Developing models to generate hygrothermal KPIs
Since mid-2013, the Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut, WKI, and the Fraunhofer Institute for Building Physics IBP have been collaborating on a joint research project. Funded by the German Federation of Industrial Research Associations (AiF), the project aims to generate hygrothermal KPIs of wood-based products for numerical simulations.
Numerical simulation is already widely used as a method in creating forecasts, for anything from weather and elections to finances and growth. It is also used to forecast the behavior of building materials and components when they are stressed by mechanical and climatic effects. When performing numerical simulations on the behavior of products made of wood and wood-based materials, however, the relatively high variance of material properties frequently leads to less than meaningful results. This variance is generally greatest for solid wood, and narrows as the materials become increasingly refined.
In addition to resistance to water vapor diffusion, the following properties are also relevant for wood-based materials: porosity, thermal capacity, thermal conductivity (also moisture-dependent), moisture retention, free water saturation and liquid transport coefficients. By measuring these properties, researchers can thoroughly describe how well or poorly a component is protected against moisture.
When it comes to wood construction, building-physical issues regarding transport of heat and moisture are worth mentioning, and for two reasons. One, when wood and wood-based products are exposed to changes in moisture, their dimensions change as well. Two, if they are exposed to a disproportionately large amount of moisture, they can begin to decompose naturally.
The problem is that the data available on the material properties needed to describe the transport of heat and liquid is still inadequate – even for tried and true products.
This dearth of information has been exacerbated by the emergence of new kinds of wood-based materials and product groups over the past few years. These positive innovations have significantly enriched the areas of wood-based materials and wood construction. Whereas just a few years ago, petroleum-based rigid foams or mineral fiber insulation were being used exclusively, ecologically and economically sound wood or wood-based materials harvested from sustainable sources are in more frequent use today.
Going to the trouble of extracting comprehensive samples for every product and every property appears to be impractical, not to mention unprofitable. For this reason, the joint research project investigates the various factors at work and their effects on the various building-physical properties of the materials. The factor that plays the largest role is probably bulk density. Other key factors have been identified, including particle size and the proportion of adhesive and means for rendering the material water-repellent; however, their effects on hygrothermal KPIs are still largely unknown.
This research project has several goals. One is to provide manufacturers of wood-based products with functional relationships between influencing parameters and thermal and moisture values. Another is to forecast those values with the help of forecast models. A further goal is to prepare a database for users of more precise calculation methods. This database is intended to make it easier for the users to identify product groups by means of the application area of the wood-based materials, without having to know precisely what the product is.