Hygrodiode

[mstellan]

I was wondering if the water permeable Hygrodiode is suited for calculations in WUFI?

Its capacity to absorb and transport liquid water is a vital property which seem to be dropped if you design the product on your own on the behalf of Sd-values only.

I am using the WUFI 1D Pro5.1 version.

Thanks for any thoughts regarding the subject!

[Manfred Kehrer]

It depends on how you define a Hygrodiode.

A real Hygrodiode is a material having a water vapor permeability depending on the direction of the driving potential, which

- can not be modeled in WUFI so far, maybe in a couple years with some tricks
- does in my knowledge not exist today.

But I think this is not really the answer for your question, is it?

[Thomas]

As far as I'm familiar with the hygrodiode, its function depends on the property that it is permeable to liquid water (conducting the water in a hygroscopic fleece layer) but relatively impermeable to vapor transport (because the vapor has to travel a long way through the fleece layer, parallel to the hygrodiode sheet).

Thus, when liquid moisture condenses on the hygrodiode, it is wicked into the sheet and transported across the hygrodiode, while for water vapor the hygrodiode acts as a vapor retarder.

In principle it should be possible to model the hygrodiode in WUFI by using the appropriate mu value to describe the vapor transport, and strongly moisture-dependent liquid transport coefficients (high value close to free saturation and almost zero for lower water contents) to describe the liquid transport when condensation occurs.

You'd need some transport measurements to properly calibrate the properties of your WUFI model. The measurements tabulated here
H.M. Künzel: Adapted vapour control for durable building enclosures
may possibly serve as a starting point. It may even be possible to model a simplified version of the hygrodiode by simply using moisture-dependent mu-values, but you'd have to check whether this simplification does work for your application...

Regards,
Thomas

[mstellan]

Hygrodiode is a vapor retarder which is composed of a synthetic fabric sandwiched between perforated polyethylene film (inside) and strips of special moisture dependent film (outside). I guess the strips are polyamide film, which changes its vapor permeability with the ambient humidity conditions. This is the new and improved Hygrodiode according to Icopal.

The old Hygrodiode was composed by a synthetic fabric sandwiched between strips of polyethylene film. Künzel has done som research with this retarder in earlier articles. For example in "Laboratory and practical experience with a novel water-permeable vapor retarder".

Im not sure if it was this product you thought of?

[mstellan]

I posted my answer without knowing you had answered Thomas. I will check this out. Thanks to you both!

[Thomas]

The old Hygrodiode was composed by a synthetic fabric sandwiched between strips of polyethylene film.

That's what I'm familiar with. If there's a new kind of hygrodiode with different kinds of plastic films on both sides, that's more difficult to model and would probably require a composite material, made from several layers with different properties. But in principle, it should be possible to model this more or less closely in WUFI.

Regards,
Thomas