetric heat capacity. The use of the VARBOOT uncertainty analysis methodology demonstrated the reliability of the former estimate, but indicated high uncertainty for the latter. The findings highlights the measurable variability and spatial dependence of key thermal properties across the wall.