This short tour through WUFI shows you a calculation example and compares the results with experimental data.

Outdoor experiment and simulation

The branch Holzkirchen of the Fraunhofer Institute for Building Physics is performing laboratory and field tests in order to assess the thermal and hygric behaviour of building materials and components. These experiments tend to be lengthy and expensive so that only a small number of variants can be examined. A suitable simulation method can replace some of these experiments. After validation and calibration by experiment it can be used to test further variants.

Experiment

External thermal insulation composite systems (ETICS) with polystyrene (EPS) and mineral wool (MW) slabs were applied to the west-facing lime silica brick walls (initial water content: 10 vol.%) of a test house. The drying-out of the masonry was monitored for three years by gravimetric testing of drill samples.

Simulation of Experiment by WUFI calculation

Component Assembly and Numerical Grid The individual layers of the component and their respective thickness are entered into a table. The component is then divided into numerical grid elements whose widths are chosen according to the temperature and moisture variation expected for the respective location. The manual grid definition is done by entering the desired number of grid elements per layer and an expansion factor which describes the ratio of the sizes of successive grid elements. Steep temperature and moisture gradients may especially be expected close to the layer interfaces. Splitting a layer into two layers allows the grid to expand and subsequently contract within a material layer. Optionally, WUFI creates an automatic grid (coarse, mean or fine) which is adequate for most applications.

Material Data The hygrothermal material data for each layer can be read from WUFI's database or entered manually. As a minimum, WUFI requires the bulk density, the porosity, the specific heat capacity, the heat conductivity (dry) and the diffusion resistance factor (dry). Depending on the object and the purpose of the calculation, additional data can be entered: the moisture storage function, the liquid transport coefficients for suction and redistribution, the moisture-dependent heat conductivity and the moisture-dependent diffusion resistance factor. WUFI can approximate the moisture storage function if provided with the equilibrium moisture content at 80% RH and the free saturation. The liquid transport coefficients can be estimated from the water absorption coefficient (A-value). For the present example, material parameters from the database were used.

Weather Data The boundary conditions acting on the building component are the temperature and relative humidity of the interior and exterior air and the rain and radiation loads, both depending on inclination and orientation of the building component. These data can be derived from measured weather data or from test reference years. For the present example weather data measured in Holzkirchen were used for the exterior climate, and the less important interior climate was approximated by sine waves with annual period. The time steps for the climate data and the calculation may be selected at the user's discretion; for most cases hourly values are appropriate.

Page created: 21 Dec 1999; last update: 30 Apr 2010