Hygrothermal performance of ventilated wooden cladding
K Nore, U Hundhausen
The risk for an attack of wooden claddings by decay and discolouring fungi strongly depends on the moisture and temperature conditions in the building envelope. The design of the cladding has a major influence on its hygrothermal performance. In the present study, different wooden ventilated cladding designs were investigated at a test house in Mid-Norway for four years. The objective was to increase the understanding of the relation between microclimatic conditions and the responding hygrothermal performance of the wooden claddings concerning their cardinal directions, the design of ventilation gap at the bottom and top of the cladding, the wood material quality, and the surface treatment. A statistical analysis revealed that air temperature, global radiation, and wind velocity were the three main factors that influenced wood moisture content (MC). Wind driven rain (WDR) was only the fourth most important factor. WDR only defines moistening and not drying, which might explain that WDR did not rank higher as explaining factor for MC. The importance of the wind velocity led to a separate computational fluid dynamics study (CFD) of the airflow in the cavities of the ventilated cladding. The study resulted in a function describing the airflow change rate of the ventilated cavities dependent on the wind velocity, wind direction, and cavity opening. The test house study shows that only 4 mm opening at the bottom and top of the ventilated cavity significantly reduces the risk of moisture problems in the wall assembly. Further opening to 23 mm did not give further improvement in a moist climate. In a dry climate, a design with closed ends of the cavity results in lowest MC. No significant conclusion can be drawn regarding the surface treatment (acryl vs. alkyd) and material quality (fast-grown spruce vs. slow-grown spruce). The test house measurements were used for validation in Heat, Air and Moisture (HAM) simulations of wooden claddings. The model showed good correlation to measurements and enables simulations to find the best-suited cladding design for different climates.