Analysis of thermal transmittance, air permeability, and hygrothermal behavior of a solid timber building envelope

A solid timber, multi-story, building envelope is developed in previous research that reduces Global Warming Potential by improving energy performance and reducing non-wood material layers. That research demonstrates that by sawing deep air chambers into timber beams and milling precise connection details, thermal transmittance and air permeability of a solid timber building envelope can be reduced. The dimensions of timber joints and other features are evaluated to determine their impact on building physics performance and develop means to further optimize the performance. Thermal resistance of air layers of various thicknesses is evaluated for thermal resistance and are shown to be up to 21 percent higher than related norm ISO 6946. Airtightness is measured and shows wide variability, demonstrating the natural variation in wood and the need to improve connections to further reduce air permeability. Hygrothermal simulations are conducted with WUFI Pro and indicate non-critical values for the wood moisture content at the outer surface and overall envelope. These simulations are confirmed by climate chamber testing on a full-scale mock-up. Based on these results, a standard timber profile is proposed which achieves a U-Value of 0.18 W/m2K.