surface diffusion in Redistribution Liquid Transport Coeff

[edgararq]

The North America ORNL Database have several materials with a moisture dependent Water Vapour Diffusion Resistance Factor, not only wood or wood based materials but also stone based materials. Is that a simplification of surface diffusion? I understand, based in the Online help, that surface diffusion should be included in the Redistribution Liquid Transport Coefficient in order to represent correctly the phenomenon of vapor diffusion and water surface diffusion going in opposite directions in a temperature gradient climate.

[Thomas]

Yes, you can consider these cases (or most of them) as simplified representations of surface diffusion. The sources from which these data were taken are providing the data in this way, and we have not attempted to modify them (beyond simple unit conversions etc.)

Kind regards,
Thomas

[edgararq]

Thank you Thomas... does it mean that, in those materials, Liquid Transport Coefficient does not represent surface diffusion at all?

Kind Regards
Edgar

[Thomas]

does it mean that, in those materials, Liquid Transport Coefficient does not represent surface diffusion at all?

At the lower end of their humidity range, the liquid transport coefficients may contain some contribution due to surface diffusion, depending on the measured humidity range, the measurement method and the measurement accuracy. So occasionally some surface diffusion may even be represented twice: in the variability of the mu value and in the liquid transport coefficients.

Usually no attempt is made to disentangle the contributions of the various transport processes in this intermediate humidity range (where diffusion and liquid transport compete as the dominant transport processes). Please keep in mind that WUFI is meant to be a simplified simulation tool, working with simplified and practically available input data. In most of the cases occurring in practical building physics, this ambiguity will be of no concern at all. If you investigate the capillary water uptake of a wall, capillary liquid transport will be far greater than the small contribution of surface diffusion or vapor diffusion. On the other hand, if the wall remains dry, you will mainly have vapor transport as described by the mu value, the variability of which allows a rudimentary - and by no means perfect - means to include surface diffusion.

If you specifically wish to investigate the details of the combined transport processes in the intermediate humidity range, you can do so, but then you need material data which have explicitly been prepared to be suitable for this purpose - material data from standard material catalogs may often not be sufficient.

Kind regards,
Thomas

[edgararq]

Thanks Thomas,

Edgar.

[Lois Arena]

Edgar,

I have been looking for this Water Vapour Diffusion Resistance Factor and cannot find one material with it in the N. American database. Can you please tell me where you see it? I have been trying to figure out how to change it for about an hour. The help file makes it seem like you change it on the Material Data sheet, but I don't see it anywhere.

Thanks,
Lois

[Christian Bludau]

Hello Lois,
if you´ve set WUFI to IP-units, then it is called Permeability... you can transform the permeability to water vapour resistance, for more information see here:
http://www.wufi-wiki.com/mediawiki/inde ... rDiffusion
Christian