Metal panels

[Michael Hurd]

Within the construction industry there are many un-insulated metal panel systems available for use on the exterior of buildings, my question with respect to these systems is two-fold:

1. When these systems are installed such that they have a continuous line of sealant around their perimeter they become (in my mind) virtually impervious to air infiltration. Typically when we use these systems we also install an air barrier such as Tyvek behind them on the surface of the exterior sheathing. When modeling such a wall what material could I use in our calculations to aproximate these metal skin materials?

2. If such a metal panel system was not sealed at its terminations but instead was installed with a metal flashing system, that could in theory allow some air behind the panels (the amount not known) what adjustments could I make in the model to aproximate this condition?

[Achilles Karagiozis]

Q1. You should create a new material if metal is not available.

Q2. In the next version of WUFI you can handle the ventilation aspects. As a very conservative approach I would model the wall with and without the metal panel system. For the system without the metal panel, I would turn off the wind driven rain option. [/b]

[Michael Hurd]

Achilles, I thought that creating a new material (my question no. 1) would be the way to go but, I wasn't sure what properties to use. I assume that porosity and permeability would be zero - correct? For the other required properties I'm not sure what values to use. Are some of the properties un-important for a metal panel situation? Do you have any further guidance in these matters?

[Thomas]

I assume that porosity and permeability would be zero - correct? For the other required properties I'm not sure what values to use. Are some of the properties un-important for a metal panel situation?

Dear Mr. Hurd,

if the metal layer is on either surface of the component (i.e. it is the outermost layer), it may be sufficient to model its vapor-retarding effect by specifying an appropriate "sd-value" (SI) or "permeance" (IP) for the respective surface. The metal layer then need not be explicitly modelled in the layer assembly. See the on-line help for the "Surface Transfer Coefficients" dialog. Please note that the rain-shielding effect of the layer must be separately accounted for by setting the rain water absorption factor to zero.

If for some reason a metal layer has to be modelled explicitly, some tinkering is needed. WUFI was originally developed to simulate heat and moisture transport in porous materials, so it was not meant to treat impervious materials like glass or metal sheets. In particular, it is assumed that every material has a certain water content (that is, its moisture storage function is not zero). While it is possible to set the moisture storage function of a newly defined material to zero, this may possibly lead to numerical problems, depending on the individual circumstances.

On the other hand, it does not matter if your 'metal' material does have some water content, as long as the water is locked in place and is not exchanged with the rest of the component. So you might try the following: take any material with a normal moisture storage function, e.g. some concrete. Leave the moisture storage function as it is, but set the liquid transport coefficients (both suction and redistribution) to zero and set the vapor permeability to a very small value (WUFI will not accept zero here).

The bulk density should have the appropriate value of the metal sheet, as should the specific heat capacity and the thermal conductivity. If the thermal conductivity is not specified as moisture-dependent, it will not be affected by the moisture content present in the material. The moisture content will slightly increase the heat capacity of that layer, but this will have negligible effect on the results. The porosity only determines the maximum amount of water that can be taken up and can remain at any value. Setting it to zero might again produce numerical problems.

In the calculation results, WUFI will then show some (constant) moisture content for this layer, but the moisture will be completely inert and can be ignored.


And regarding the ventilation issues: as mentioned by Achilles, handling ventilated air layers must be left to the next WUFI version, but question #20: "Ventilated curtain walls" in the Q&A section of the online help may provide a few hints of what little can be done with the current version, if that is of any help to you.

Regards,
Thomas

[jarrett davis]

So with all of this said of nearly six years ago. What is the best way to model a metal rainscreen system? Is it still to not necessarily spell out the metal panel and turn off the wind driven rain function, or to assign a material that is metal, but to "ventilate" air cavity?

[Rasava]

So with all of this said of nearly six years ago. What is the best way to model a metal rainscreen system? Is it still to not necessarily spell out the metal panel and turn off the wind driven rain function, or to assign a material that is metal, but to "ventilate" air cavity?

This is a good question. I am working on a model where we have a rain screen metal panel that has open joints. If I include the metal panel in the model, the assembly will not pass ASHRAE 160. If I remove the metal panel, it does pass 160. What way is the correct method?

[jarrett davis]

I never really got an answer. So if anyone has one that would be great.

[Daniel]

I think best thing is to simulate the metal panel with suitable material properties: especially no rain water absorption and close to zero vapor permeability, realistic radiation Exchange on the surface... the rest is of little relevance.

Behind the panel a ventilated air layer - I'd try to vary the ventilation rate (constant values) between min around 5 h-1 and Maximum maybe 50h-1. Reality should be inbetween.

Daniel

[holtzcjd]

Has this issue been answered? I'm looking to model an aluminum panel that serves as a rain screen using WUFI Pro 6.1, is there an answer yet? I did not find one on the forum.

thanks
david

[Daniel]

What information is missing for you in my answer directly above your post?

[arnaldosh-eme]

Hi Daniel

Thank you for your comments. I wanted to follow up on this discussion thread. How do you adjust the ventilation rate of the rainscreen to stimulate a ventilated air layer?

Thanks
Arnaldo

[Christian Bludau]

Hi arnaldosh-eme,
for taking in account ventilation, you have to insert a ventilation source in the airlayer which you like to ventilate.
For this source you can set the needed strength.

Maybe you will find our tutorial "Handling of typical constructions" interesting, where inserting an ventialtion source is described.
https://wufi.de/en/service/downloads#ty ... structions
Christian