IES-VE User Forum

Hi everybody,

I was wondering which is the proper way to set internal and external emissivity for each layer of a glazing unit.

When I insert a different emissivity value (both on the outer and inner face of the internal pane) in the CDB dialogue, nothing is going to change at all.

It doesn't make sense!

Is there an alternative and effective path to follow?

Many thanks

Ornella

Another related question guys..

Is there a manner to simulate an ETFE rooflight?

I have this doubt because I don't understand whether IES realizes that ETFE, differently from glass, transmits part of the longwave radiation.

So that I was wondering in which way I can give this kind of input..Otherwise I should simulate ETFE incorrectly as a pane of glass..

Many thanks

Ornella

...nobody??

I thought that this forum was more popular than that...

Sorry to hear about your poor experience on the forum. I too have had questions go unanswered which has prompted me to reply.

It looks to me like changing the emissivity effects the U-value. The effect does appear small. To directly answer your question, I might suggest leaving the values at their default unless specific data is provided by a manufacturer.

From a thermodynamic point of view I speculate that changing the emissivity simply changes the mode of heat transfer and not the total energy exchanged through the construction. For example, by decreasing the emissivity I suspect that the energy is simply now being transferred by convection rather than radiation. So at the end of the day it probably doesn't have a large effect on the overall performance of your structure or even the window.

As far as ETFE rooflights I don't know what those are so I can't help there.

Hope that helps.

Hi,

With any type of "glazed unit" or ETFE or polycarbonate, I would just look to match the U-value and G-value to represent its general thermal performance.

Even if a manufacturer gives you every piece of data you can think of, if you put it into IES the calculated parameters never seem to match. You can easily waste a lot of time.

I have had a look through some of the calculations in standards like EN 410, it is very messy for anything other than single glazing. I would aim to keep things simple.

I hope this helps.

Ross