U-frame and Condensation

[Angelica Maria Fernandez Rubio]

Good morning,

I have this problem: I have two glazing configuration: 1/4" Solarban 72 clear on Surface #2 + 1/2" Argon 95% + 1/4" Optifloat Clear, and the other one is 1/4" Solarban 72 clear on Surface #2 + 1/2" Argon 95% + 1/4" Energy Advantage on Surface #4 .

When you compare the Optifloat clear and the Energy Advantage, you can see that the second one has better thermal and optical properties therefore the whole Center of glass U-value will be better (attached below). it is supposed that if the glazing system is better, the window detail will be better. But the problem is that when I run the same head (sill and jambs) with the two glazing system, the system that was supposed to be the worse, turn out that has better Frame U-factor and does not present condensation. 

What happen here? Why the best glazing system has more heat transfer than the other one?

Therm 7.7 and Window 7.7

Thanks for your time, 

Head with: 1/4" Solarban 72 clear on Surface #2 + 1/2" Argon 95% + 1/4" Optifloat Clear

Head with: 1/4" Solarban 72 clear on Surface #2 + 1/2" Argon 95% + 1/4" Energy Advantage on Surface #4 .

[Robin Mitchell]

without seeing the actual THERM files, it's hard to say. 

Robin

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Robin Mitchell

Building Technology and Urban Systems Division
Energy Technologies Area
Lawrence Berkeley National Laboratory

[Angelica Maria Fernandez Rubio]

Hi Robin,

i forgot to attach the files, This is the one with the Optifloat Clear.

Thanks

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[Angelica Maria Fernandez Rubio]

And this is the one with energy advantage

[yalin uluaydin]

Angelica,

This is quite simple. You have a low-e coating on the interior side of the IGU. Although this will give you a better center of glass uvalue it also will reduce the inside surface temperature of the glass.

As you know the purpose of low-e is to reduce the radiation heat transfer between bodies. By reducing the radiation transfer tonthe glass from the interior you have less heat loss to the surface (better uvalue) but because you reduce the heat loss to the surface it reduces the surface temperature.

It is similar to putting insulation inside of wall or curtain wall layers. It results in the indoor heat being isolated from the outer layers and the outer layer temperature getting colder.

Yalin

[Angelica Maria Fernandez Rubio]

Hi Yalin,

Thank you for your response. Even though I understand your point, I am not complete sure about that because my inner glass layer should be sufficient to stop the condesation on the interior surface of the glass, caveat the isotherm could be offset to the the right. This kind of logic would justify that a "better" glass layer to generate condensation when it should be in the contrary.

Thanks for your time,

[yalin uluaydin]

Angelica,

It is pretty basic knowledge and commonly known in the industry that interior Low-e lowers the surface temperature.

Because of the lower glass temp it affects the frame by conduction and gray body radiation exchange between the frame and the glass which would lower your frame temperature. Because the frame emissivity is 0.9 and has “normal” radiation transfer to the inside and the temperature of the frame is lower than the without the low-e on #4 there is more heat loss through the frame. Radiation heat transfer is a function of the temperature difference to the fourth power so nonlinear and can’t be thought of or rationalized as linear as you are thinking.

If you condsider the combined stabdard film coefficient of 10W/mK used in Europe or 1.46 btu/hr.F.ft2 in the US about 2/3 of the combined film coefficient can be attributed to radiation. By using a low-e on the inside you cut out a lot of heat to the glass and radiation. I came across this about 10 years ago when inside low-e was introduced to the market as a product and had similar reactions as you did but unfortunately this is the facts.

There’s info online about this if you want to search for it.

https://www.dwmmag.com/experts-discuss-risks-versus-rewards-of-surface-4-low-e-coatings/

Yalin

[Ventura, Ed Ramir]

Angelica,

You say Solarban 72, but the file is using SB 70XL? just want to be clear which is correct?

What would happen if you do not flip glass 2 such that low e coating is on surface #3 instead of #4?

Thanks,

Ed

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[Robin Mitchell]

Yalin

Thank you for your excellent response (as usual!) to this question, as well as the link.

Robin 

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Robin Mitchell

Building Technology and Urban Systems Division
Energy Technologies Area
Lawrence Berkeley National Laboratory

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[Angelica Maria Fernandez Rubio]

Hi Yalin,

I understand so in this particular case, where the dew point isotherm is near to the interior boundary, it is worse to use de low-e in the interior surface, but if you have a good thermally broken system (the dew point isotherm away to the interior surface) and also you have to decrease the U-value, is it tent to use the low-e in interior surface? or people just prefer to use other methods? I ask that because in this case the energy advantage on 4th surface just decreases the temperature a few degrees (at this time it was enough to create condensation). Sorry, I pretty new in the industry and this forum helps me a lot.

Thanks for your time!

[Angelica Maria Fernandez Rubio]

Hi Ed,

The correct glazing was in the file, I made a mistake. I will try to that.

Thanks for your time,

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[yalin uluaydin]

Angelica,

You can use the low-e on interior if the indoor conditions don't result in too much condensation. If there's condensation at the center of the glass it defeats the purpose of the window as you can't see outside. If indoor conditions allow it then surely you can use but if you are getting condensation basically doesn't work.

For better glass performance the typical is to use a gas in the gap like argon of krypton or use three panes of glass and two gaps.

You can also try and improve the frame u-value to get better performance. I see one issue that may help is that you have a big air gap that sees both the extreme cold and extreme warm boundary conditions. Because the heat transfer is related directly to the temperature difference you will have large losses and will somewhat bypass you thermal breaks. If you look at some European high performance frames they will tend to break up these air gaps.

Secondly in your model you are using thermally improved NFRC boundary. You have thermal breaks larger than that required in NFRC 100 so you can consider this thermally broken and use the thermally broken frame boundary which improves you frame. From 0.7973 the u-value becomes 0.790

You can break up the large cavity per ISO15099/NFRC100. Frame u-value becomes 0.77. But the large cavity still has a large Nu value of 6.25.

You can break up the cavity with thermal break fins or gaskets and brings the frame u-value to 0.61 which is a 30% improvement. I'm sure there's other ways to break up the gaps and a cellular gasket would give you more flexibility in production.

Yalin