menbranes porosity's

[Raymond]

Hello,

In Wufi materials database, the vapor retarders have their porosity set to an arbitrary value (eg 0.001) in order to represent their low porosity.

We want to represent the behavior of the rain shield Delta-Vent N/S (Doerken) between two materials layers but we don't know its porosity.
We see in Wufi materials database that weather resistive barriers have a porosity also set to 0.001

If I well understand, rain shields have a "porosity value" enough big to let the vapor migrate through the membrane, but as in Wufi the Sd value is used to represent the vapor migration and the porosity to represent the maximal water content, we set the porosity to a very small value to represent the behavior of such hygrophobic materials (that can't retain water).

Thus even if we don't know the Delta-Vent membrane porosity we can set it to a very small value of 0.001.

Thanking you by advance.

Greetings.

[Thomas]

If I well understand, rain shields have a "porosity value" enough big to let the vapor migrate through the membrane, but as in Wufi the Sd value is used to represent the vapor migration and the porosity to represent the maximal water content, we set the porosity to a very small value to represent the behavior of such hygrophobic materials (that can't retain water).

Thus even if we don't know the Delta-Vent membrane porosity we can set it to a very small value of 0.001.

Hi Raymond,

yes, the vapor permeability of a material in WUFI is described exclusively by its mu-value (which, for a layer with a given thickness, results in a certain sd-value for this layer).

The porosity, on the other hand, is only used as a parameter which describes the maximum water content.
If the material in question uses a tabulated moisture storage function, the moisture contents below free saturation are used as tabulated (and thus independent of the porosity value), whereas moisture contents between free saturation and maximum saturation are affected by the value chosen for the porosity (because the maximum water content is 1000 kg/m3 * porosity).
On the other hand, if no moisture storage function is explicitly tabulated, and WUFI therefore uses the default moisture storage function, all the moisture contents will be affected by the choice for the value of the porosity, because the porosity is used as a parameter determining the shape of this default curve.

Thus, for materials without explicitly tabulated moisture storage function (as is usually the case for membranes), the value for the porosity can be used as a parameter for adjusting the desired moisture storage function; this does not change the vapor permeability of the material.
(By the way, most plastic membranes have no pores anyway, the water molecules are diffusing through the plastic material itself. Therefore the porosity value in WUFI does not have any real physical meaning in the case of these materials.)

So yes, the value of 0.001 used for the porosity of most membranes in the database is more or less arbitrary and is just intended to ensure low moisture contents in the plastic membranes.
There is no point in pondering the real amount of pores in a given plastic membrane - in the case of membranes the 'porosity' is just a numerical parameter affecting the moisture storage function, and the vapor permeability is described by the mu-value as a different and independent parameter.

Note: if you are simulating a thin membrane as part of the construction assembly you might consider the possibility to enter the membrane with greater thickness than the real membrane and to use an appropriately modified mu-value which (together with the modified thickness) results in the correct sd-value of the membrane.
The advantages of this approach are discussed in the WUFI on-line help in the topic Reference | Material Data | Special Materials | Membranes.

Regards,
Thomas