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# Adjust U-Value of 4 layer ETFE Cushion

Hi all,

I´m working on a project with an ETFE envelope, both for the facades and roof. These are made of 4-layered cushions with air gaps between them. To model the cushions in E+ I´ve used the detailed specs from the manufacturer as WindowMaterial:Glazing WindowMaterial:Gas objects.

E+ outputs show a resulting U-Value of 1.28 W/m2K which according to the local technical standards needs to be increased by 0.5 W/m2K to account for the curved nature of the cushion (and varying air gap thickness). Essentially a "safety factor".

Is there a way of doing so?

I´m aware of the WindowMaterial:SimpleGlazingSystem object, but that would change the thermal interaction between the ETFE layers, and affect the resulting internal cushion temperature - which is key for the comfort on the last floor, right below the roof.

It seems quite tricky to account for this safety factor while maintaining the precision that WindowMaterial:Glazing brings in this particular case. Has someone faced something similar?

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If there is someone following the topic, the solution ended up being relatively easy.

I just adjusted the thickness of the air gaps between the ETFE films until I got the corrected U-value. The resulting U value can be checked in two ways, either in Window7.7, where I had first assembled my construction or in E+ (this last option requires running the model and checking the output files). I found the iterative testing faster in Window7.7, then followed by a final check on E+ output to confirm the value.

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May I ask what is the U value now? And what climate conditions dictated 4 layers?

( 2021-03-01 03:11:46 -0500 )edit

What's the gap now? What's the U-value? We are developing a similar etfe structure and really need to understand the flexibility we have with the U. Thanks

( 2021-03-01 03:15:47 -0500 )edit

This is for a project in South west France, close to the Atlantic coast. The U value according to the manufacturer was 1.8, which included a safety factor of 0.5 W/m2K according to NF EN ISO 673. From a simulation perspective, we got all the technical details from the ETFE film manufacturer, but I cant share those unfortunately. Hope this helps.

( 2021-03-01 15:22:48 -0500 )edit