| Conceptual Reference Database for Building Envelope Research |
Prev Next |
| Essay:Glaser method, moisture profile method, dewpoint methodThe Glaser method as detailed in German standard DIN 4108 has been a common method to assess the moisture balance of a building component by considering vapour diffusion transport in its interior. However, this method does not allow for the capillary moisture transport in the component, nor for its sorption capacity, both of which reduce the risk of damage in case of condensation. Furthermore, since the method only considers steady-state transport under heavily simplified boundary conditions, it cannot reproduce individual short-term events or allow for rain and solar radiation. It is meant to provide a general assessment of the hygrothermal suitability of a component, not to produce a simulation of realistic heat and moisture conditions in a component exposed to the weather prevailing at its individual location. (source: WUFI http://www.hoki.ibp.fhg.de/ibpe_sof1_fr.html)
The Glaser-method was designed to calculate the amount of interstitial condensate formed during a cold winter period and the theoretical amount of evaporable water in a cold summer. If the amount of condensate does not exceed specified limits and, if it is lower than the evaporable amount of water, the building assembly is considered to be on the safe side. The Glaser-method does neither account for hygroscopic sorption nor for liquid transport. Therefore its application is more or less limited to light-weight structures. Since hygroscopic materials are present in most building assemblies steady state boundary conditions should be employed. Transient conditions require correct moisture storage terms e.g. a sorption isotherm. From that aspect block conditions for winter and summer are better suited than monthly mean values. It should be noted that Glaser-calculations are no simulation of reality, but a tool to determine the risk of interstitial condensation only. Problems with other moisture loads, such as construction moisture, precipitation, summer condensation or rising damp are beyond the scope of the Glaser-method. [Sources Kunzel, H. M., 2000]
"Glaser's thermal diffusion model is based on Fick's Law, and it is still used widely in civil engineering, for condensation risk analysis and in defining the quality specifications which the constructive elements must satisfy, due to its ease of use graphically. It presents a great simplicity as it only lets the unidirectional moisture transfer intervence in the vpaour phyase and assumes a steady state. This method allows on to predict condensation. However, it considers that the liquic phase resulting from condensation does not have any subsequent movement. The comparison of the theroetical and experimental results (Crausse, P. "Edude fundamentale de transferts couples de chauer etĄ ; J. Van De Kooi, Moisture transport in auclaved aerated concrete roofs. Thesis, Eindhoven University of Technology, Waltman, Delft, 1971) shows that Glaser's theory is insufficient to predict the condensation phenomena" Freitas and Crausset, 1996, Moisture migration in building walls - analysis of the interface phenomena Glaser, H. 1959. Graphisches Verfahren zur Untersuchung von Diffusionsvorglinge. Kalfetechnik 10: 345-349.
Related: WUFI-ORNL/IBP ASHRAE moisture profile (dewpoint) method Summary by: Sherwood, G. E., 1983, Condensation potential in high thermal performance walls ¨C cold winter climate "The most commonly used design method is the moisture profile or dewpoint method which is described in the ASHRAE Handbook (ASHRAE 1977). The method is based entirely on diffusion theory, thus ignoring any convection effects. Assuming steady-state conditions, the temperatures can be calculated at points within a wall or ceiling from indoor and outdoor temperatures and the thermal resistances (R-values) of each layer of material. Each temperature corresponds with a saturation vapor pressure at that point. Similarly, actual vapor pressures at those points can be computed from indoor and outdoor vapor pressures and the vapor flow resistances (Rep-values2). If the calculated vapor pressure is above the saturation vapor pressure, condensation occurs. The most likely locations for condensation in a wall are the inside (warm) surface of the sheathing (Sherwood 1983) and the inside surface of the siding. It is not always clear which is the actual location. Multiple locations are also possible. The moisture profile method requires the user to choose a location for condensation, usually at the interface between two layers of material. Here the vapor pressure per definition must equal the saturation vapor pressure, which changes vapor pressure differentials across the rest of the wall, and consequently the vapor pressure profile should be recalculated to check if condensation is possible at any other location. If other possible locations are found, the process is repeated. This sometimes leads to elimination of the first location (see following example). The process is repeated until the user has identified all locations for condensation which can exist simultaneously. The rate of moisture accumulation may then be calculated from the difference between vapor flow to and from the condensing surface."
| |||||||