Volume 6, Issue 9 (October 2009)
Vapor Permeability Measurements: Impact of Cup Sealing, Edge Correction, Flow Direction, and Mean Relative Humidity
Water vapor permeability is a basic hygrothermal characteristic of a porous material. At first sight, the cup test used for measuring the property looks simple. Conditions to be fulfilled, however, are that the vapor flow crosses the sample only, is one-dimensional, and that flow direction has no impact on the result. Flow through the sample only demands a perfectly vapor-tight edge-high seal between sample and cup. That condition was tested using different sealing solutions. The results learned that more precautions than expected were needed. If the flow is not one-dimensional, for example, because such perfect seal is only realizable for a sample larger than the cup mouth, an edge correction must be applied to the value measured. As far as direction is concerned, measurements on a couple of hygroscopic materials gave different vapor permeability results for inward and outward flows. Long-lasting sorption after a fast initial hygroscopic moisture uptake appeared to be the reason for that. Dry, wet, and very wet cup testing shows that vapor permeability of hygroscopic materials increases with relative humidity. Some attribute that effect to surface flow from higher to lower relative humidity in the adsorbed water layers once beyond 50 % relative humidity. If that hypothesis was correct, apparent vapor permeability of hygroscopic layers, in situations where partial water vapor pressure and relative humidity gradients oppose, should be much lower than that measured with a wet cup. During the cold season opposing gradients are present in assemblies with a hygroscopic layer at the outside, covered by a vapor retarding finish. Yet, hot box testing on that kind of assemblies did not reveal vapor permeabilities lower than the wet cup value, although interstitial condensation between the hygroscopic and vapor retarding layer was a fact.