WUFI Design Tool
Educational PC Programs for Calculating the Coupled Heat and Moisture Transfer in Building Components
WUFI-ORNL/IBP - What's that?
WUFI - Oak Ridge National Laboratory (ORNL)/Fraunhofer Institute for Building Physics (IBP) is a menu-driven PC program which allows realistic calculation of the transient coupled one-dimensional heat and moisture transport in multi-layer building components exposed to natural weather. WUFI-ORNL/IBP is based on the newest findings regarding vapor diffusion and liquid transport in building materials. The underlying model has been validated for over 20 years.
Besides the thermal properties of a building component and their impact on heating losses, its hygric behavior has to be considered, too. Permanently increased moisture content in the component may result in moisture damages. Elevated surface moisture levels in living rooms can lead to hygienic problems and health risks due to mould growth.
In addition, thermal and hygric behavior of a building component are closely interrelated as well as an increased moisture content favors heat losses. The thermal situation affects moisture transport. Therefore, both have to be investigated together in their mutual interdependence; the research field of hygrothermics is dealing with these problems.
Out of Date: Dew-Point (Glaser)
The Dew-Point method as detailed in American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) handbook has been a common method to assess the moisture balance of a building component by considering vapour diffusion transport in its interior. However, this method does not allow for the capillary moisture transport in the component, nor for its sorption capacity, both of which reduce the risk of damage in case of condensation. Furthermore, since the method only considers steady-state transport under heavily simplified boundary conditions, it cannot reproduce individual short-term events or allow for rain and solar radiation. It is meant to provide a general assessment of the hygrothermal suitability of a component, not to produce a simulation of realistic heat and moisture conditions in a component exposed to the weather prevailing at its individual location.
Up to Date: WUFI-ORNL/IBP
The menu-driven PC program WUFI-ORNL/IBP developed by the Holzkirchen branch of the Fraunhofer IBP and ORNL validates using data derived from outdoor and laboratory tests, allows realistic calculation of the transient hygrothermal behaviour of multi-layer building components exposed to natural climate conditions.
WUFI-ORNL/IBP is based on the newest findings regarding vapor diffusion and liquid transport in building materials. WUFI-ORNL/IBP only requires standard material properties and easy-to-determine moisture storage and liquid transport functions.
WUFI-ORNL/IBP can use measured weather data - including driving rain and solar radiation - as boundary conditions, thus allowing realistic investigations on the behaviour of the component under exposure to natural weather.
WUFI-ORNL/IBP can be used for assessing: the drying time of masonry with trapped construction moisture; the danger of interstitial condensation; the influence of driving rain on exterior building components; the effect of repair and retrofit measures; and the hygrothermal performance of roof and wall assemblies under anticipated use or in different climate zones.
WUFI-ORNL/IBP is a tool for developing and optimizing building materials and components. For example, it was used as a development tool for the smart vapor retarder.
WUFI-ORNL/IBP is directed at manufacturers of building products, consultants, designers, engineering offices and experts in the field of hygrothermics. WUFI-ORNL/IBP may be used as a teaching aid or advertising tool because of the instructive visualization of its calculation results.
WUFI-ORNL/IBP runs on PCs under Windows 7, 2000, XP, and Vista. The comprehensive on-line help and documentation amounts to 200 pages. WUFI-ORNL/IBP is available in 10 languages. A free Research and Education version for USA and Canada is available for download.
Proper application of WUFI-ORNL/IBP requires experience in the field of hygrothermics and some basic knowledge in the use of numerical calculation methods.
Outdoor Experiment and Simulation
Oak Ridge National Laboratory (ORNL) and the Holzkirchen branch of the Fraunhofer Institute for Building Physics is performing laboratory and field tests in order to assess the thermal and hygric behavior 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.
External thermal insulation composite systems with expanded polystyrene (EPS) and mineral wool (MW) insulation were applied to the west-facing lime silica brick walls (initial water content: 10% vol.) of a test house. The drying-out of the wall was monitored for three years by gravimetric testing of drill samples.
Simulation of Experiment by WUFI-ORNL/IBP 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-ORNL/IBP creates an automatic grid (coarse, mean or fine) which is adequate for most applications.
The hygrothermal material data for each layer can be read from WUFI-ORNL/IBP's database. As a minimum, WUFI-ORNL/IBP 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 used: the moisture storage function, the liquid transport coefficients for suction and redistribution, the moisture- and temperature-dependent heat conductivity, the moisture-dependent diffusion resistance factor, and the temperature-dependent Ethalpy. For the present example, material parameters from the educational database were used. The users are cautioned when using material properties from this educational database. ORNL is working on providing a reliable database with the new hygrothermal laboratory facilities.
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 a database. ASHRAE provided the raw data for the development of a moisture design year for 64 cities.
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.
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After entry of a few remaining data like surface transfer coefficients, initial conditions, etc., the calculation can be started. WUFI-ORNL/IBP then computes the temporal evolution of the temperature and the moisture field in the component. Typically, a calculation spanning one year in one-hour steps takes less than one minute. WUFI-ORNL/IBP offers experimentally verified default values in a separate Material Database. During the calculation, WUFI-ORNL/IBP optionally displays the newly computed temperature and moisture fields after each step, allowing you to watch the processes in the component as a 'film'. This film display is, of course, somewhat slower, so that you should perform lengthy calculations without the film; on the other hand, you can immediately see whether a test calculation or a parameter study conforms to your expectations and stop it if necessary. The direction and magnitude of the heat and moisture flows across the interior and the exterior surface as well as across the inner material interfaces are indicated by corresponding arrows.
Calculation Results and Comparison with Experiment
Displaying the Results
After the calculation, the results - stored in a binary result file - are available for display and analysis. WUFI-ORNL/IBP lets you display the curves of courses in time and cross-sectional profiles as graphics, compare them with measured data, edit and print them. You can also view graphics of the climate data. You can watch the film after the calculation at your leisure; you can export it, together with an external viewer, to a CD. If you want to process the results on your own, you can export them to ASCII files.
For the entire simulated time span WUFI-ORNL/IBP produces courses which describe the temporal behavior of the following quantities: the heat flux densities through the interior and exterior surface, respectively, the temperatures and relative humidity at an arbitrary number of freely selectable monitor positions, the mean moisture content of each material and the total moisture content of the entire building component. The diagram for the present example shows the resulting courses of the moisture content, averaged over the cross-section of the lime silica brick masonry, and compares them with the gravimetrically measured values. It takes the wall with mineral wool insulation somewhat over one year to reach the normal equilibrium moisture of 2.5% vol. and the wall with EPS insulation two and a half years.
In addition, for points in time selected by the user WUFI-ORNL/IBP provides profiles which show the distribution of the following quantities across the component: temperature, relative humidity, moisture content. The diagram shows a comparison between the measured and the calculated moisture content profiles for four different points in time. Evidently, good agreement between measurement and calculation can be achieved for EPS insulation (top) as well as MW insulation (bottom). The shape of the moisture profile indicates that in the case of the EPS insulation most of the initial moisture dries out towards the room side (right), whereas the Exterior Insulation Finishing System (EIFS) with the more permeable mineral wool also allows considerable drying to the outside which results in quicker overall drying.
(Click photo to enlarge)
WUFI-ORNL/IBP also writes a film file during the calculation which contains all the computed profiles and which - displayed as a 'film' - conveys a dynamical impression of the thermal and hygric processes in the component.
This film is ideal for gaining insights into the hygrothermal processes and for developing a 'feel' for the situation in the component. The reactions of the different materials to the changing climatic conditions can be watched directly.
In the present example, a good correspondence between calculation and experiment is obtained, so that the calculation method in general as well as the material parameters used for this specific example can be considered valid. Therefore, it is now possible to perform a purely computational examination of variants and extrapolations of this experiment.