Effect of wood modification and weathering progress on the radiation emissivity

The research reported here is a part of the BIO4ever project, which aims to develop numerical models simulating performance of the bio-based cladding materials in relation to the exposure time or so-called "weather dose". The value of emissivity is one of the thermodynamic material constants, highly affecting heat transfer calculations. The lack of reliable emissivity data for several investigated materials was discovered during model's preparation, especially in a case of modified woods and coated samples. It was especially important since the emissivity is directly affecting the surface state/condition, and might change due to the weathering process. The overall objective of this research was therefore to determine actual emissivity coefficients for diverse cladding materials (various wood species and modification processes) in different ambient conditions (temperature). The thermographic (radiometric) measurements were conducted with the thermal camera FLIR T200, covering a spectral range from 7.5 to 13 µm. Tests were carried out on preselected samples to highlight the differences between diverse materials. The emissivity coefficient was determined at different surface temperatures by conditioning samples in climatic chambers for a period of at least six hours. The emissivity values obtained experimentally were used for improvement of finite element method models of the solar irradiation and of the surface moisture content changes for all investigated bio-materials. Subsequently, simulation of the façade appearance implemented within BIO4ever project, considering simultaneously time of service, geographic location, local microclimate and intrinsic material characteristic, become more realistic and trustworthy.