Assessing a unitised facade

[Nick Devlin]

Hi,

I'm modelling a unitised facade. I'm aware that with non porous materials this doesn't fit what WUFI was originally designed for. The external wall assembly (from inside to out) is:

Plasterboard
vapour control layer
Air cavity (with SFS / metsec stud) (no additional moisture storage capability)
1.5mm steel
141mm mineral wool
1.5mm steel
41mm mineral wool
2mm aluminium.

The facade panel achieves the target u-value for the project, but only just satisfies the flanking noise acoustic reduction requirements and the acoustician recommends adding 50mm mineral wool into the air cavity / SFS layer to improve matters. I am investigating the condensation risk / moisture balance for the client. The model includes a high moisture load to 15026 and air infiltration into the SFS / insulation zone on the cold side of the vapour control layer (tested at both 5m3/m2 and 7m3/m2/hr@50Pa).

My first models included each of the metal layers, but balances 1 & 2 were wildy different, which I assume indicates a convergence issue (or similar) in the calculation. Interestingly, in these scenarios, the calculation suggested that the moisture was accumulating in the 41mm mineral wool layer, even though this is outboard of 2x 1.5mm steel layers. The water content of individiual layers was low, but the whole assembly never reached dynamic equilibrium.

Next, I removed the middle steel layer, (between the 41mm and 141mm insulation layers) to assess the impact of this on the calculation. The numerical balances are now within acceptable tolerances. The water content of the SFS / cavity mineral wool layer is <2 mass% and between 1.4-2.6 mass% for the mineral wool between the metal sheets after 5 years but continues to increase slowly for 20 years.

So, my questions are:

1: Is it acceptable or reasonable to remove the middle metal layer from the model to simplify the model to obtain better numerical balances (my assumption is yes).

2: The steel sheet layers should in effect be almost complete vapour barriers, so is the accumulation of moisture in the outer mineral wool layers realistic or a function of the numerical simulation that is not realistic.

3: Could the unitised facade panel element be better modelled as a fictitious homogenous material with very high vapour resistance factor, zero moisture storage (representative of the metals) and low thermal conductivity to reflect the insulation?

Many thanks in advance -

[Christian Bludau]

Dear Nick,

I guess, that the interesting question for you is, if there may be problems in the construction interior the metal sandwich.
As you write, the sandwich is tight on both sides - so if you put in an infiltration source there, you will get an accumulation of moisture (as you found in questions 2?)

Indeed I would leave out the middle steel sheet. Further I would not place and infiltration source in between the sheets. If you want to do that, you have to use some effective µ-value for the sheets including the connection between the elements. I would set the sd-value of the outer metal layer in the boundary conditions instead of building it in the construction as a layer. That will speed up the calculation and give you better convergence as well. Switch off the rain doing that.

The convergence will be maybe better if you split the air layer in the cavity as follows (assuming the air layer is 50mm, if it is different, use the thickness you have:
1 mm "air layer 50 mm"
48 mm "air layer 50 mm without additional moisture capacity"
1 mm "air layer 50 mm"

This will allow a realistic transport of the moisture through the layer and if dew water forms, the 1 mm layer with the higher moisture storage function can keep that.

Please also note, that if your air layer connects to a metal, you have to change the effective µ and lambda-values because of the changed long wave radiation which is included in this both effective values. Please see WUFI program help for further information on how to do that. Search for "air layers".

Christian

[Nick Devlin]

Hi Christian,

This is fantasitically useful. I will implement the measures you suggest and see what happens. I was placing the air leakage into the cavity on the warm side of the sealed unitised facade panel rather than into the panel itself.

Regards

Nick