Instrumentation and real-time hygrothermal monitoring of a double-layer Cob prototype and condensation risk assessment

Earthen construction is becoming increasingly popular as a low-carbon alternative to conventional materials, but it’s in situ hygrothermal behavior in instrumented prototypes remains poorly documented, particularly regarding the combined monitoring of temperature, humidity and heat flux across multiple wall orientations. As part of the CobBauge project, a prototype with a two-layer wall (structural material and insulation) was built in Normandy and continuously monitored across its four orientations. Analysis of two three-day periods, selected in summer and winter, shows that, in the absence of heating, the indoor temperature remains stable between 23-29°C in summer and 13-15°C in winter, despite outdoor variations of 13-33°C and -4 to 11°C respectively, while indoor relative humidity remains between 50 and 60%. Temperature profiles across the wall thickness reveal a marked contrast between the four facades (which differ in orientation and thickness), with the east and west facades exhibiting the highest temperature amplitudes. The interior and exterior superficial thermal resistances, estimated from these profiles, remain generally consistent with the standard’s default values, with more pronounced deviations on the exterior side, likely influenced by local wind and solar radiation. An assessment of the risk of condensation using the Glaser method reveals no risk, either on the surface or within the structure. A result that contrasts, however, with the relative humidity recorded in the insulation of the north-facing wall, reaching 90% in winter, exceeding the commonly accepted mold risk threshold of 80%. This discrepancy highlights the limitations of the Glaser method when applied to hygroscopic materials and it underscores the necessity of combining theoretical analyses with in-situ measurements to reliably assess the hygrothermal behavior of earthen structures.