Surprisingly, even in these days results of standard tests of the thermal and moisture behaviour of building materials measured in different laboratories can vary considerably. This applies especially to the measurement of water vapour permeability, but also sorption properties or thermal conductivity. The Building Physics research team at CTU UCEEB identifies causes of errors in these experiments, leading to higher measurement accuracy and thus more efficient use of materials in building practice.
The accompanying picture shows a test revealing changes of the air mass inside cups caused by variations of air density and pressure in the laboratory during the measurement of water vapour permeability of different materials. The results helped increase the accuracy of the experiment by up to 10%. Although it is not much, we are the first to quantify this phenomenon.
Another significant cause of error in the measurement of water vapour permeability, especially for highly hygroscopic materials, is the sorption hysteresis, which can lead to up to twice the observed diffusion resistance factor under the same test conditions. Moreover, both values are relevant and describes the actual behaviour of the material based on its hygro-thermal history. This may be particularly relevant for wood and wood-based materials. Our findings are regularly published in scientific journals.
Detecting errors in standard tests is often complicated because they usually apply differently for different material groups. On the other hand, it's often all the more fun.