Scientists get to grips with moisture

Research in focus September 2013

A building’s users tend not to worry about damage to its structure until it has actually occurred – too late, because repairs involve a lot of work and are often very expensive. For this reason, one of the fundamental activities in building physics research is that of testing building materials, evaluating their properties, identifying their weak points and finding ways to overcome them before the materials are used for building. In the Hygrothermics department at the Fraunhofer Institute for Building Physics IBP, Dr. Cornelia Fitz leads a research group that studies the hygrothermal properties of materials and their behavior under moisture load conditions. “Builders and their clients need to be reassured,” says Fitz before going on to explain: “For example, moisture that seeps into brickwork when it rains and is unable to dry out will cause damage to the wall in frosty weather.” This and other forms of degradation are something that both manufacturers and building users would prefer to avoid. The test facilities at Fraunhofer IBP therefore include five controlled climate chambers and a large outdoor testing site, allowing the hygrothermal behavior of building materials to be investigated both in the laboratory and under real climatic conditions in the field.

One of the main areas of work for which Fitz and her team are responsible is determining the characteristic moisture properties of different building materials. Such data are required, for example, for the certification of new building materials and are also being used to an increasing extent for computer simulations in the planning phase of construction projects. Fitz and her team of scientists carry out tests on an extremely diverse range of materials – from cellular concrete and wood to composite thermal insulation systems.

Interest in methods of internal insulation has been growing in recent years, especially in connection with the renovation of historic buildings. Installing insulation materials on the inner surface of walls gives rise to a different set of questions than those that apply in the case of external insulation – a subject that Fitz and her team also deal with regularly. “We have developed a new laboratory technique specifically for investigating vapor transport mechanisms in capillary-active insulating materials. It enables us to characterize the materials’ hygrothermal performance under environmental conditions that correspond closely to those encountered when the product is used as internal insulation in real buildings,” says Fitz. The first step involves preparing small test samples of the material to be tested. A sealant is then applied around all four edges of each sample and to its rear surface. The rear face of the sample is affixed to a cooling element, which serves to bring its temperature down to below the dew point of the indoor air. The front surface is exposed to the controlled environment of the climate chamber. As water vapor moves by diffusion from the air in the climate chamber toward the rear of the sample, the water content increases, especially in the cooled area at the back. The molecules of water vapor are captured by the pores in the material. Above a certain level of humidity, the fluid transport mechanism in the capillaries begins to take effect, acting in the reverse direction to the diffusion process. Capillaries are pore-like structures with a very small internal diameter, in which the so-called surface effects are more pronounced than in larger pores. Water rises in capillaries as a result of surface tension; it also condenses more easily because its boiling point is lower in a capillary. The opposing action of the fluid transport mechanism in the capillaries causes less moisture to accumulate in the material; if the capillary action is strong enough, the diffusion process into the material can even be halted entirely. To determine how much moisture a sample has absorbed, and how it is distributed inside the material, the scientists weigh the samples at regular intervals throughout the test and scan them in a magnetic resonance imager. Finally, a hygrothermal simulation is carried out to determine the fluid transport characteristics.
Fitz cites one of the many reasons why the question of moisture transport is so important: “When an external wall is insulated on the inner surface, it receives less heat from inside the building. If it rains, the wall gets wet – just as it did when there was no insulation – only now it is unable to dry out as easily as before and retains more moisture. This can lead to various undesirable consequences such as frost damage or algal growth.” The ability to transport moisture is often a decisive factor in the choice of building materials. Another important parameter used to evaluate the hygrothermal performance of building materials is water vapor permeability, because this determines their drying rate. Consequently, another of the important tasks that the scientists in the laboratories of the Hygrothermics department at Fraunhofer IBP are regularly called on to carry out is conducting tests to establish the water vapor diffusion resistance coefficient of building materials. This coefficient represents the water vapor diffusion resistance of a particular material compared with that of air at normal atmospheric pressure.
The test procedure is as follows: a flat, circular sample of the material to be tested is placed like a lid over a glass vessel and fixed to its upper edge to form a vapor-proof seal. Depending on whether the dry-cup or the wet-cup method has been chosen, the vessel contains either a desiccant or a saturated salt solution. The dry-cup method is used to establish the material’s diffusion resistance to water vapor in an atmosphere with a relative humidity of between 0 and 50 percent, and the wet-cup method for measurements at a relative humidity of between 50 and 100 percent. In each case the relative humidity inside the vessel is constant once it has been sealed. The glass vessels are then placed in a controlled climate chamber in which the temperate is set to a constant 23 degrees Celsius and the relative humidity to 50, 65, or 80 percent. Here too, regular weighing of the sample is essential, because the partial pressure gradient between the air spaces adjacent to the surface of the material under test allows water vapor to penetrate the sample by diffusion. Once a stable state has been reached in which repeated observations confirm that the weight of the vessel is changing at a constant rate, it is possible to establish the diffusion rate.
Not all of the tests conducted by Fitz and her team take place in the laboratory. For over 60 years, Fraunhofer IBP has also carried out field tests at its outdoor testing site. The interior insulation materials, small samples of which are subjected to moisture permeability tests in the IBP’s climate chambers, are also tested in a real-life environment. Entire building components or individual materials can be installed interchangeably along the east and west faces of the 100-meter-long test facility. This allows them to be tested under naturally varying weather conditions, as opposed to the tests inside the building, in which – just as in laboratory testing – they are exposed to constant temperatures and levels of humidity. Fitz uses this experimental facility to test the various interior insulation materials currently offered on the market. “We do this work on our own initiative,” she emphasizes. “This is not contract research but an important part of our fundamental research that serves as input to regulatory and standardization processes.” The present study is scheduled for completion by the end of 2014. Data gathered during the winter season are of particular importance: “We need at least three years to obtain reliable results from this kind of study. The first year is devoted to setting up the experiments and solving any problems encountered at this stage. In the second year, we can then proceed with the field tests under authentic conditions and obtain reliable measurements. The third year delivers additional data that enable us to validate our previous findings.”
Depending on the questions to be answered, instead of installing building components on the test building, the researchers and their industrial clients also have the possibility of constructing entire, full-scale wall elements and exposing them to natural weathering effects in order to observe their thermal and moisture behavior over an extended period of time.
“Demands on the quality of building materials are constantly increasing. Our tests help manufacturers to improve their existing products and develop new ones, and at the same time help building authorities to define standards and guidelines to serve as a basis for certifying building materials,” says Fitz. For this reason, and in view of the growing use of building components with high insulating properties, this type of research is becoming increasingly important, she adds.


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