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A new study on variable diffusivity membranes

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Vapour barriers and retarders are often needed to improve the hygro-thermal performance of the building envelope. Their use is particularly important in prefabricated timber façades, especially when critical boundary conditions occur.

However, considering the growing interest and use of such timber facade elements, an analysis of the performance of integrated membranes is needed in order to improve the material function curves available in the data­sheets to enable the correct design of the whole wall structure.

Thus, the novelty of the work completed by Eurac Research, Free University of Bolzano and Rothoblaas, and recently published in the Journal of Façade Engineering and Design lies in the validated analysis of a building envelope sample that integrates membranes with a variable vapour diffusivity.

The focus has been more related to the experimental set-up, and particular attention has been paid to the development of a rela­tively simple testing procedure to analyse the behaviour of such integrated membranes. The study seeked to investigate the behaviour of an envelope component integrating a hygro-variable membrane and a breathable membrane by using computer simulation and experimental facilities.

A thermo-hygrometric analysis of the element has been performed in Delphin, and an experimental methodology aimed to validate the numerical model, measuring the temperature and relative humidity in different layers. Two sets of boundary conditions have been accurately chosen, as they are critical for the building component in terms of thermal and humidity transmission.

Results show very good agreement for one test condition. For the second condition, the measurement uncertainty was greater. One possible reason for this was the presence of condensation in the measurement box frame caused by the first test run. The experimental set-up developed is a relatively easy-to-replicate layout for the validation of similar complex packages. Compared to previous studies, the experimental set-up used in this research is simpler and less expensive.