[hamed1387]

Hi all,
I have a question in need of answering very soon. I am very grateful if someone could help me.
We have a single pane window, which has a specific emissivity, say 0.84. I add a low-e (emissivity) film on one side, say with an emissivity of 0.1. Now I want to know the emissivity on both sides is 0.1, or the other side, which is uncoated, has still an emissivity equal to 0.84.
Thank you very much in advance.

[LuckyTran]

The uncoated side still has an emissivity of 0.84. It helps a lot if you remember to call it surface emissivity, you'll remember that it's a surface property.

[Simbelmynë]

Out of curiosity, is this something that is worthwhile? I would guess that the radiation losses due to emission are rather small compared to convection losses as well as transmittance of radiation.

[hamed1387]

Originally Posted by LuckyTran

The uncoated side still has an emissivity of 0.84. It helps a lot if you remember to call it surface emissivity, you'll remember that it's a surface property.

Thank you a lot dear LuckyTran,
I guessed that emissivity is a surface property. But, I still have a problem with this concept. We attribute the emissivity property of objects to their ability to absorb radiation, even some authors reported exactly the same value for both: emissivity and absorpitivity, while as far as I know absorption occurs inside the volume of objects and is a volume-based quantity. Maybe, I am wrong. How does this make sense?
Thank you again for you help.

[Simbelmynë]

Originally Posted by hamed1387

even some authors reported exactly the same value for both: emissivity and absorpitivity,

That would probably be Kirchhoff's law, which is valid then the two bodies that exchange radiation also have the same temperature.

[hamed1387]

Originally Posted by Simbelmynë

Out of curiosity, is this something that is worthwhile? I would guess that the radiation losses due to emission are rather small compared to convection losses as well as transmittance of radiation.

As far as I know, radiation is sometime important, especially when we limit the effect of convection by insulation. In this situation, part of the heat loss occurs through radiation. Sometimes, even the thermal effect of radiation is not what we try to find. In fact, in some cases the optical matters is the key point.

But I should add that since I am relatively new in this field, I am not necessarily someone whose opinions are totally valid

[LuckyTran]

emissivity = absorptivitiy is indeed Kirchoff's law.

There is emission and absorption for both surfaces and volumes. But the volumetric ones have units of 1/length. We rarely talk about emissivity as a volumetric property, but all atoms emit thermal radiation by Planck's law. It also doesn't help that we call it by something else entirely. It's called emission coefficient due to its prevalence in spectroscopy.

Originally Posted by Simbelmynë

Out of curiosity, is this something that is worthwhile? I would guess that the radiation losses due to emission are rather small compared to convection losses as well as transmittance of radiation.

I have low-E fims on every single one of my windows, I applied it myself. The films themselves are rather expensive but they easily lower your HVAC load by 20-30%. It's quite significant and practical. They are part of building standards in many places. They're one of the levers we have for net-zero and I wouldn't be surprised to see required on every single new window installation by 2050, at least for G20 nations.

[Simbelmynë]

Originally Posted by LuckyTran

I have low-E fims on every single one of my windows, I applied it myself. The films themselves are rather expensive but they easily lower your HVAC load by 20-30%. It's quite significant and practical. They are part of building standards in many places. They're one of the levers we have for net-zero and I wouldn't be surprised to see required on every single new window installation by 2050, at least for G20 nations.

Is it really a low emission film that you have or is it a reflective film that prevents thermal radiation from exiting the room? Reflection (of thermal radiation in the non-visible spectrum) makes more sense to me. Especially if we are talking about a situation with a single pane window (as the OP mentions).

Where I live single pane windows are non-existing, we only have double and triple pane windows. A few years ago I installed an extra glass (lowE, hardcoat) on all our windows, converting our 2 pane windows to 3 pane. Even though it is not filled with argon and the frame is still from the 1950s, I got a sizable reduction in the heat losses. I left one window with 2 panes to have as reference for measurements (or maybe it was because I was clumsy and broke the glass during installation, but let's say it was for science!). It shows a reasonable difference on the inside glass temperature of 3-4 degrees C, between the reference and the new setup, when the temperature is a couple degrees C below zero outside.


Lowering the total HVAC cost by 30% by just installing a film on all windows is marketing BS in my humble opinion (unless we are talking about living in a green-house).

[LuckyTran]

I don't get it. You're questioning the practicality of low-E surfaces but you yourself have installed one... Are you trolling and trying to start a flame war or something?

Originally Posted by Simbelmynë

Lowering the total HVAC cost by 30% by just installing a film on all windows is marketing BS in my humble opinion (unless we are talking about living in a green-house).

You say it's marketing BS yet you have the data that shows it works. Why did you even bother asking about low-E surfaces if you already have the answer. I don't get you people.

The single pane window in the original post is just an example for discussion purposes. For the sake of discussion... Otherwise, the post would have been I have a building with 1124 windows. Some of them have triple panes. Soem of them double panes. Some of them dbl panes. Some are coated. Others are not. Some are not even windows. Some have ppl walking in front of them. Others have bird poop on them. It is 12pm. The sun is shining. There is a cloud partially blocking the sun. A wind blows from the south east at a speed of 8 mph. There is a 8% chance of rain. These details are superfluous when you just want to know what is the surface emissivity of a second surface and how that is (un)related to another surface being coated.

[Simbelmynë]

@LuckyTran


No I am not trying to start a flame war (over LowE coating? not sure what you mean actually). The reason for installing such windows was easy since the added cost was small and the marketing promised reduced heat losses. However, when thinking about the topic I am a bit confused, as should be clear from my questions regarding emitted vs transmitted radiation. This should be a viable topic for discussion here without you trying to pin some trolling on me.



In my opinion, the major part of the heat loss reduction was not due to the lowE coating on the glass that I installed, but rather because I reduced convective losses by creating two different convection cells in the window. This is in part enforced by my previous installment of acrylic plastic between the two panes (which refracted the visible light in a very strange way, thus forcing me to remove it). So even without the lowE coating I got similar reductions in the inner glass surface temperatures, by reducing the convective losses through the window.


With a single pane window I suspect that the convective losses will be huge, dwarfing any emission from the glass surface. With triple pane windows, perhaps the inner glass (side turned outwards in this case?) would be reasonable to have the film on since it will have a much higher temperature compared to the outer glass.


As for the marketing BS, I think any reasonable person would understand that the total HVAC losses are a function of surface area and that the walls and roof of most houses contribute to quite a substantial part of the total surface area. You do not change this by installing a film on your windows so.... But hey, you most likely have the data, so if you got a reduction of 30% then I congratulate you to an extremely cost efficient solution!




EDIT: So after some more reading and doing some napkin calculations, it seems that there is a sizable reduction to be had for lowE coating, by decreasing the emissivity. Especially between two glass facing each other. For single pane window it depends on the convective heat transfer coefficient, but the emitted radiation losses can be substantial (same order as the convection losses) in still air. Transmittance in the IR spectra seems to be really low, so reflecting radiation back into the room is not really an issue. There, I answered my own question