Experimental evaluation of the time lag and decrement factor of a raw earth wall for various climates: Comparison of different methods

The current environmental context, as well as the major societal issues linked to the ecological and environmental transition, mean that more thought needs to be given to improving the energy efficiency of buildings. Indeed, the ability of materials to respond to different climatic fluctuations is becoming an important issue to study. This study deals with the assessment of dynamic thermal properties, such as time lag and decrement factor, of a wall made from cob (raw earth + straw), a geo-sourced material. The main aim of this investigation was to evaluate the response of the wall to several climatic stresses and compare several calculations methods of dynamic thermal properties. Three different climates were simulated (tropical, desertic and oceanic) and the response of the wall at different depths was analysed. It appeared that the cob wall was able to attenuate the thermal transfer for hot climates such as Douala and Riyadh. The same statement was made for the oceanic climate even if the climatic stresses were less extreme. Thereafter, the experimental results were used to determine the dynamic properties using various methods, in particular that of Asan et al., Fourier series decomposition. These two methods were also compared with the analytical method of standard NF EN ISO13786. Significant discrepancies were found between the standard NF EN ISO 13786 method with that of Asan et al. and Fourier for the time lag, particularly in tropical climates, where mass transfer has a strong influence on heat transfer. Combining raw earth with bio-based materials could improve its performance under extreme climatic conditions.