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The conditions at the exterior surface of building components with high insulation are almost independent of the indoor climate. The tiny heat flow from the interior is generally not sufficient to prevent a temperature drop below ambient conditions by long-wave emission during nighttime. Apart from energetic consequences, this temperature drop may lead to surface condensation and subsequently to soiling or microbial growth. Another factor resulting in surface temperatures below ambient conditions is the evaporation of precipitation moisture.

In order to obtain realistic surface conditions by numerical simulation, the heat and moisture transfer processes at the surface have to be modeled accurately, taking into account convective and radiative exchange as well as evaporation and condensation heat. This requires hourly climatic data including air temperature and humidity, solar and sky radiation, precipitation, wind speed and direction. These data from selected years recorded at the meteorological station of the Fraunhofer-IBP in Holzkirchen serve as input for a hygrothermal simulation tool called WUFI®. The calculated results are compared with measured surface temperatures of walls and roofs with different orientation at the Fraunhofer-IBP test site. From this comparison appropriate surface transfer coefficients for simulation tools can be deduced and the different surface humidity sources may be quantified.

Citation: Thermal Performance of Exterior Envelopes of Whole Buildings IX