Capillary reverse transport capabilities in capillary-active interior insulation

A new test method

© Fraunhofer IBP

Test set-up to determine the capillary activity of insulation materials.

In many cases, the only way to improve a building’s thermal protection is to install interior insulation. The standard way to avoid condensation forming on the back of the insulation is to fit a vapor retarder or moisture barrier to the insulation on the side facing the building’s interior. The catch is that this gets in the way of the insulation drying out in dry weather.

This dilemma spawned the trend of using vapor-permeable designs in the form of capillary-active insulation – in other words, materials whose structure enables them to take on and redistribute accumulating moisture. In this way, these materials can help achieve moisture balance. The data typically used to monitor moisture behavior and how such materials perform are provided by tests that expose materials to the effects of wind and weather. The trouble is that these data are often not precise enough.

In response to this situation, researchers at the Fraunhofer Institute for Building Physics IBP in Holzkirchen near Munich have developed a new laboratory test, which they dubbed "Kapi-Test" (set-up shown in illustration 2). By determining precise values for capillary reverse transport in interior insulation, the Kapi-Test enables the researchers to make reliable statements about exactly how insulation can regulate its own environment.

To do this, the researchers recreate the conditions in which insulation is installed – they determine capillary activity in interior insulation without exposing it to liquid water and under non-isothermal conditions, with the result that vapor and liquid move in opposite directions.

For the test, the researchers seal the sides and back of a prism-shaped test sample so that only the front comes into contact with the atmosphere inside a climate chamber. 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 diffusion set in motion by the interior climate leads to an increase in relative humidity toward the back of the sample, which in turn causes more and more vapor to move in the other direction, toward the warm – and still drier – front area of the sample. Depending on the conditions and material properties, if there is sufficient opposing action by the capillaries then the movement of moisture in opposite directions creates a state of equilibrium.

Moisture absorption and distribution are determined using regular gravimetric analysis and magnetic resonance spectroscopy. The actual determination of values for liquid transport is then done using hygrothermal simulations.

Diagrams 1, 2 and 3 show the measurement results of a Kapi-Test performed on calcium silicate – a hydrophilic, mineral-based material with extremely fine pores. These results are contrasted against the results of the numerical simulation featuring vapor transport coefficients based on the new as well as on conventional measurement methods.

Especially at the beginning of the test, we see a marked increase in weight (diagram 1). Looking at the results for moisture distribution (diagram 2) we see that moisture levels increase first on the chilled rear side of the sample. As the test continues, we observe less and less water build-up as moisture is dispersed over a larger area of the sample. After about three to four weeks, a state of equilibrium is established at a total water content of around 49 kg/m³.

In the numerical simulation, the values provided by the Kapi-Test (blue curve) were largely in accordance with the measurement results in terms of relative moisture – both in relation to the increase in absolute water levels and to the moisture distribution (diagram 3). In contrast, simulations based on values provided by wind-and-weather tests (water absorption test, redistribution tests) show significantly lower moisture levels and that a state of equilibrium is reached more quickly.

The measurements show that with the help of the new test method, it’s possible to achieve more accurate values for calculating the opposing action of capillary reverse transport. Using hygrothermal simulations based on these values, it’s also then possible to test interior insulation designs while taking into account measured, non-static climatic conditions. Since this gives a realistic picture of the behavior and functionality of these designs, we are now much better equipped to assess application safety as well as how systems may be refined.


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Dr. Cornelia Fitz

Fraunhofer Institute for Building Physics IBP
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