I think this would be a case of conduction: heat flow from excited air molecules to cold glass surface molecules.
I would also think radiation is not the significant factor. How much energy does room-temperature air emit in the form of infrared compared to the amount lost to air-glass conduction?
Then too, difference between air temperature at window and further into room may be great enough to set up a convection current.
This is clearly homework and something that you will have to investigate yourself, but I will try to point you in the right direction.
I will bet my engineering license that it is not conduction. Similarly, assuming that it is a "poor seal" is both incorrect and a cop out for blaming it on poor construction. All engineering calculations assume proper construction with a factor of safety on top of that- the burden is on the architect/ engineer in this case.
The answer is far more fundamental- you need to do two things to solve this problem:
1. Have a fundamental understanding of how the three modes of heat transfer work and how they differ from one another.
2. Analyze the environmental conditions of the interior and exterior environment, particularly with respect to temperature.
You will also need to make a few justifiable assumptions. I would start by assuming that the question is implying that it is a winter scenario and the building is losing heat.
Can we also assume the building is insulated (such that the opaque wall assembly has a higher U-value than the window), that the widows are double pane, and heat transfer via air movement is negligible?
When making technical assumptions, it is always best to reference the most common implementations of what you are analyzing.
Semantic point- you may want to double check your units. Is a high U value indicative of more insulation and resistance to heat flow?
Assuming that this window is set within a typical wall assembly, I would agree that in most common cases, opaque walls typically resist heat transfer better than window glazing.
In most common cases, are windows double pane? Maybe in your house, but it is debatable for *all* buildings. If an assumption can be easily debated, it is invalid. Since there is a lack of information given in the question regarding window type, one can only conclude that it must be about a phenomenon that occurs with all glazing.
If you must assume proper construction and there is no mention of mechanical systems, can you think of any other plausible reasons convection would occur? If not, then this assumption is valid.
You are starting to ask the right questions, which is half of the battle
I'm not the OP, just interested in clarification for the sake of personal edification... so I'm saying case in point is double pane, and let's go from there. The OP can elucidate if the situation is single pane.
The different conductivities of wood and glass were mentioned in response to Glitter Centaur, for the sake of argument. I feel like the argument that glass isn't all that great a thermal conductor is just a rationalization for modernist wall of glazing, but that's another issue.
Let's assume it is night time as well, so there is no solar radiation through the glass which might induce convection of the air at the interior side of the window.
I can't think of any other things that might cause convection, so I'm still at radiation of air mass through window via thermal radiation or conduction through glass....
modern aluminum framing in a commercial product (be that a curtainwall or a unit window) will be thermally broken (inside metal does not touch the outside part), and glazing will have vacum or argon gas between its panes. Not as good as 6 inces of polyiso and rubberized air membrane, but still quite good.
I am actually amazed at how many LEED candidate projects are entirely wrapped in curtain walls.
@ rustystuds:
this is good point. I do also wonder if the embodied energy / lifespan / additional up-front cost of a relatively high-tech assembly such as a vacuum insulated curtain-wall is off-set by reduced heating costs... And, can a glazed envelope possibly make sense in a cooling climate?
aslo, just a general fyi from your friendly neighborhood spec writer:
-glass has an amazing insulation quality given that the material itself has no insulation characteristics whatsoever. How is the insulation rating achieved you aks? Through the thin layers of air that builds up on both sides of your glass pane. Take your hand and put it very close against the glass (assuming it's much colder outside). You will feel the cold air. That cold air is what gives your glass assembly an R rating of any kind. That cold air is the insulation and a critical portion of your overall assembly.
"I do also wonder if the embodied energy / lifespan / additional up-front cost of a relatively high-tech assembly such as a vacuum insulated curtain-wall is off-set by reduced heating costs... And, can a glazed envelope possibly make sense in a cooling climate?"
It depends on quality of design. Modern curtainwalls are extremely air tight (you can test them upon installation). You can also get all kinds of coatings and laminates on glass that will control solar gain.
Passivehouse uses large glazed storefronts (triple glazed) that meet the strictest of global energy standards.
I'm no fan of a building entirely clad in curtainwall, but glazing assemblies can be used quite effectively by capable designers.
FWIW I don't post/ read here often, it just happens to be a slow day at work.
@glitter centaur: That is a fritted glass louver system, not a window. What is your background again?
Ignoring your snark altogether, you should reread what I said initially: In engineering we assume proper construction for calculation.
This is clearly an AE/ building technology question asked by a student in an academic setting.
Having written (and taken) many of these questions for exams, I am going to assume the intended answer to this question is a little more complex than what equates to "blame the contractor."
@rustystuds: Glass has both a lower heat capacity and thermal conductivity (as in the variables C, k) than wood. That is what I was referring to.
@hbrain: I'll just end the suspense for everyone then :D
I think what you may be assuming (correct me if I am wrong) is that a double pane system creates a thermal break preventing conduction. This isn't entirely accurate. Using our winter case, here is how heat would flow across a two lite (double pane) system.
Configuration:
Interior air
Interior lite of glass (with interior surface is closer to vacuum/gas)
Vacuum/gas
Outer lite of glass (with interior surface is closer to vacuum/gas)
Exterior environment
Heat Transfer:
1. Warm interior air will interact via convection with the outer surface of the interior lite of glass.
2. Heat will transfer throughout the cross section of the interior lite of glass until it reaches the surface exposed to the vacuum or gas.
3a. If a vacuum: the heat will transfer across the vacuum via radiation to the inner surface of the outer lite.
3b. If a gas: heat will transfer via a combination of radiation and convection across the space from the inner surface of the inner lite to the inner surface of the outer lite. The more massive the gas, the higher the heat capacity and the lower the convective heat transfer.
4. Heat will conduct through the outer lite until it reaches the outer surface of the outer lite.
5. Convection and radiation transfer the heat from the outer lite exterior surface to the environment.
Given that the original question was why does the area *near* a window feel cold, one can assume that you arent touching it- so direct conduction from you to the environment through glass is impossible. Remember conduction is only for direct physical contact.
Furthermore, the majority of convection only really occurs within a small boundary layer (the outer and inner air film) of the entire window assembly. It occurs, but it is by no means the dominant method of heat transfer. If convection was dominant, it would feel like a fan was blowing every time you walked by a window.
That leaves radiation. The only thing radiation cares about is the temperature differential between surfaces. From the interior, our body only "sees" two things for radiative heat transfer- the interior surface of the inner lite of glass is one, and the entire viewable exterior environment (which behaves like a surface far away) as another. The temperature of the exterior environment significantly dominates the temperature of the interior surface and is the controlling factor.
The only thing that having a double pane system will do is resist heat transfer slightly more, therefore creating a slightly higher surface temperature on the innermost surface of the window unit.
Both single and double paned glazing systems still boil down to:
If you want to split hairs, you can count radiation losses to each of the four assembly surfaces plus the exterior environment "surface"- but I assure you this will be negligible. The innermost surface and the environment are the ones that are used in engineering energy modeling.
So to get back to the original question, it is a rather simple answer. Glazing offers a direct view to the exterior environment that allows for radiative heat transfer to take place while an opaque surface does not. The inner surface will be significantly colder than a proper wall assembly (or slightly colder if double/ triple pane) due to its material properties, but the controlling factor is that the exterior environment will act as a surface for radiation and will essentially steal the heat from a person walking by.
My undergrad and first masters were in architectural engineering; I didn't get to enclosure science and design until the end of my graduate courses. It is by no means trivial, and quite frankly the most challenging area of AE.
Absorbtive chiller explanation... another time. That one is messy and requires lots of diagrams.
Good question about the reflected thermal radiation. The best example that I can give is think of a chord in music and how it is made up of different notes- each note is a different frequency (measured in hz). Sometimes you can put cool effects on say an electric guitar to block certain frequencies of sound from being played through the amplifier.
Light behaves very similarly to sound. The sun is essentially emitting chords of energy as light, and like musical notes they are made up of hundreds of waves each with a unique frequency. Every color that you can see has its own distinct frequency- as do what you can't see like microwaves, x-rays, gamma rays, and what have you. Remember our old friend from 7th grade science, the electromagnetic spectrum? Yeah, he's back.
Quick tangent:
If you are wondering why some colors look better under certain lighting, it is because that lighting is emitting more light in its "chord" closer in frequency to what you perceive as the true color of the object. Think of how the only thing that looks good under sodium lamps are things the same color as... sodium lamps. No other color frequencies are present. This is the color rendering property you see on most fluorescent bulbs. If you find yourself in lighting design, you may even look at a spectral power distribution graph to see exactly what frequencies are being emitted and which lighting will go best with your scene.
End digression.
So as our "chord" of light passes through matter, certain frequencies will be reflected, transmitted, or absorbed. Whatever combination of the three is, its sum must equal the total amount of light going into an object.
Glass typically reflects the higher frequency, shorter wavelength frequency bands (ranges). Most glazing has a UV filter applied to it to make this reflection very effective. Unfortunately, glass is terrible at reflecting low frequency, long wavelength solar radiation like infrared energy. As this is transmitted through glazing and comes out the other side, its frequency changes to one that is reflected by most glass- becoming trapped and producing said 'greenhouse effect' like you see in your car on a sunny day.
"If you are wondering why some colors look better under certain lighting, it is because that lighting is emitting more light in its "chord" closer in frequency to what you perceive as the true color of the object."
Never ever review and approve color samples late at night. Wait till morning to have another look see. The owner will make you pay for the corrective paint job.
It's funny. I feel like I learned that high-energy wavelengths pass through glass easily (without any filter), but low-energy long-wavelength infrared gets reflected back. But, essentially, you are saying that both hi and low energy waves both pass through typical glass, but maybe low-low-energy infrared does, in fact, get reflected back?
Somebody should invent a glass product that, like the earth's atmosphere, reflects infrared. Can we sequester Sulfur Dioxide in glass?
Shortwave infrared can penetrate glass but most 'typical' glasses like soda, float and even some decorative glasses are infrared opaque.
Specialty ceramics (like glass cooktop), fused quartz and various uncommon doped glasses are infrared transparent.
An interesting note here is that 'crystal' [leaded glass] and certain kinds of crown glass [optical lenses] are infrared and UV transparent. The adding of chemicals such as lead, barium and magnesium increases their refractive index (minimum Abbe number of 50). This is what gives cut crystal like Swarovski its fire and brilliance.
The physics behind it are known as "bond stretching." When energy strikes a crystalline structure, the atoms and molecules of that structure vibrate. When the frequency of the light matches the vibrational frequency, the atom or molecule can absorb the energy of the light and either retransmit the light or radiate the adsorbed energy as heat.
Since glass lacks a crystalline structure and that the molecular structure of glass is under such intense pressure, low energy radiation cannot excite the atoms or molecules within the glass.
Doping glass, like leaded crystal or crown glass, with a number of compounds increases the 'coherency' of glass changing its optical, thermal and other physical factors.
Glass with a high-level of iron is nearly totally infrared opaque. This is what gives many glazing glasses a bluish-green tint when seen from the side. When chromium is added, the effect is even greater-- this is primarily the formula for wine and dark liquor bottles (deep green).
That's why motion detectors do not work through most glass.
@rusty That is a good bet. The sun emits all bands of visible light, whereas man made lighting may not.
@hb I had to rush that last paragraph and just realized how poor my copy/ paste job was. Allow me to clarify.
Most construction glazing has the coating that allows the high energy waves to be reflected. Glazing without this coating allows pretty much any short wave radiation through without any resistance. It is more or less frowned upon not to have this coating on facade glazing if people are going to be on the other side for more than a few minutes.
The umbrella term of infrared radiation has both short and long wave versions contained within the same general frequency region. Relative to gamma rays, IR is pretty long-wavelength and can still go through glazing without a problem. I didn't really articulate my frame of reference- my mistake on that.
Within the realm of infrared, you are very correct in your statement that short wave IR passes through. However as some of this is absorbed, it re-emits as long wave IR back into the interior space. Similarly, the transmitted short wave radiation is passed into the space and is absorbed by interior objects. This is also re-emitted as long-wave infrared back into the interior space.
This long wave version of IR is the one that is reflected and causes the greenhouse heating effect.
And yes, that is why the sky is blue. It sort of sucks the fun out of the question, no?
hhbrian is probably correct, there are convective loops forming due to the reduced temperature directly at the window (due to poor performance) and the warmer air in the room. poor air sealing can also be a culprit. and some windows can actually leak significant quantities of air.
you can have significant conduction through double pane windows.
the UV wave/glass thing is why they had to use hummingbirds in the Biosphere II to pollinate the plants; bees couldnt navigate inside of the building because they glass blocked the UV rays
In each case, lets call our skin one surface. In radiation, the other surface would be the exterior environment (with some contribution from the inner surface of the glazing) while in convection it would only be the inner surface of the glazing.
Radiation will be the dominant mode of heat transfer because a) the temperature terms are raised to the fourth power and b) there is a much greater temperature difference between you and the exterior environment than you and the interior glazing surface. Don't mistake this to mean that convective losses do not occur.
Convective losses are easier to mitigate, which is one reason why we have perimeter heating. We can mitigate unwanted radiant heat gain in the summer much easier by implementing exterior solar shading (one of the radiation surfaces becomes the interior of the shading device).
It is possible to block radiant heat loss in the winter, but if you have ever seen those reflective curtains from the 60s, the aesthetic is dubious at best.
Also keep in mind that the closer you get to the window, you are exposing yourself to a more of an effective area of the exterior environment. The farther away you get, the temperature of other surfaces will begin to have a greater impact.
Think of it like a weighted average of the surface temperatures of everything in your line of sight.
This might be a technicality, but isn't there no way to transfer energy from air to glass via convection (convection would require a fluid continuum)? Rather, convection will deliver hot air to the cold window surface (due to different densities of hot and cold air), and the energy in that hot air will then conduct across the glazing?
Yes and no, depending on semantics and your frame of reference.
All of this occurs within a thermodynamic region called the fluid boundary layer. How much heat is transferred depends on the thickness of this layer, which is in turn largely dependent on the velocity of the fluid. Forces between the molecules of the fluid and surface cause the fluid particles to stick to the surface, creating a thin layer where the fluid is moving at zero velocity and setting up what is known as a no slip condition.
All fluid particles not in the ‘no slip’ range are free to move. These transfer their heat to the ‘stuck’ molecules, and because they are both technically air and air is a fluid, this is convection in the truest sense.
If you look only at the no-slip interface region, this is a bit of a gray area. The molecules are fluid and they are receiving heat via convection, but at the same time they are also in physical contact with the glazing surface and losing heat via conduction. This area is known as the convective-conductive interface, and it is experiencing both forms of heat transfer. This is the phenomenon that you have mentioned.
The actual math of this is well out of my area of expertise, but it is made up of particularly nasty differential equations (look up the Navier-Stokes equations if you are into that sort of thing) and not really used by anyone except pure mechanical engineers (I am architectural).
Radiation also has some goofy effects with fluids- in fact on the exterior surface of the glazing we often use a combined radiation-convection term that I can explain later if I haven’t bored you to tears already.
As an aside, I hope this type of info is useful to other students in whatever M.Arch I program I eventually get into. I’d hate to be that crazy old dude in the corner rambling about nothing.
If this is actually built and not a theoretical question, I would first verify the construction (simple) and then analyze the environmental conditions (the bulk of what I talked about)
The area near the window is cold, why?
Is this caused by the conduction between the interior and exterior or the space around the window is not sealed well?
It could be either/both of those. Check first and see if it's open.
Seriously?
Radiation, one of the three major ways by which heat moves?
magnets, how do they work?
I think this would be a case of conduction: heat flow from excited air molecules to cold glass surface molecules.
I would also think radiation is not the significant factor. How much energy does room-temperature air emit in the form of infrared compared to the amount lost to air-glass conduction?
Then too, difference between air temperature at window and further into room may be great enough to set up a convection current.
Except that glass is a poor conductor of heat and that explanation would only work if the window was single-paned.
Since this person's homework does not make any clear mention of the window's construction... it's obvious it's conduction from the wall assembly.
Unless of course it is just a drafty window seal or an improperly installed window frame.
But I didn't go to architecture school.
?
This is clearly homework and something that you will have to investigate yourself, but I will try to point you in the right direction.
I will bet my engineering license that it is not conduction. Similarly, assuming that it is a "poor seal" is both incorrect and a cop out for blaming it on poor construction. All engineering calculations assume proper construction with a factor of safety on top of that- the burden is on the architect/ engineer in this case.
The answer is far more fundamental- you need to do two things to solve this problem:
1. Have a fundamental understanding of how the three modes of heat transfer work and how they differ from one another.
2. Analyze the environmental conditions of the interior and exterior environment, particularly with respect to temperature.
You will also need to make a few justifiable assumptions. I would start by assuming that the question is implying that it is a winter scenario and the building is losing heat.
Can we also assume the building is insulated (such that the opaque wall assembly has a higher U-value than the window), that the widows are double pane, and heat transfer via air movement is negligible?
And thereby reduce the possible means of heat transfer to radiation through glass or conduction at glass air interface?
And glass is still three times more thermally conductive than, say, wood.
When making technical assumptions, it is always best to reference the most common implementations of what you are analyzing.
Semantic point- you may want to double check your units. Is a high U value indicative of more insulation and resistance to heat flow?
Assuming that this window is set within a typical wall assembly, I would agree that in most common cases, opaque walls typically resist heat transfer better than window glazing.
In most common cases, are windows double pane? Maybe in your house, but it is debatable for *all* buildings. If an assumption can be easily debated, it is invalid. Since there is a lack of information given in the question regarding window type, one can only conclude that it must be about a phenomenon that occurs with all glazing.
If you must assume proper construction and there is no mention of mechanical systems, can you think of any other plausible reasons convection would occur? If not, then this assumption is valid.
You are starting to ask the right questions, which is half of the battle
Glazing is certainly more conductive than wood. Given how conduction operates, is that knowledge applicable in this question?
I'm not the OP, just interested in clarification for the sake of personal edification... so I'm saying case in point is double pane, and let's go from there. The OP can elucidate if the situation is single pane.
The different conductivities of wood and glass were mentioned in response to Glitter Centaur, for the sake of argument. I feel like the argument that glass isn't all that great a thermal conductor is just a rationalization for modernist wall of glazing, but that's another issue.
Let's assume it is night time as well, so there is no solar radiation through the glass which might induce convection of the air at the interior side of the window.
I can't think of any other things that might cause convection, so I'm still at radiation of air mass through window via thermal radiation or conduction through glass....
depends.
modern aluminum framing in a commercial product (be that a curtainwall or a unit window) will be thermally broken (inside metal does not touch the outside part), and glazing will have vacum or argon gas between its panes. Not as good as 6 inces of polyiso and rubberized air membrane, but still quite good.
I am actually amazed at how many LEED candidate projects are entirely wrapped in curtain walls.
Yes, these windows couldn't possibly be leaking since windows can't be poorly sealed.
@ rustystuds:
this is good point. I do also wonder if the embodied energy / lifespan / additional up-front cost of a relatively high-tech assembly such as a vacuum insulated curtain-wall is off-set by reduced heating costs... And, can a glazed envelope possibly make sense in a cooling climate?
glitter centaur is that a window or a transparent adjustable louver?
aslo, just a general fyi from your friendly neighborhood spec writer:
-glass has an amazing insulation quality given that the material itself has no insulation characteristics whatsoever. How is the insulation rating achieved you aks? Through the thin layers of air that builds up on both sides of your glass pane. Take your hand and put it very close against the glass (assuming it's much colder outside). You will feel the cold air. That cold air is what gives your glass assembly an R rating of any kind. That cold air is the insulation and a critical portion of your overall assembly.
How does this happen? Magic.
Window, hbrain.
In the warmer climates down south, many older buildings have "louver windows."
They're great on the days when the air temperature outside isn't 98 degrees. Otherwise, they just mostly let cockroaches and black flies inside.
I thought the air filmhad to do with magnets.
It depends on quality of design. Modern curtainwalls are extremely air tight (you can test them upon installation). You can also get all kinds of coatings and laminates on glass that will control solar gain.
Passivehouse uses large glazed storefronts (triple glazed) that meet the strictest of global energy standards.
I'm no fan of a building entirely clad in curtainwall, but glazing assemblies can be used quite effectively by capable designers.
jalousie, glitter? I do declare.
FWIW I don't post/ read here often, it just happens to be a slow day at work.
@glitter centaur: That is a fritted glass louver system, not a window. What is your background again?
Ignoring your snark altogether, you should reread what I said initially: In engineering we assume proper construction for calculation.
This is clearly an AE/ building technology question asked by a student in an academic setting.
Having written (and taken) many of these questions for exams, I am going to assume the intended answer to this question is a little more complex than what equates to "blame the contractor."
@rustystuds: Glass has both a lower heat capacity and thermal conductivity (as in the variables C, k) than wood. That is what I was referring to.
@hbrain: I'll just end the suspense for everyone then :D
I think what you may be assuming (correct me if I am wrong) is that a double pane system creates a thermal break preventing conduction. This isn't entirely accurate. Using our winter case, here is how heat would flow across a two lite (double pane) system.
Configuration:
Interior air
Interior lite of glass (with interior surface is closer to vacuum/gas)
Vacuum/gas
Outer lite of glass (with interior surface is closer to vacuum/gas)
Exterior environment
Heat Transfer:
1. Warm interior air will interact via convection with the outer surface of the interior lite of glass.
2. Heat will transfer throughout the cross section of the interior lite of glass until it reaches the surface exposed to the vacuum or gas.
3a. If a vacuum: the heat will transfer across the vacuum via radiation to the inner surface of the outer lite.
3b. If a gas: heat will transfer via a combination of radiation and convection across the space from the inner surface of the inner lite to the inner surface of the outer lite. The more massive the gas, the higher the heat capacity and the lower the convective heat transfer.
4. Heat will conduct through the outer lite until it reaches the outer surface of the outer lite.
5. Convection and radiation transfer the heat from the outer lite exterior surface to the environment.
Given that the original question was why does the area *near* a window feel cold, one can assume that you arent touching it- so direct conduction from you to the environment through glass is impossible. Remember conduction is only for direct physical contact.
Furthermore, the majority of convection only really occurs within a small boundary layer (the outer and inner air film) of the entire window assembly. It occurs, but it is by no means the dominant method of heat transfer. If convection was dominant, it would feel like a fan was blowing every time you walked by a window.
That leaves radiation. The only thing radiation cares about is the temperature differential between surfaces. From the interior, our body only "sees" two things for radiative heat transfer- the interior surface of the inner lite of glass is one, and the entire viewable exterior environment (which behaves like a surface far away) as another. The temperature of the exterior environment significantly dominates the temperature of the interior surface and is the controlling factor.
The only thing that having a double pane system will do is resist heat transfer slightly more, therefore creating a slightly higher surface temperature on the innermost surface of the window unit.
Both single and double paned glazing systems still boil down to:
Interior environment
Interior surface
[glazing system]
exterior surface
exterior environment
If you want to split hairs, you can count radiation losses to each of the four assembly surfaces plus the exterior environment "surface"- but I assure you this will be negligible. The innermost surface and the environment are the ones that are used in engineering energy modeling.
So to get back to the original question, it is a rather simple answer. Glazing offers a direct view to the exterior environment that allows for radiative heat transfer to take place while an opaque surface does not. The inner surface will be significantly colder than a proper wall assembly (or slightly colder if double/ triple pane) due to its material properties, but the controlling factor is that the exterior environment will act as a surface for radiation and will essentially steal the heat from a person walking by.
My undergrad and first masters were in architectural engineering; I didn't get to enclosure science and design until the end of my graduate courses. It is by no means trivial, and quite frankly the most challenging area of AE.
Succinct post cwj - thanks for taking the time to type out the building science.
cjw,
you completely omitted to mention magnets in your explanation.
I question your qualifications.
@207
Haha. Brevity was never my strong suit! I had the rare opportunity to geek out for a bit. Sorry for the length.
@rusty
Oh, but don't you see? Magnets are the reason why the heat flows from hot to cold in the first place!
No, in some places those are actually used as windows. Hello, sunbelt!
My point was this: there's no mention of construction in the question at all.
So, "In engineering we assume proper construction for calculation." ... Well' that's a really bad assumption.
And that's the exact reason why technical support people start out the question with "is the device plugged in?"
Because by your standard of assuming everything is nominal then obviously it couldn't be something as simple as "I don't know, is the window open?
Thanks for this detailed answer. I love this kind of information.
I thought it was the clinamen that caused energy to flow from hot to cold....
Can we discuss how an absorptive chiller works now?
WAIT, what about the green house effect? I thought that glass reflected thermal radiation?
hbrain, now why would you start blaming something that Al Gore made up?
its obvious that lazy unemployed people living it up on welfare are causing the cold area near the window....
thats where the ice sculpture is?
@hbrain
Absorbtive chiller explanation... another time. That one is messy and requires lots of diagrams.
Good question about the reflected thermal radiation. The best example that I can give is think of a chord in music and how it is made up of different notes- each note is a different frequency (measured in hz). Sometimes you can put cool effects on say an electric guitar to block certain frequencies of sound from being played through the amplifier.
Light behaves very similarly to sound. The sun is essentially emitting chords of energy as light, and like musical notes they are made up of hundreds of waves each with a unique frequency. Every color that you can see has its own distinct frequency- as do what you can't see like microwaves, x-rays, gamma rays, and what have you. Remember our old friend from 7th grade science, the electromagnetic spectrum? Yeah, he's back.
Quick tangent:
If you are wondering why some colors look better under certain lighting, it is because that lighting is emitting more light in its "chord" closer in frequency to what you perceive as the true color of the object. Think of how the only thing that looks good under sodium lamps are things the same color as... sodium lamps. No other color frequencies are present. This is the color rendering property you see on most fluorescent bulbs. If you find yourself in lighting design, you may even look at a spectral power distribution graph to see exactly what frequencies are being emitted and which lighting will go best with your scene.
End digression.
So as our "chord" of light passes through matter, certain frequencies will be reflected, transmitted, or absorbed. Whatever combination of the three is, its sum must equal the total amount of light going into an object.
Glass typically reflects the higher frequency, shorter wavelength frequency bands (ranges). Most glazing has a UV filter applied to it to make this reflection very effective. Unfortunately, glass is terrible at reflecting low frequency, long wavelength solar radiation like infrared energy. As this is transmitted through glazing and comes out the other side, its frequency changes to one that is reflected by most glass- becoming trapped and producing said 'greenhouse effect' like you see in your car on a sunny day.
In the words of Jesse Pinkman, "Yeah science!"
Never ever review and approve color samples late at night. Wait till morning to have another look see. The owner will make you pay for the corrective paint job.
It's all those illegal Mexicans hiding between the walls stealing hard-earned heat from honest, jobless Americans!
It's funny. I feel like I learned that high-energy wavelengths pass through glass easily (without any filter), but low-energy long-wavelength infrared gets reflected back. But, essentially, you are saying that both hi and low energy waves both pass through typical glass, but maybe low-low-energy infrared does, in fact, get reflected back?
Somebody should invent a glass product that, like the earth's atmosphere, reflects infrared. Can we sequester Sulfur Dioxide in glass?
Now we can extrapolate why the sky is blue too.
No, you're right.
Shortwave infrared can penetrate glass but most 'typical' glasses like soda, float and even some decorative glasses are infrared opaque.
Specialty ceramics (like glass cooktop), fused quartz and various uncommon doped glasses are infrared transparent.
An interesting note here is that 'crystal' [leaded glass] and certain kinds of crown glass [optical lenses] are infrared and UV transparent. The adding of chemicals such as lead, barium and magnesium increases their refractive index (minimum Abbe number of 50). This is what gives cut crystal like Swarovski its fire and brilliance.
The physics behind it are known as "bond stretching." When energy strikes a crystalline structure, the atoms and molecules of that structure vibrate. When the frequency of the light matches the vibrational frequency, the atom or molecule can absorb the energy of the light and either retransmit the light or radiate the adsorbed energy as heat.
Since glass lacks a crystalline structure and that the molecular structure of glass is under such intense pressure, low energy radiation cannot excite the atoms or molecules within the glass.
Doping glass, like leaded crystal or crown glass, with a number of compounds increases the 'coherency' of glass changing its optical, thermal and other physical factors.
Glass with a high-level of iron is nearly totally infrared opaque. This is what gives many glazing glasses a bluish-green tint when seen from the side. When chromium is added, the effect is even greater-- this is primarily the formula for wine and dark liquor bottles (deep green).
That's why motion detectors do not work through most glass.
@rusty That is a good bet. The sun emits all bands of visible light, whereas man made lighting may not.
@hb I had to rush that last paragraph and just realized how poor my copy/ paste job was. Allow me to clarify.
Most construction glazing has the coating that allows the high energy waves to be reflected. Glazing without this coating allows pretty much any short wave radiation through without any resistance. It is more or less frowned upon not to have this coating on facade glazing if people are going to be on the other side for more than a few minutes.
The umbrella term of infrared radiation has both short and long wave versions contained within the same general frequency region. Relative to gamma rays, IR is pretty long-wavelength and can still go through glazing without a problem. I didn't really articulate my frame of reference- my mistake on that.
Within the realm of infrared, you are very correct in your statement that short wave IR passes through. However as some of this is absorbed, it re-emits as long wave IR back into the interior space. Similarly, the transmitted short wave radiation is passed into the space and is absorbed by interior objects. This is also re-emitted as long-wave infrared back into the interior space.
This long wave version of IR is the one that is reflected and causes the greenhouse heating effect.
And yes, that is why the sky is blue. It sort of sucks the fun out of the question, no?
Cjw:
Thanks for your education.
After reviewing your post, I don't know what to ask further. My window is double pane but it seems doesn't matter in my case. What is your best guess?
Don't you guys have similar problem or just me? It is so cold to pass by that window on the winter.
it's not radiation.
hhbrian is probably correct, there are convective loops forming due to the reduced temperature directly at the window (due to poor performance) and the warmer air in the room. poor air sealing can also be a culprit. and some windows can actually leak significant quantities of air.
you can have significant conduction through double pane windows.
glitter
the UV wave/glass thing is why they had to use hummingbirds in the Biosphere II to pollinate the plants; bees couldnt navigate inside of the building because they glass blocked the UV rays
holz
It is a combination of both radiation and convection. Radiative losses will be much greater in magnitude than convective.
In the basic equations for heat transfer:
Qradiation = [some constants] * [(Tsurface1)^4 - (Tsurface2)^4]
Qconvection = [some constants] * [(Tsurface1) - (Tsurface2)]
where Q is heat and T is temperature.
In each case, lets call our skin one surface. In radiation, the other surface would be the exterior environment (with some contribution from the inner surface of the glazing) while in convection it would only be the inner surface of the glazing.
Radiation will be the dominant mode of heat transfer because a) the temperature terms are raised to the fourth power and b) there is a much greater temperature difference between you and the exterior environment than you and the interior glazing surface. Don't mistake this to mean that convective losses do not occur.
Convective losses are easier to mitigate, which is one reason why we have perimeter heating. We can mitigate unwanted radiant heat gain in the summer much easier by implementing exterior solar shading (one of the radiation surfaces becomes the interior of the shading device).
It is possible to block radiant heat loss in the winter, but if you have ever seen those reflective curtains from the 60s, the aesthetic is dubious at best.
Also keep in mind that the closer you get to the window, you are exposing yourself to a more of an effective area of the exterior environment. The farther away you get, the temperature of other surfaces will begin to have a greater impact.
Think of it like a weighted average of the surface temperatures of everything in your line of sight.
This might be a technicality, but isn't there no way to transfer energy from air to glass via convection (convection would require a fluid continuum)? Rather, convection will deliver hot air to the cold window surface (due to different densities of hot and cold air), and the energy in that hot air will then conduct across the glazing?
@cwj - (may posts back) My compliment was sincere. Sure the post was a bit long, but I think you did well to pack in a ton of good info.
@207
thanks- to me it looked a bit lengthy!
@hb
Yes and no, depending on semantics and your frame of reference.
All of this occurs within a thermodynamic region called the fluid boundary layer. How much heat is transferred depends on the thickness of this layer, which is in turn largely dependent on the velocity of the fluid. Forces between the molecules of the fluid and surface cause the fluid particles to stick to the surface, creating a thin layer where the fluid is moving at zero velocity and setting up what is known as a no slip condition.
All fluid particles not in the ‘no slip’ range are free to move. These transfer their heat to the ‘stuck’ molecules, and because they are both technically air and air is a fluid, this is convection in the truest sense.
If you look only at the no-slip interface region, this is a bit of a gray area. The molecules are fluid and they are receiving heat via convection, but at the same time they are also in physical contact with the glazing surface and losing heat via conduction. This area is known as the convective-conductive interface, and it is experiencing both forms of heat transfer. This is the phenomenon that you have mentioned.
The actual math of this is well out of my area of expertise, but it is made up of particularly nasty differential equations (look up the Navier-Stokes equations if you are into that sort of thing) and not really used by anyone except pure mechanical engineers (I am architectural).
Radiation also has some goofy effects with fluids- in fact on the exterior surface of the glazing we often use a combined radiation-convection term that I can explain later if I haven’t bored you to tears already.
As an aside, I hope this type of info is useful to other students in whatever M.Arch I program I eventually get into. I’d hate to be that crazy old dude in the corner rambling about nothing.
@ny
If this is actually built and not a theoretical question, I would first verify the construction (simple) and then analyze the environmental conditions (the bulk of what I talked about)
Wow. This was a very informative thread.
so conceptually, for our case, it's: input energy to fluid boundary layer = convection and output energy to adjacent solid material = conduction.
hb, I think the 'no-slip' region would occur on both sides of the glass.
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