U-values for metal panels

[Lampham]

Hello everyone. I would love to ask, for glass we can get COG (Center of Glazing) in Window software. However, I wonder if it is acceptable or correct to do the same with metal panels, as per my image below

Thank you all. 

[Collin Robinson]

Hi, one issue I see is that the metal panel "glass" sheets have a low-e coating. In reality they would not have low emissivity unless they were polished on the interior. The U-factor you show looks reasonable for truly dead air space, but I doubt the panel will be perfectly sealed. 

I once attempted something similar with insulated spandrel panels, to get an assembly value with mullions. 

I am curious what others think about this, too. 

These slides at link below have some good info on calculating insulating value according to ASHRAE 90.1. Also see attached portion of ASHRAE handbook, which has some similar results. The quantity is resistance not transmissivity, however. 

https://www.continuousinsulation.org/sites/default/files/designpptairspacervalue.pdf

Best,

Collin

[Lampham]

Dear Collin, 

Thank you very much for your answer. It was very helpful.
In addition, I would like to ask: I’m trying to model a metal sheet in WINDOW to obtain the center-of-panel U-value. Is this an acceptable approach? I couldn’t find any specific guidance on this in the NFRC documentation. Even in NFRC 100, Table 4.3, which lists the representative products for simulation, there is no mention of metal panels. 

Thanks & Best Regards!

Phạm Viết Lâm

Email: Lamph...@gmail.com

Mobile: 0868213013

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[Collin Robinson]

Hi Pham, 

I can see why you would want to use WINDOW this way, but it's really for fenestration only. That's why it doesn't appear in NFRC 100. There are established physical tests such as ASTM C518 (center of panel) and ASTM C1363 (assemblies with joints) for insulated metal panel. THERM is well-suited for both center and assembly, assuming you can't get data from the manufacturer. The Therm user manual is a great place to start. 

I made a simple model in Therm 8.0, which is 1mm galvanized steel each side separated by 100 mm of air space. It's showing U-factor of 1.6184 W/m²-K. See attached. 

Best,

Collin 

[Lampham]

Dear Collin, 

Thank you so much for your support. It helps me a lot.

Thanks & Best Regards!

Phạm Viết Lâm

Email: Lamph...@gmail.com

Mobile: 0868213013

Skype: Lampham1593

To view this discussion visit https://groups.google.com/d/msgid/LBNL-THERM/c52a0a9c-2bd5-4c54-8377-833b67bf6ef6n%40googlegroups.com.

[yal...@gmail.com]

Collin,

The air gap table is for a horizontal air gap. The panel is vertical in the LBLWindow screen clipping. 

The panels have an emissivity of 0.2, so possibly anodized finish. This would be an effective emissivity of 0.11. Reading from the air gap table for 90mm vertical air gap (Close enough to 100mm) with horizontal heat flow, mean air temp 10C, and temp diff. 16.7C the resistance of the air gap is R=0.33m^2K/W. Adjusting for the gap width the conductivity of the air gap is [1 / R] * 0.09m = 0.272 W/mK conductivity. 

The effective conductivity of the air gap in Window is Gap 1 Keff and reads 0.288 W/mK so would say the calculation in Window is reasonable. There's some small nuances with the ashrae table you can interpolate the air gap mean temperature which would show the numbers converge.

There's a good point about NFRC applicability. A lot of the frame boundary conditions are based on window frames so now applicable. Unless this panel is part of a curtain wall or fenestration system NFRC might nor be applicable. You can use ASHRAE boundary conditions, I think these are in the base installation of THERM. If you click the "load lib" button in the BC window the BC-Arch.lib will have ASHRAE 90.1 boundary conditions to use and import into LBLWindow via manual input and creation of new BC's.

For all practical purposes a hermetic seal is not needed for the air gap values to be applicable. 

I would be mindful that the air gap material property in THERM is not the same as an air gap in WINDOW. Therm is a cavity that is generally enclosed all sides at its perimeter as a simple explanation. The layer gap in Window is planar and can be assumed to have adiabatic perimeter or a large planar cavity like a metal panel or IGU. The equations for THERM frame cavity and WINDOW air gap are different so are not necessarily interchangeable.

Yalin

[yal...@gmail.com]

Collin,

Try making the model longer, i.e. extend the air gap to be 500mm tall, use 0.2 emissivity for the panels as per the original post Window screen clip. You will get close to the window calculation.

Yalin

[Collin Robinson]

Yalin,

Thanks for your detailed feedback on this! I revised my THM model to show longer section and corrected the table from Fundamentals, both attached. That's great to know that WINDOW could get similar results if the boundary conditions are minded appropriately. I learned a lot from this thread. In the revised THM model:

With bright galvanized steel, ε=0.20, the 100mm cavity had Keff of 0.27 W/mK, with overall panel U-1.56

With oxidized iron/steel, ε=0.80, the 100mm cavity had Keff of 0.50 W/mK, with overall panel U-2.73

Getting the metal finish right makes a big difference. 

Collin