External aluminium layer in a roof

[Coverd]

Hi,
I need an answer to this question.
How can I model a roof with an external aluminium layer?

I've just made a structure with an aluminium layer, but this material causes any problem.

Also, the value of the surface temperature is too low, in my opinion (with the insulation of summer i havent seen higher values ​​than forty degrees °C on the aluminium surface).

I need to know: how it is possible and why I am wrong.

Thanks

Alberto

[Christian Bludau]

You can model the aluminium as an external sd-value on the surface or as a alyer. The layer can cause convergence problems, which you can avoid by using a finer grid for example. Switch on the adaptive time step control. If the balances in the table after the calcutlation are quite the same, you can ignore the convergence failures.

The temperature mainly depends on the vfalues you set for short wave absorptivity and long wave emissivity. If your material has an uncoated black surfaces, you will have an low emission, what causes the material to get warmer. For that you have to switch on the explicit radiation balance.

Christian

[fostertom]

What are the generally applicable guidelines for deciding whether the balances, and the no of convergence failures, are good enough? What principles - e.g. error 'not too bad' relative to what? I don't find any written advice that explains the principles (if any) and/or specifies the acceptable limits. I have no idea of the scale of "quite the same".

[Christian Bludau]

Hello Tom,
please check the programs online help Topic 76 "Dialog: Status of Last Calculation" and see the handouts from the WUFI-Workshop "Evaluation of hygrothermal simulation results".
Christian