Center for Light and Environmentally-Friendly Structures ZELUBA®

Research project

Generation of hygrothermic characteristic values of wood-based materials for the numerical simulation of the component behavior

In this research project, the entire hygrothermic characteristic values of all wood-based materials were examined for the first time. Generic material datasets were thereby generated which, in the evaluation of the results, tend to be on the unfavorable side compared to the measured datasets. The generic datasets can be utilized for the dimensioning of constructions and should produce representative results in the majority of cases. The project has, however, also raised new questions which necessitate further investigation and which are intended to stimulate further discussions.

© Fraunhofer WKI | Norbert Rüther
The diagram shows the w-value in kg/(m²*h^0.5) plotted against the proportion of hydrophobic agent in percent. In the lower representation, both the axes are represented logarithmically.
© Fraunhofer WKI | Norbert Rüther
Values for thermal conductivity, measured with the plate tool, plotted against the bulk density of the samples.
© Fraunhofer WKI | Norbert Rüther
Values from Figure 2, with three additional progressions of the thermal conductivity at 15% wood humidity and 20% wood humidity (solid lines) and values for thermal conductivity determined using the THB procedure.

One important question concerns the significance of hygrothermic characteristic values. Whilst the strength and stiffness values for safety-related building materials are determined over the course of large series of tests and statistically evaluated, most hygrothermic characteristic values ​​are determined through random sampling on a few randomly selected samples.

Due to the wide diversification of the material parameters and the differing installation situations, in which different respective material parameters can be critical, it cannot be ensured in each case that the generic datasets achieve less favorable results than the measured data sets for a specific product. In the case of application in a sloped, ventilated roof, the moisture in the materials results from, for example, the high humidity level of the outdoor climate in winter. The water content increases when the level of the sorption isotherm is raised. In flat roofs, however, the moisture comes from the interior area via the vapor barrier. The quantity barely changes in relation to the properties of the materials above the vapor barrier. The previously more critical high level of the sorption isotherm now results in the same quantity of moisture leading to a smaller increase in the relative humidity in the outer region of the component and the generic materials remaining slightly more favorable. A generation of material datasets, which are critical in all cases, is therefore not possible. Furthermore, the respective generic datasets could only be generated and verified on the basis of one to three measured datasets.

The generic datasets can be utilized for the dimensioning of constructions and should produce representative results in the majority of cases. Particularly in the case of hygrothermically-favorable results, slight differences between the generic and individual specific materials can be expected at most. In contrast, if results are obtained which are close to critical limits, moisture levels will be reached in the materials in which the differences between the generic and individual measured data can be greater. If, in individual cases, it is not possible to improve the component in an appropriate manner, the use of specific material characteristic values for the real product should be attempted in order to enable a more reliable evaluation.

Project partners

Fraunhofer-Institut für Bauphysik IBP

Funding

Funding body:
German Federal Ministry for Economic Affairs and Energy (BMWi)

 

Project management:
Arbeitsgemeinschaften industrieller Forschungsvereinigungen (AiF) via the “International Association for Technical Issues related to Wood (iVTH)“

 

Project duration:
01.07.2013 to 31.12.2015