Numerical and experimental study on the impact of humidity on the thermal behavior of insulated timber walls

The push towards a lower environmental impact of the building sector has led to the widespread adoption of renewable organic materials such as wood and other bio-based insulation materials. These materials are characterised by a solid matrix with voids that can be filled with moist air and water and in which complex heat and mass transfer occurs, influencing the global thermal properties. Indeed, due to different heat transfer mechanisms involved, the apparent thermal conductivity varies both with temperature and moisture content because of water thermal conductivity is higher than air. However, many Building Simulation programs (e.g., EnergyPlus and TRNSYS) still adopt constant thermal properties for simulating the heat transfer in buildings, causing inaccuracies in calculations of heat flux and thus, in the prediction of the energy consumption of a building. The aim of this work is therefore to analyse experimentally and numerically the impact of both moisture and temperature on the thermal behaviour of an insulated timber wall. An experimental activity was conducted at the Building Physics Laboratory at the Free University of Bozen-Bolzano on Cross-lam (XLAM) and wooden insulation specimens to measure thermal conductivity at different temperatures and moisture contents by means of a heat flux meter (i.e., HFM) and a climatic chamber. A 1D model for coupled heat and mass transfer across the studied wall was then developed and calibrated against the experimental data. Finally, by simulating with both nominal (10 °C and 23 °C reference temperature) and actual thermal conductivity, hourly heat fluxes and energies were compared, taking into consideration climatic files relating to the Italian peninsula.