Custom emissivity values for reflective membranes

[Bonilauri]

Hello,
I'm modeling a roof assembly with a reflective aluminum membrane included in it (see screenshot).

I need to adjust the emissivity value of the internal surface of the membrane, so that the radiative exchange through the unvented cavity is correct, but I can't seem to find a place to customize the emissivity.

Where can I do that?

Thank you,
Enrico

[Manfred Kehrer]

Hello Enrico,
Quick answer, you cannot!

Radiative energy transport is only accounted for at the exterior surface. Within the assembly the way to consider this effect is to change the Therm. Conductivity of the air to include the radiativ effect. As the radiation depends on the 4th power of the temperature, you can optimize this linearization only for on temperature.

Another option could be to program a dll plugin which calculates and applies a heat sink on one side and a heat source on the other side on an hourly bases.

[Bonilauri]

Thank you Manfred!

[Christian Bludau]

Hi Enrico,

the emissivity is included in the effective material data for the air layers. Please see the WUFI Pro program help concerning the air layers. There is described how they are calculated.

Further the European standard EN 673 offers includes equations to calculate effective values for µ and lambda also depending on the emissivity of the boundary surface.

Christian

[Bonilauri]

Hello Christian,
Thank you for the additional info. I've been using ISO 6946 (Annex A) to calculate the surface resistance for low emissivity surfaces, which is valid for steady-state conditions.

However, in a room assembly dynamic conditions the temperature will vary on a short-term basis (e.g. one hour), so the surface resistance will also vary as well depending on the hour of the day.

The roof assembly described above is for a roof in Australia, so it is subject to a broad range of temperatures over the span of a day in Summer.

[Christian Bludau]

Dear Enrico,

However, in a room assembly dynamic conditions the temperature will vary on a short-term basis (e.g. one hour), so the surface resistance will also vary as well depending on the hour of the day.

I completely agree. Unfortunately we can not take that in account with the air layers currently available. For the air layers close to the inside the influence is small, but but closer to the outside this may have a significant influence.
We are working on an implementation of the air layers, which are closer to reality, but thats gonna take some time.

Christian

[Bonilauri]

Christian,
Thanks for your reply. I understand it must be a lot of work!

Cheers,
Enrico