Do I really need coated glass for north-facing windows?
Sum_Guy
| Posted in Energy Efficiency and Durability on
I understand that coatings that block external IR on south or west facing windows are very effective at preventing solar heat gain in the summer (but I guess they suck for blocking useful heat from getting into a northern house in the winter).
But for north-facing windows on a northern (Canadian) house, are the coatings that reflect heat back into the house really that effective? What surface are they applied to (and are they durable or can they be scratched / wiped off over time?). What effects do they have on visibility? Do they make it harder to see into the house during the day or cause glare / reflection issues?
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Yeah, the coatings are worth having even on north facing windows, but not all of them. I would use LoE-180 on a north facing window since it is better for visible light transmission compared to the more aggressive coatings, and solar gain usually isn’t an issue with a north facing window. The more aggressive coatings are more noticeable in terms of reduced visible light transmission (things look darker), and their advantages with reduced solar gain are only a benefit for windows that will see direct sunlight.
All of the coatings help with the insulating value of the glass, and that helps to keep heat inside your home in the winter. You get most of the benefit for better U value with the LoE-180 coating, the more aggressive 270 and 340 coatings only gain you a very small amount of additional insulating value compared to 180 since the main benefit of the more aggressive coatings is reducing solar gain.
Bill
It's not clear to me how an infesimally thin layer of (of what?) can "insulate" the glass from the cold. If I want to "insulate" the glass from the cold, without affecting it's optics, why not use that plastic they laminate glass with? It's got to be thicker than the loe-180 stuff and hence have better "insulating" capability - no? Surely that would greatly reduce a glass pane's thermal conductivity - no?
Or why even bother with glass - why not have acrylic panes? They seem pretty clear, would be lighter than glass, have better thermal properties than glass. Maybe have a triple-pane window, where the center "pane" is acrylic?
From doing various web-searches I find this:
"Low-e windows have U-value and emissivity values that range from 0.02 to 0.20, which means that 80 to 98% of the room’s heat is reflected back into the room in the winter. Very low-emissivity windows offer the best improvement you can obtain in window insulating performance. Low-e windows offer the warmest glass temperatures in winter, which offers highest occupant comfort."
Wow. Coatings that "reflect" 80 to 98% of a room's heat back into the room.
So tell me - why are we building homes with walls a foot thick instead of painting them with coatings that "reflect 80 to 98% of the room's heat back into the room" ???
Something doesn't add up here.
Tell me what makes for a warmer interior pane surface, a large picture window, all else being equal, when it's midnight some night in january and it's -10 C outside and the air in the middle of the room is 18C. (a) Double-pane, argon filled, each pane is 3 mm thick, and has the best coating. (b) Double pane, argon filled, each pane is 4 or 5 or 6 mm thick, no coating.
Will I get more thermal bang for the midnight-winter buck with thicker panes or with a fancy micron-level coating?
Thinner panes with loE coatings will outperform thicker panes. The thickness of the glass has little to do with the insulating value of a glazing assembly, the thickness of the air space between panes as well as the type of gas fill used has far more impact. You can actually do even better with vacuum insulated panels, but there are physical challenges to use those in windows.
Even the best window (maybe R10-13 or so) can easily be outperformed by a basic wall (usually at least R15 these days). The old saying goes “the worst wall is better than the best window” for insulating performance. But you don’t want to live in a house with no windows so we accept some thermal loss to gain outdoor views.
Thermal transfer has to do with radiant emissions, mostly infrared light, and transfer of molecular motion via conductance. You can also have convective currents in fluids like air. The air space in double or triple pane windows deals with the second two, the first is handled by the loE coatings. Since most of the thermal energy is in wavelengths outside the visible spectrum, the coatings act as filters, transmitting visible light but reflecting light outside the visible range.
A fun fact for you: you’re using thin film optical filters right now while you use the Internet. Modern DWDM optical networks use thin film filters to allow multiple wavelengths of light to carry different data steams down the same physical optical fiber. Optical amplifiers can then amplify all these wavelengths simultaneously. These technologies are the primary reason internet speed has gotten so much cheaper in recent years.
I do optical network design as part of my regular work :)
Bill
> Thermal transfer has to do with radiant emissions
Um, that's only partially true. Heat transfer mechanisms are (broadly):
- Conduction
- Convection
- Radiation
Heat transfer THROUGH A SINGLE PANE is going to happen by conduction. The thicker that pane is, the lower/slower the conduction. A microscopic coating of low-e will do nothing to alter conduction.
Convection is how heat is transfered from the air (or the room generally) to the interior pane surface, with air as the fluid transfer medium. Convection is also happening between the 2 panes of glass, and yes having a wider pane-spacing is better but there have been studies on that in labs and once you get beyond 12 mm and certainly 19 mm you get very little improvement. And you won't find many window makers that will make double-pane with, say, 1 inch of separation. 3/4 inch maybe. 1/2 inch is standard.
Radiant transfer: does not require a medium. This is radiant energy. A dark room in the middle of a winter night will not have much radiant energy, but will have a lot of thermal energy. Sunlight (in winter or summer) has a lot of radiant energy. But you don't have the sun inside your house that you want to keep all it's radiant energy inside by using a low-e window coating. Maybe you have a fireplace that is emitting a lot of IR radiant energy, or maybe some incandescent light bulbs, but besides that I see nothing on the inside generating radiant energy that can be reflected back into the room.
My understanding is most warm surfaces continually absorb and emit radiant energy at some wave length, unless the surface can optically transmit that radiant energy, like glass does. So the radiant energy from your houses warm surfaces "bounces" around inside your house until it escapes out a window. I don't know how much you lose, but I'm sure it's not insignificant.
So what happens when I close the blinds in front of the window? Where is all this mysterious dark radiant heat in the room going then? It's going to hit the blinds, then what? Again, we are totally not concerned about applying heat or radiant reflective coatings on our walls - but for some reason we are for windows.
I'd like someone in a lab to take a set of windows and test their thermal / radiant conductivity in a controlled way. One window has low-e coating. One is painted black. Another is painted white. Make the paint layer really thin, so that it blocks 99% of light transmission. Keep the air on the inside side of the window at room temperature, expose the outside side to -10 c. Make the air space on the inside side of the windows large - to correctly model a room situation. And keep that space dark - no lights or IR heat sources. Just some duct work to bring in fresh heated air. Tape a thermocouple to the center of the inside side of the window and get a reading.
The paint on the painted windows should do an excellent job of absorbing / blocking the so-called "radiant" energy in the room and by this low-e logic the painted window surfaces should have a higher temperature than the low-e glass.
But I still say that plain glass vs painted glass vs low-e glass in this test would all perform the same (have the same surface temp) in this test because the temperature of the inside side of the glass surface is determined by the thermal resistance of the panes and the air space between the panes. Just like you model a wall's thermal resistance to keep the interior surface temp. as high as possible by, more or less, making the wall thicker.
The same concept does exist for walls as far as I am aware, insulation with reflective or radiant barriers add insulation value if provided with an air space.
If I remember correctly, I have read a number of times Martin say that it adds about an R2 value to the insulation properties of an assembly.
I cannot give you a precise answer as to why it isn’t done throughout a wall. I presume it has to do with cost and relative improvement - small decrease in u value is much more significant for a window than for a wall. However, they are just presumptions.
> painted windows should do an excellent job of absorbing / blocking the so-called "radiant" energy in the room
The paint would absorb radiant energy from the interior and then re-radiate/conduct it back out. So it still leaves.
There would be some benefit to low-E wall coatings - but it's far more practical to just add more insulation. Being transparent, windows are different.
Radiant loss is important - a #4 (interior) side low-E coating can take the same window from U value = .24 to .20. It also increases radiant comfort and decreases draft comfort (since the inner pane surface *is* colder).
http://nesea.org/sites/default/files/session-docs/room_side_low_e_coating_as_good_as_it_sounds.pdf
I want to see hard data under correct test conditions. I think low-e coatings to improve glass winter performance is hogwash. I totally get that it's great for summer and reducing A/C load in the south, for tall office/appartment buildings, for south/west facing residential windows.
I think visibility (altered optics, interior reflection and other effects) of low-e for northern climate, especially for winter, makes low-e a useless option.
Use thicker glass. Use wider pane spacing. Why does nobody talk about that? Why is it practically impossible to get 1 inch pane spacing for double-pane IGU's?
Why does nobody talk about the effect of drapes / blinds at limiting heat loss through windows? Who leaves their blinds / drapes open at night in northern climate areas?
>" I think low-e coatings to improve glass winter performance is hogwash."
You're simply mistaken.
A single hard-coat low-E coating such as indium tin oxide will convert a double pane on a north facing wall from a net heat loser to a net heat GAINER in winter compared to clear-glass double-panes, even in on dark north facing walls in the dim northern wilds of Canuckistan. This is well modeled by the physics, and well demonstrated in the field.
>"I think visibility (altered optics, interior reflection and other effects) of low-e for northern climate, especially for winter, makes low-e a useless option."
Soft coat low-E designed for solar heat rejection have a bigger impact on visible light transmission than than the hard coat low-E materials used in windows for cold climates. Even double low-E hard coat glass (surfaces #4 and #2) can still pass more than 75% of the visible light, with solar heat gains coefficients north of 0.5 even at U-factors less than 0.22. eg. Cardinal Glass LoE180 + i89, with argon fill:
https://www.cardinalcorp.com/products/coated-glass/loe-i89/
The limiting factor for double low-E hard coat is the cooler temperature of surface #4 (the side facing the room, in contact with the room air), which can have copious condensation when the outdoor temps drop below -25C or so. Water has very high IR emissivity, so once the condensation begins the performance drops to that of a single low-E window. Several US vendors have this glass as an opttion. It's a good choice relative to much more expensive triple panes in Canadian locations where it only drops to -25C or lower a handful of times per year.
If you want to calculate the effects of a particular low-E coating for yourself you'd need to know it's spectral emissivity/reflectivity as well as the spectrum of the incident radiation. But a stripped down somewhat easier model can be found here:
https://windows.lbl.gov/sites/all/files/Downloads/tarcog-mathematical-model.pdf
> copious condensation when the outdoor temps drop below -25C
This number changes significantly with conditions. For example, at 25% RH and reasonable other conditions. Payette reports no condensation even at -40C. So don't rule these efficient windows out.
https://www.payette.com/glazing-and-winter-comfort-tool/
That's right- indoor conditions matter.
I suppose those folks who blast a humidifier to maintain 50% RH+ could run into condensation issues at -10C. :-)
> Why is it practically impossible to get 1 inch pane spacing for double-pane IGU
Because they lose more heat (at test conditions) than windows with closer spacing.
I hesitate to post this because we are clearly in tin foil hat territory, but here goes: According to you every glass manufacturer, window maker, the scientific community and every government in the industrial world is part of a vast conspiracy to defraud the public over glass coatings. Well, they're not.
Everything emits radiant energy proportional to its temperature. That's why thermal imaging cameras work.
Glass itself has little insulating value. because of it's high density heat on one side is easily conducted to the other. Making glass thicker does little to raise thermal resistance. Air is a fluid, and like other fluids is subject to surface tension. As air molecules get closer to the glass surface they cling ever more tightly. In still air there is an effective film of air about 2mm thick clinging to each glass surface. This is where glass gets its insulating value. Your body surfaces exhibit the same phenomenon. That's why when wind scrubs the air film off your skin you feel colder. Beyond 2mm convection currents more easily scrub molecules from the air film. After 19mm there are rapidly diminishing returns to wider spacing.
>"Everything emits radiant energy proportional to its temperature. "
No- only highly emissive surface emit radiant energy in proportional to it's temperature (aka "black body radiation" to us physics nerds.) Shiny metals and other material surfaces that reflect most of the radiated energy also emit very little energy with temperature. (Try taking the temperature of a copper plumbing on a hot water heating system with an infra-red thermometer sometime.)
>"That's why thermal imaging cameras work."
They don't work . Thermal imaging cameras are a hoax, designed to deceive, just like that "telescope" thingy Galileo was promoting that supposedly makes Jupiter look like sphere with some other blobs orbiting around it. It's a scam, folks. Whomever is promoting a "thermal imaging" or "camera" hoax deserves to rot in the same jail that he did.
So, when I melt my shiny aluminium for casting that red glow is not radiant energy? Feels like it to me. Granted, an IR thermometer will not give as accurate a reading as the side of the crucible but it is emitting.
Besides mirrors and polished metal few objects in a home are highly reflective. They will give reasonably accurate readings from IR thermometers or cameras.
>"So, when I melt my shiny aluminium for casting that red glow is not radiant energy? "
Low-E isn't NO-E, and emissivity varies with spectrum & to some extent the temperature of the material. Aluminum doesn't have the same emissivity of iron, rust doesn't have the same emissivity of bright clean iron.
From this table you can see that the emissivity of unoxidized aluminum at 500C is about 3x as much as it is at 25C:
https://www.engineeringtoolbox.com/radiation-heat-emissivity-aluminum-d_433.html
(So when the roof deck is 500C the radiant barrier isn't doing as much for you as it is at cooler temps, not that you'd even have a roof deck by the time the RB got that hot. :-) )
... but from this table you can see that even the 500C unoxidized aluminum would still be an order of magnitude less emissive than concrete or wood:
https://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html
Liquid molten copper is about 20x as emissive as bright polished/scoured copper, but even molten copper is only about 1/4 as emissive as ceramic (p3 & 4):
http://www-eng.lbl.gov/~dw/projects/DW4229_LHC_detector_analysis/calculations/emissivity2.pdf
Back here at home heating/cooling temperature levels you don't really need to make those adjustments. The emissivity at +25C isn't very different from what it is at -25C or 40C. Just rest assured that there are WIDE differences of emissivity among different materials in this temperature range.
> According to you every glass manufacturer, window maker,
> the scientific community and every government in the
> industrial world is part of a vast conspiracy to defraud
> the public over glass coatings.
Window coating were first and formost developed to reduce heat load in the summer. That's a big deal in the huge market that is the USA. Glass coatings for northern climates are a mixed bag as far as keeping out desirable winter sun (the little that some areas get when it's not cloudy).
> That's why thermal imaging cameras work.
Thermal imaging cameras, which still cost thousands of dollars and really only commercially available a decade ago. A sophisticated sensor to detect the PALTRY amount of radiant energy leaving a surface like a person's face/body or a house. The thermal imaging cameras tell you the temperature of the object or surface you're pointing it at by the "color" of the IR leaving the surface. How that surface came to have that temperature is a function of what the temperature is on the other side of that surface or material and the thermal resistance (of the wall, or glass, or wood, or brick, etc).
For a window, lots of thermal energy being carried away by the wind, but radiated IR from the house interior? Not enough to justify an interior reflective coating I say.
And yea, I get that still air close to an object is an insulator, but the only still air in this situation is inside the IGU. In a house with forced air heating, no still air. Outside - usually no still air. You want still air next to the window -> close the drapes or the blinds! Again, no response here to the alternative to a coating which is the drapes or blinds (which you're practically garanteed to have in a residential situation, but probably not for commercial).
I asked earlier why aren't we using acrylic for the panes instead of glass. Even putting an interior acrylic pane inside an IGU should dramatically improve it's thermal performance and give no durability or scratch issues, and no UV discoloration issues either.
I know that everything inside a room has thermal energy. The air in the room, that the furnace is heating, is being cooled only because it is in contact with the walls and window. Does the air itself give off radiant IR? I don't know. Does a chair that's 5 feet in front of the window radiate IR? Yea, probably. Is there a lot of energy in the IR being radiated by the chair towards the window? I'd bet it's pretty low. But that's the energy you people are claiming is being prevented from escaping to the outside by the window coating. I say that energy is paltry compared to the heat being thermally transfered from the air in the room to the IGU and then through the IGU to the outside.
Another angle is lighting. Not too long ago most residential lighting was incandescent. Some IR was leaving a home through the window (because you don't close the blinds, right?). But now we have CF and LED's and hardly any IR to worry about.
>"Thermal imaging cameras, which still cost thousands of dollars and really only commercially available a decade ago. [snip] The thermal imaging cameras tell you the temperature of the object or surface you're pointing it at by the "color" of the IR leaving the surface. "
Actually, no, they don't cost thousands of dollars anymore. Seek offers a little one for your phone that is under $200. It's not as high performance as more expensive models, but it's functional. Flir makes some nice ones for $400-500. I use these often to find hot spots (loose connections, mostly) in electrical panels.
>"but the only still air in this situation is inside the IGU. In a house with forced air heating, no still air. "
That's not correct. There is ALWAYS a film near the surface, you only reduce the thickness of that film with forced air. Even rapidly moving surfaces have a film of air clinging to them, that's what keeps the head of your harddrive suspended above the platters (the "bernoulli effect", in this case).
>"I asked earlier why aren't we using acrylic for the panes instead of glass. Even putting an interior acrylic pane inside an IGU should dramatically improve it's thermal performance and give no durability or scratch issues, and no UV discoloration issues either."
Acrylic costs more, is less durable, and has a MUCH shorter life. You do get yellowing over time, and within an IGU just means LESS UV exposure. In a way, Alpen has sorta kinda done what you're suggesting though, and they also put LoE coatings (their versions) on those suspended films! Why do they do this? It gives their windows better performance characteristics.
>"But that's the energy you people are claiming is being prevented from escaping to the outside by the window coating."
Not prevented from escaping, just the amount escaping is reduced. Physics is funny that way, you almost never completely stop anything. You just reduce things, and eventually there is so much reduction so as to be almost immeasurable. This is how the decibel scale works. There is a reference level, usually 1 milliwatt, specified for 0dBm. you can then go infinitely higher or infinitely lower than that. -60 dBm for example, would be one millionth of a milliwatt. A whole lot less energy, but there's still some there. You never get it reduced to zero.
>"Another angle is lighting. Not too long ago most residential lighting was incandescent. Some IR was leaving a home through the window (because you don't close the blinds, right?). But now we have CF and LED's and hardly any IR to worry about."
This is the main advantage to the newer lights. Incandescent lost much of the input energy to heat, basically IR radiation that didn't do anything useful, which in this case is emitting visible light. All of that extra energy that you are claiming is miniscule is where those watt savings are coming from with LED lights. If you doubt the radiated enery is miniscule, I suggest holding your hand near both an LED and an incandescent lamp. Physics is nice that way, many concepts are very easy to very quickly demonstrate.
Why are you so doubtful of the value of the LoE coatings anyway? There is a LOT of information out there about them, and you can do your own tests. I've even seen demonstration setups with heat lamps where you don't feel the heat on the other side of the coated glass but you do through uncoated glass.
Bill
I don't point heat lamps towards my window in the winter. Nothing in a typical room in my house gets warmer than room temperature except the coffee I happen to be drinking.
I made the claim that the amount of IR energy being emitted by objects in the room and passing through an uncoated window is paltry and not worth the expense of the coating option. Nobody has challenged that statement (in a credible way). I made the claim that the thermal heat loss is what you really want to improve, and a coating will do nothing in that regard (unless the coating is really thick, which low-e coatings are not).
If there is ANY info that gives something useful (like the IR energy in a typical room that can pass through an uncoated IGU) and give it in useful terms (like BTU, etc) then I'd like to see it. I don't give a damn about blocking outside IR from the sun on a north-facing window in Canada in the summer. I want to know how much radiant energy vs thermal energy is being transferred through an uncoated IGU to the outside in the winter.
>"I don't give a damn about blocking outside IR from the sun on a north-facing window in Canada in the summer."
Neither do I.
Not all low-E windows are good summertime heat blockers- some are fairly HIGH gain.
Not all window gain is from direct insolation- there's a LOT of gain from backscatter from ground/snow, or even "bright cloud" days from all directions. It's also not all in the IR spectrum. Only about half the solar spectrum reaching the earth's surface is IR, with a large fraction of the energy in the visible & UV spectrum. When the visible & UV is absorbed by materials indoors, it radiates back in the IR spectrum, while reflecting back visible spectrum. Across the spectrum most of the light energy that makes in into the house is retained by a low-E window (even a single l0w-E) as indoor heat, but clear glass loses something half, with each clear-glass pane retaining only 10-15% of the near- IR and visible spectrum. Compare the transmittance of clear soda-glass to the solar spectrum- something like a third of the total energy in the solar spectrum is in the visible region (look at just the red part of the solar spectrum) :
https://www.trzcacak.rs/myfile/detail/94-944672_soda-lime-glass-typical-transmission-spectrum-window-glass.png
https://www.researchgate.net/profile/G_Graaf/publication/271891094/figure/fig1/AS:295086937133058@1447365616452/Solar-spectrum-at-the-top-of-the-atmosphere-and-at-sea-level-1.png
A high gain hard coat low-E coating reflects most of the DEEP IR spectrum (~2000 nm or longer) and passing the bulk of it, while only knocking back the transmission visible spectrum and near infra red by about ten percent. But the heat radiating back out of the house is essentially the black body radiation at 20C, where unlike the solar spectrum essentially ALL of the radiated energy from indoors is longer wave than 1000 nm, and reflected back toward the room:
http://www.edscave.com/images/InfraredThermopile_Spectrum.png (Take a look at the red curve , blackbody radiation at 25C. The first tick is at 5 um which is the same as 5000 nm.)
The transmission curve of indium tin oxide hard coat (a common Low -E high gain coating) looks like this, where even at 1100nm it's reflecting back half the radiation, even more at longer wavelength.:
https://www.researchgate.net/profile/John_Walls/publication/259507813/figure/fig7/AS:267689230860338@1440833494973/The-optical-transmission-spectra-of-a-238nm-thick-indium-tin-oxide-thin-film-deposited-on.png
(The green line, ITO = Indium Tin Oxide)
That makes it something of an energy-diode: The radiated energy coming at the window from the outdoors is primarily from a shorter spectrum that is mostly passed by the low-E glass, whereas most of the radiated energy coming at the glass from indoors is in the deep IR spectrum, and mostly reflected back into the room.
A high gain gain U0.20 double-Low-E argon filled double pane such a like Cardinal LoE 180 + i89 glass (there are other vendors with similar product) blocks only about 20% more visible light than a clear-glass uncoated double pane (due to the pair of hard coat low-E coatings), but blocks LESS heat gain, only about 17% compared an uncoated window. This is different from run-of-the-mill single low-E double panes which block more than half the incoming energy in the solar spectrum.
But the combined radiated and conducted heat LOSS of that glass is less than half that of a clear-glass double pane.
There is also the comfort factor benefit of the coating on surface #4 reflecting body heat back at exposed skin. That makes it FEEL warmer near that window than it actually.
Even when facing north that type of glass gains more heat during the winter than it loses. Whether the upcharge for the better glass is "worth it" on a present-value-of-future-energy-cost-savings over the lifecycle of the house basis depends on your fuel cost, discount rate, and climate, but it's both cheaper and higher performance than triple panes, and "worth it" on a comfort basis when it's below -10C outside for most people.
But is it going to be "worth it" for Sum_Guy?
I kind of doubt it, given the expressed "hogwash" characterization, and who still thinks IR cameras only showed up 10 years ago (I designed some of the electronics for precision instrumentation IR camera than 25 years ago, and it wasn't new-tech then) and somehow cost thousands of dollars rather than a couple-hundred (if you don't count the cost of your phone or tablet computer used for the display):
https://www.flir.com/products/flir-one-gen-3/?creative=249917783646&keyword=flir%20one&matchtype=e&network=g&device=c&gclid=EAIaIQobChMI8eL0w9WU5AIVy16GCh3JHAa0EAAYASAAEgJKzPD_BwE
You can lead a fish to water, but that doesn't make it a horse.
For what it's worth, I learned a lot from what you just wrote! Thanks for the in-depth explanation, very interesting stuff! And I had you pegged too young to be designing IR cameras 25 years ago. Age begets wisdom I suppose...something to look forward to.
>"... I had you pegged too young to be designing IR cameras 25 years ago. "
I mentioned Martin a week or so ago that just because I sometimes ACT like an 11 year old isn't an indication of my chronological age! :-)
>"Age begets wisdom I suppose..."
Don't count on that: https://vimeo.com/166290068
Hey, that looks like Batawa Ski Hill!
That joke would probably go over better with the locals...
>"Hey, that looks like Batawa Ski Hill!"
Close! It's actually WaWa! (Which is the affectionate diminutive monicker for Wachusett Mountain in MA.)
This thread is quickly headed down the triple wythe brick road...
Complete with a flying-monkey troll too? :-)
For the record, the U-factors of glass made in the US is the collective total relative heat-loss (radiated + conducted) at 0F (-18C) outdoors, 70F (21C) indoors. If you look at the actual mathematical model you'll see that there are factors which are relative to the fourth power of the temperature-Kelvin, so it's not a constant with temperature, but in most of Canada the performance at -18C is relevant.
A typical lower cost single low-E double pane runs about U0.32-U0.36 depending on coating, gas fill type and glass spacing. A typical clear glass double pane runs about U0.5-U0.6. So even a cheap double pane only loses between half and 2/3 as much heat as any clear-glass double-pane. Even those are net heat gainers (take in more heat than they lose in winter) even on the north side, though not nearly as effective as multiple coating multiple pane windows.
And a more sophisticated (but still not ridiculously expensive) U0.20 double-panes only lose about a 33-40% as much heat as a clear glass double pane.
...all at -18C, of course.
The relative performance at different different temperatures, but there is no temperature at which a clear-glass double pane is as effective as a low-E double pane. The differences in solar gain and net heat gain varies by a lot with coating type, which is why the low-E coatings appropriate for Miami aren't the same as those that would be appropriate in Whitehorse. Even though all low-E coatings cut into the raw solar gain at least a bit, it's the NET solar gain that matters.
Feels like I'm kickin' a dead fish here (or maybe it's really a horse?)
How many $ worth of natural gas heat is passing through my uncoated double-pane argon-filled IGU via long-wave radiated IR vs thermal conduction to the outside air in the winter.
Do all the analysis you want about what percentage of this or that wavelength is bouncing this way or that way by this coating or that coating. Tells me nothing about how much actual $ worth of BTU is represented by those waves.
I've read how too much internal IR reflection can make the interior glass colder than without a coating. This is big for me, because I don't to fight condensation any more like I've been doing the past 20 years with these original Pella 2-pane windows with removable inner pane.
I still say that the long-wave IR that is radiated from a room interior through an uncoated IGU represents a paltry amount of heat energy when compared to the heat energy being sucked out of the window through thermal flow caused by the temperature difference between interior and exterior glass surface in the winter.
If there is so much heat energy in the room-radiated IR, don't I want that energy to leave the room in the summer, instead of being reflected back in? You can't have it both ways. You can't say that a coating designed to reflect heat BACK INTO A ROOM is great for the winter and then disregard how bad that is in the summer. If you say that it's not really a big heat load for the AC to handle then it also means it's not a lot of heat supplement for the furnace to overcome in the winter.
In terms of reflecting external IR (which as I've stated I have no need to be concerned about for my north-facing windows), read this:
https://www.wral.com/-the-next-asbestos-scientist-says-low-e-windows-cause-dangerous-reflection/17553558/
There are many such stories appearing now. People that can't enjoy their patio's, chairs and plants that melt and burn up, etc.
?>"How many $ worth of natural gas heat is passing through my uncoated double-pane argon-filled IGU via long-wave radiated IR vs thermal conduction to the outside air in the winter."
As stated previously, the $ of fuel that it's saving is a function of the local climate and your cost of fuel. Do you're own math- even a first-order approximation using the U-factor at -18C is good enough.
>"I've read how too much internal IR reflection can make the interior glass colder than without a coating."
Then you have clearly MIS-read too much. The conditioned space surface (surface #4, in window-speak) of clear glass double panes is substantially colder than that of single l0w-E double panes. Over a wide range of outdoor temperatures the surface #4 of an argon filled single l0w-E window will be 9-12 F/5-7/C warmer than a clear glass double pane:
https://www.efficientwindows.org/img/condensation.gif
Surface #4 of double low-E double panes are somewhat cooler than single low-E windows, but well above that of clear glass double panes. So if condensation is a primary issue being addressed lower performance U0.30-ish single low-E double pane is better than a high gain double l0w-E window, and WAY better than clear glass double-E.
>"You can't say that a coating designed to reflect heat BACK INTO A ROOM is great for the winter and then disregard how bad that is in the summer."
Sure I can disregard it since the paradigm case under discussion is "Do I really need coated glass for north-facing windows" in a northern latitude heating dominated climate. Unless you're north of the arctic circle there is effectively no DIRECT solar gain, only gain from the ambient light and the blackbody radiation of the landscape. Radiant cooling that can take place through the window is at night, and when the temperarature of the surfaces outdoors are lower than the the surfaces indoors. While there will be days when there's a cooling load during the day outdoor temperatures drop below the indoor temperatures at night, the temperature difference is relatively small, on the order of 5C, which isn't enough of a difference to matter- the radiational cooling is very very small at delta-Ts that small. The solution on those night is to open the window.
On nights when the outdoor temperature is still higher than indoors, the low-E is saving you from that small amount of radiated gain.
>"I still say that the long-wave IR that is radiated from a room interior through an uncoated IGU represents a paltry amount of heat energy when compared to the heat energy being sucked out of the window through thermal flow caused by the temperature difference between interior and exterior glass surface in the winter."
Based on what?
The U-factor takes into account ALL heat transfer mechanisms, including the convective transfer in the gas between the panes, not just radiated energy. U0.33 single l0w-E glass loses a third less heat (by all transfer mechanisms) than U0.50 clear glass double panes, and U0.20 double low-E glass loses 60% less. Insulated glass designed too meet a particular U-factor based on the physics & engineering of the materials and gases used, and yes, it is actually tested to verify whether it met the design spec. If it's being marketed at one value and doesn't actually meet that value you can bet their competitors will step up to point out the fraud.
>"In terms of reflecting external IR (which as I've stated I have no need to be concerned about for my north-facing windows), read this:
https://www.wral.com/-the-next-asbestos-scientist-says-low-e-windows-cause-dangerous-reflection/17553558/
There are many such stories appearing now. People that can't enjoy their patio's, chairs and plants that melt and burn up, etc."
Really "...many such stories..."? How many? Do you have any idea just how RARE those instance are? While those instances do occur it requires a unique optical geometry of window curvature./focal length, solar angles, and distance to the receiving surface to happen, and a material on the receiving end sufficiently absorptive in the reflected spectrum that is also easily damaged or ignited by the temperatures. I challenge you to come up with five unique instances of siding damage, or even one actual fire. The article asserts (without citation) that:
"The reflections have already started a number of fires, and homeowners want something done."
I'm going to hazard that "...a number..." fewer than 10 house fires EVER have been started by low-E coatings on windows.
This is one of those topics that come up from time to time even on this forum:
https://www.greenbuildingadvisor.com/question/are-low-e-windows-dangerous
Dana, to anyone believing that low-E coatings can somehow make for "dangerous reflections", I would point out the following:
A reflection can contain no more energy that the energy arriving at the surface of the window (in the case of a reflection off of a window). It is not possible to reflect more energy than was received. Glass will reflect ALL of the received light when the angle is past the critical angle, and that would be a worst-case scenario with ALL received energy being reflected and non transmitted through the window.
There is no way a Low-E coating can somehow reflect any more energy than the above example when the angle is right for total reflection to occur. Basically the same "dangerous" conditions have always existed, Low-E does nothing to make things any "more dangerous". It's no different than leaving a mirror outside in the sun and being annoyed when the angles are right to blast your face with sunlight. Even water shows this phenomenon of total reflection when the angles are right -- that's where the "sparkles" come from on the edges of waves.
I think there is a lot of misunderstanding out there with people reading scientific articles and not fully understanding what they're reading about. I commend you for your detailed replies here trying to explain these things to everyone.
Bill
> It is not possible to reflect more energy than was received.
Except that what matters is not total energy but energy in terms of watts/sq meter - concentration is possible and it does (very rarely) start fires.
Concentration would require a curved surface, or multiple flat surfaces at slightly different angles directed towards the same spot. A single flat plate should keep the total concentration the same. Either way, Low-E or no-Low-E, the result is the same.
Bill
Yes, it requires curvature (which sometimes occurs in windows). More reflectivity makes it worse.
That's right. A curved mirror that has 10 square feet of area that focuses a solar image down to a few square inched can definitely create some high temperatures if it's reflecting more than half the incident radiant energy.
That's another reason to go with high-solar gain glass, rather than a high reflectivity heat rejecting soft-coat low-E, which is more commonly sold only in cooling dominated climates. In Canada a very-low gain soft coat low-E window would have to be special ordered, and wouldn't do as well from a heating energy use point of view compared to the more typical hard coat low-E windows, which are (appropriately) the typical type of window sold in cooler climates.
The only windows that develop a short-focus curve are those using thinner glass and argon or krypton gas fill between the panes. While the mono-atomic noble gases give it it performance boost, the smaller molecular size of Ar or Kr compared to N2 & O2 molecules of dry air gives them a tendency to leak a tiny bit of gas out of the seals when heated up in the summer sun. Since the larger N2 & O2 molecules don't get by the seals (or at least not at the same rate as Ar or Kr) when the window gas cools off the window stays under negative pressure, causing it to be slightly concave after 100s of cycles. The thinner the glass in each pane, the more potential there is for creating a concave focusing surface.
" Choose ECOGLASS/HM – Dual Plus, Quad or Superquad. We manufacture ECOGLASS/HM with one, two or three Heat Mirror films to achieve insulating values up to R-20. It’s your choice, of course. No Other Glass Does That!"
https://www.ecoglass.ca/products
"2” ECOGLASS/HM Super Quad – R-20 ECOGLASS/HM Super Quad is made with three layers of Heat Mirror film laid between two glass lites creating four air spaces. Filled with krypton and xenon gas, you can achieve insulating values up to R-20. (U-value 0.05). It’s the most energy-efficient glazing product in the world."
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So tell me this. A 2" IGU that is claimed to be R20.
Why aren't we putting these IGU's in our walls instead of fiberglas or roxul?
I'm going out to get me some 14" wide, 8 feet long IGU's and replace all my wall insulation with these IGU's because this magical micron-thick reflective layer is giving me this incredible R value.
Why are we putting 1 or 2 inch insulated foam-board polystyrene on the outside of our houses (we get what - maybe R8?) when we can put up these 2" IGU's and get double the thermal insulation from them!
>"So tell me this. A 2" IGU that is claimed to be R20.
Why aren't we putting these IGU's in our walls instead of fiberglas or roxul?"
You COULD actually do that, but it would be QUITE expensive compare to the fluffy stuff. It's the expense (and relative fragility) , not the thermal performance that makes it a non-starter.
Don't believe all aspects of the marketing claims- it's different from an engineering spec. A claim of U0.05 for a whole window or even a whole insulated glass unit is somewhat dubious, even for the quad film and xenon-krypton gas fill. That might be the center glass/film value, but the higher thermal conductivity of the edge seals & frame cut into that a bit, just as the higher conductivity of wood framing undercuts the performance of the cavity insulation from a "whole-wall-R" basis.
Even just as a window I'd like to see the visible transmission, solar gain and other specs for the ECOGLASS/HM Super Quad. Third party testing of the U0.05 claim would also be worthwhile. Even best in class triple-panes sold in the US have U-factors in the low 0.1_s (whole-unit, not center-glass), and just one more layer isn't going to cut that by half:
https://www.energystar.gov/products/most_efficient/fixed_or_picture_windows
> > Why aren't we putting these IGU's in our walls instead of fiberglas or roxul?"
>
> You COULD actually do that, but it would be QUITE expensive compare to the fluffy stuff.
So you're saying that even inside a wall cavity, that interior room IR is getting past the drywall into the cavity where it would hit the IGU (If I put an IGU in my wall) and the low-e coating would reflect the IR back into the room (or the backside of the interior drywall) ?
I still don't have a good "feel" for how much room heat-energy is carried away by long-wave black-body IR vs thermal conduction/convection to the outside. I still think that at exterior walls and windows that thermal conduction dominates and that long-wave IR is not a significant percentage because we are talking about room temperature (300 K) not thousands of K (ie - the sun).
If there really is a lot of "energy" in room-temperature IR radiation, then it would make sense to have a foil-based vapor barrier (or a combination foil layer bonded to conventional polyethylene vapor barrier). Maybe such a product exists, but I haven't seen it at Home Despot. Even a foil backing sheet attached to insulation batts - ? Why doesn't the building codes mandate that? Or - when you have 2 materials in direct contact (ie - drywall and vapor barrier) with no air gap then you don't have a space where radiant "waves" can propagate and thus conduction dominates over radiation?
Maybe that's it. Maybe in a room with no air (hypothetically - a vacuum) the only way for heat to leave the room through a window is radiant black-body IR radiation, and the low-e coating is definately doing something to reflect the IR back into the room (because there is no thermal conduction because there is no air). But when you have air and room air currents, heat loss through thermal conduction dominates (I would think) and radiant IR losses are trivial in comparison (again, 300 kelvin IR).
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Well I probably really messed up. I had clear glass double paned Marvin fiberglass windows installed on the south side of my home. I'm having low e on the north side of my home. There are only windows on the north and south sides. I live where there is a great deal of sun in the winter time. The house never overheats in the summer even without air conditioning. I use thermal curtains that allow light through but keep heat and cold out or in. But now in reading more since, I wonder if the trade off didn't add up. I was hoping to maximize solar gain in the winter and control heat loss and excess solar gain with the window coverings. And, if I did mess up, can I use window inserts to save the day as there is ample ledge space for placing them.