Thermal metrology of water sensitive materials: experimental and theoretical approaches

In France, the thermal insulation of buildings is part of the main recommendations of Grenelle Environment Forum for the energy transition. Bio-based insulators are part of solutions implemented in building insulation. As bio-sourced insulators are hydrophilic, the classical non-water sensitive models of thermal metrology require being adapted when applied to these materials in wet atmospheres.

To understand the influence of the humidity on thermal characterization of bio-based insulators, this study focuses on thermal metrology of hydrophilic and hydrophobic materials by an experimental approach based on the asymmetric hot plate method and a theoretical approach of coupled heat and humidity transfers. The antagonistic distinction between hydrophilic and hydrophobic character of studied media highlights the absolute need of coupled heat-humidity transfer models when characterizing water sensitive materials.

In this study, the experimental approach shows that the thermal conductivity of a bio-based hydrophilic material increases with rising humidity. Results show for example that, at 20 ∘C, the thermal conductivity of hydrophilic insulator increases up to 20% when the relative humidity RH increases from 30% to 90%, i.e. from 0.028 ± 10 − 3 to 0.033 ± 10 − 3  W.m − 1.K − 1. However, in the case of a hydrophobic materials, such as a phase-change polymer, the measured thermal conductivity is almost constant (0.218 ± 10 − 3  W.m − 1.K − 1) for the RH between 30% and 90%.

Furthermore, we develop a theoretical model that coupled equations of moisture and heat transfer and solved by the COMSOL Multiphysics® software. The results show a good agreement between the theoretical and the experimental approaches in both cases of the hydrophilic and the hydrophobic materials.