Product Development with the Help of Hygrothermal Simulations

DampfbremseProduct development is often a continuous process in which existing materials and/or components undergo incremental improvements over potentially long periods of time. The optimization of bricks and mortar to improve the thermal resistance of monolithic walls is one example of continuous product development.

The development of entirely new materials usually begins with an idea that will address a practical problem. The next stage of the development usually draws on existing experience and knowledge, but the success and speed of the development will depend on a number of factors. For new building materials and components, hygrothermal simulations can aid in the development process.

This document will describe the development of “smart,” humidity-controlled vapor retarders. Since their market launch in 1997, smart vapor retarders are now well established in the building industry. The development of smart vapor retarders was aided by hygrothermal simulations.

The Idea

Investigations of many cases of moisture-related damage in buildings have shown that it is nearly impossible to use water/vapor barriers to keep components completely dry. For this reason, the latest thinking in building science is that it is critical for components to be able to dry. A limited absorption of moisture from rain or condensation is considered harmless, as long as there is an interim drying processes and that there is no long-term moisture accumulation. This drying concept is particularly important for “fachwerk” (half-timbered timber framing) walls and steep roofs. These constructions are vapor-tight on the exterior and need to be able to dry out to the interior. If vapor barriers are used on the interior and there are even small defects in the overall construction, significant moisture damage can occur.

To prevent damage and mold growth from excessive moisture within building components, Dr. Hartwig Künzel, from the Fraunhofer Institute for Building Physics realized that a new type of vapor retarder was needed. The vapor retarder would need to be impermeable when there was high likelihood of moisture ingress from condensation and highly permeable when there were favorable conditions for drying.

First Steps

To determine the optimal permeability characteristics of this new vapor retarder, Dr. Künzel and his staff first used WUFI Pro® to analyze the moisture behavior of specific constructions. In particular, roof structures and fachwerk walls were analyzed in Central European climate conditions. It became clear that a vapor barrier with an sd-value of 2 to 5 m was the best compromise between the need to prevent moisture ingress while still allowing drying. However, simulations showed that although the drying potential with the given sd-values was better than with conventional vapor retarders, the drying potential was not good enough to ensure that moisture-related damage would not occur.

Subsequent WUFI Pro® simulations performed by Dr. Künzel and his research team showed that a virtual vapor retarder that becomes 10 times more permeable in the summer would be a good solution for moisture management in the roof and facherk wall constructions being considered. Thus the team needed to develop a vapor retarder with increased vapor permeability during in the evaporation period (summer) and with no permeability in the condensation period (winter). The permeability should therefore be variable and this variability must a function of a climatic state variable that clearly distinguishes between evaporation and condensation periods. The relative humidity on the two sides of the vapor barrier is such a climatic state variable.

The Implementation

Having used simulations to determine what properties were needed, the team began the process of finding a membrane that would change vapor transmission characteristics based on surrounding relative humidity. The team tested various polymer films in the laboratory before finding a polyamide film (PA) film which had the moisture-dependent permeability of the virtual membrane used in the simulations.

Being stable at high temperatures and aroma-tight, PA films have been used primarily for food packaging and for baking/roasting food. PA films also have good mechanical properties and are easy to manufacture and glue. Through a robust development process it was found that PA films could also be manufactured with vapor transmission properties that could vary by 10 times, depending on the surrounding humidity. Thus this smart, humidity-controlled vapor retarder was patented, and several companies licensed it for mass production.

Moisture content in a steep roof

Calculated moisture content of a steep, north-south oriented roof with a pitch of 28 degrees. Compared to a conventional vapor retarder with a constant sd-value of 2m, the smart, humidity-controlled vapor retarder shows significant improvement in summer drying. In fact, moisture accumulation has been prevented and the roof construction is becoming slightly dryer with increasing numbers of years .

Diffusion resistance dependent on relative humidity

Measured data of diffusion resistance of a polyamid foil of 50 micrometers thickness (see figure below) dependent on relative humidity. The figure above shows the diffusion resistance as thickness of a stagnating air layer with the same resistance.