The Film on the Windows: Newsletter Challenge (10/02/07)

The film will also assist in keeping in the heat during the winter. leaveing it on will save your fuel bills and keep you wife happy. stubbing your toes on the furniture occasionally is a small price to pay for a happy wife. But then again if she's threatning to banish you to the workshop if you remove the film. Check that the heater is working and the fridge is full of beer and tear it off

That is probably true. It is a bi-directional shield, I think, and I also think that it is the infrared radiation that you want to block. UV will not be a thermal issue, but it does cause sun fading of material.

Another consideration, winter days are shorter and the Northeast typically does not have a lot of days that are sunny; compounding the problem of insufficient solar radiation. Over the course of an average winter day the percentage of sunshine will probably be outweighed by the heat dissipation from inside to outside the glass, even with the film off.

I'll bet Coconutpete has the correct answer.

Standard plate glass does not allow for the transmission of UV light. Howmany times have you gotten a sunburn by sitting in a sunny living room, or driving down the highway?

This is not quite true. The addition of iron oxides to plate glass-this is what gives it the slight greenish cast does inhibit some uv but not all or even most. When using uv cure adhesives on plate glass there is no problem at all of placing the uv lamp or sun on one side and gluing glass or other objects to the subsurface. On the other hand, laminated glass completely blocks uv. The plastic inner layer has uv inhibitors that prevent uv rays from passing through it. Thus no sunburn through the car windshield.

I know that your comment represents the norm, BUT I GOT SLIGHTLY SUNBURNED ON A TRAIN FROM PORTSMOUTH TO LONDON, I HAVE A VERY SENSITIVE SKIN!! IT IS POSSIBLE!! I EVEN HAD WHITE BITS AROUND MY EYES FROM MY GOOD SHADES.....

The film acts to reflect the longer wavelengths of energy which includes heat. It should work to reflect heat back into the house but it will also inhibit the sun from contributing to the heating of the house. As the sunlight is seriously diminished during the winter, the amount of heat contribution from the sun might be negligible anyway.

In the end it might be better to have deciduous trees around the house so it is shaded in the summer and has full sun in the winter.

Kris,

Don't feel bad. I am in the US and am not familiar with the term "metal oxide" as applied to window films. However it is apparent (if not transparent ) that he is referring to a film which blocks infrared (heat) radiation from the sun, yet allows visible light to pass through relatively unimpeded. Your "cling-film" which provides an air gap to reduce convective and conductive heat loss in the winter time is a different matter altogether. I suppose there may even be some blockage of radiation by the film, but generally it would be negligible unless your film also had an IR reducing tint.

Actually, the answer to the question is quite simple, "It depends". If the "metal oxide" film act strictly as an IR blocker, which reduces A/C load from the summer sun's infrared heating, then it would be desirable to leave it up in the winter time as well to retain heat that might be lost by radiation through the windows, since the transfer of IR through a window or "black hole" (in the non-astronomical sense) is always from an area of higher temperature (inside surfaces) to one of lower temperature (objects and air molecules outside and surrounding the house, with the reduced solar radiation being an exception (outside to inside). As noted earlier, the nights are longer in winter and the days are shorter, so there is more potential for loss than for gain without the film.

Ideally, an easily removable film which could be switched day and night would work better in both winter and summer. Perhaps a return to the roll-up window shades of old, instead of the now ubiquitous "mini-blinds", or in addition to them if possible, would work in this case. However, the roll-up shades would be made of material similar to or coated with the "metal oxide" film. That way you could "have your cake and eat it, too!", i.e. let the sun help warm the house during milder and sunny winter days, when net IR transfer is inward, by rolling up the shade and removing the IR blocker, but roll it back down and put the IR blocker in place at night and during cold overcast days when the net IR transfer would otherwise be a loss for the interior.

I know the last paragraph does not answer the question, which seeks a simple yes or no answer, but as is often the case, the answer is "depends". That is, the answer depends on which direction is the average IR transfer through the windows, which usually would be out on average during the winter when you actually want to keep the house warmer, and so "leave it alone" is the better answer, I believe.

Thanks STL, You've added clarity to the Question. I understood what the questioner was referring to with the use of film ( and the distinction between that and a secondary glazing gap), but felt like a mild snipe because of the lack of further detail.

I agree with your comments about the length of day and IR heat transfer being factors. The question leaves us to assume the building is in America, but even that's not very informative as to the overall question. Taking the question 'as is', it seems to be querying the manner of heat transfer across the window. The oxide film will function to block radiant heat, but what of it's effect on conductive heat ? Glass with film will have a different u value to glass on it's own, albeit not much for a few microns of Oxide. At present I'm thinking 'leave the film in place during Winter' because less radiant heat will escape the building than could be gained by having lower conductance of heat via an oxide film. < Also I'm lazy and wouldn't want to go around filming the windows every year.>

You got to realize a couple of things here.

In the higher latitudes the sun is hitting the earth at a lower angle than summer, so there will be more light entering the window.

Secondly the light entering the window is at shorter wavelengths and will not get converted to the longer wavelengths until it strikes the interior. In the summer the angle is high and less light enters the window but the shorter wavelengths created by the light striking the ground outside can not enter through the film. So there can be a little bit of an advantage. In the summer though, the light coming through the window is still going to heat the room but there will be less of it since it is at a higher angle.

Please explain how shorter wavelengths are converted to longer, I am fascinated to learn how this happens!

Electromagnetic waves are emited by all objects unless their temperature is at absolute zero. The radiation is broadband, but peaks at shorter wavelengths for hotter objects and at longer wavelengths for cooler objects. The temperature of the surface of the sun is about 6000 degrees C so its peak radiation is at shorter wavelengths than the objects in your living room which will radiate at longer wavelengths because the temperature of the living room is lower than the temperature of the sun. To learn more about this topic look at Planck's law in Wikipedia http://en.wikipedia.org/wiki/Black_body_radiation or search on greenhouse effect http://en.wikipedia.org/wiki/Greenhouse_effect.

MZE

Andy,

This is what the greenhouse effect is all about. It isn't the infrared energy from the sun that heats things up, so much as the UV and visible light. Objects (such as people and bricks and asphalt) that neither transmit UV and visible (UV-VIS) light, nor reflect UV-VIS (like mirrors), will absorb that UV-VIS energy.

Once absorbed, the energy of the UV-VIS light causes the object to heat up. Everything that has temperature emits light. Even you! You just can't see it, because it's infrared. The warmer an object is, the closer the infrared output gets to the visible realm... which is why very hot object start to glow red, and then orange, and then yellow, and then white.

The UV-VIS passes through glass easily, and is absorbed by objects in the room. Those objects heat up, and radiate infrared. Glass is opaque to infrared! Therefore the infrared gets trapped in the room, and the whole place gets warmed up. Just like a greenhouse!

If the glass is laminated, only the VIS light can enter the room. The UV is blocked by the vinyl layer. Therefore there's less energy to heat up the room, so the room stays cooler.

I hope that helps!

Shawn

It was a kind thought, but it did not really help.

I understood that UV also does not get through glass either or is severely attenuated at best.......!!!

Shawn,

I agree with most of what you said, and I think that this goes along with my example of the fluorescent light tube, where the coating absorbs UV and re-radiates visible light, which is lower in frequency and has a longer wavelength.

However, and not to quibble, because you make excellent points, but I have to disagree with:

Glass is opaque to infrared!

If this was the case, when I drove by the box company warehouse fire many years back in my car, I would not have felt the intense heat hitting my face through the window glass. The fire did not emit UV and very little visible light, mostly from a few flames that were visible intermittently and the red glow of the burning embers. It was night time, so my pupils were dilated by the dark and therefore my eyes were already more sensitive, yet this light was not even intense enough to cause me to need to shade my eyes. Granted, rolling my window down increase the intensity of the IR I felt, but I still received considerable IR with the window rolled UP!

Let's say that Glass is "Semi-opaque" to infrared, OK? Certainly IR encompasses a wide band of frequencies and some may pass through more easily than others, depending on the composition of the glass. Others contributors here have noted how some glass windows are more IR reflective than others.

Otherwise, good job.

Hang on, you got me playing with razor blades and oranges Rick. In Winter, the incident angle will be lower and so less light hits the window. Isn't that what makes Winter 'Winter' ? Maybe I'm missing something in the phrasing here.

On the second bit, how does the short wavelength get converted to long wavelength (and also loose heating power) ? I'm probably being obtuse, bu can you explain a bit more.

In Winter, the Oxide is going to enable the window to conduct more heat ( my guess). My interpretation is that it would increase convection around the room by increasing emissivity of the window. This question was no doubt phrase to ignite debate about terms etc, but I'm interested to hear more. In city's over here, 'tinted' ( I use that to include glass with added thermal properties) glass is not unusual - I'm guessing not just for the privacy effect, but also for energy efficiency. I observe no majority opinion when looking at buildings in London. My assumption is that the jury is out, ie there is no cover-all rule of thumb. Do a high percent of building in NYC or elsewhere in the USA use coated glass ? Accepting that some buildings do it for aesthetic/security reasons, it doesn't seem that many use it for thermal reason.

Thanks MZE for elaborating on the wavelengths. I just thought everybody knew that little tidbit.

If the sun is at a high angle the light comes straight down on the building and not into the windows. With a low angle the sun will hit more of the windows, especially those with a southern exposure (Northern exposure for those folks in the southern hemisphere.)

As for coatings, those are more popular in the south and on commercial buildings. I used to work for PPG and they made coated glass. The machine sprayed the glass before it went into the Lehr where it got baked on. Very toughy machine which broke down often, but when it was running, it made a ton of money.

Very toughy machine which broke down often

Hmmm...wouldn't a toughy machine by tough and NOT brake down often? I guess you meant touchy, right?

As always, I am in complete agreement with STL!

This site provides two paragraphs ("Reflex glass reduces solar gain" and "Reflex glass retains heat") that support STL's contention.

Clearly, STL must own stock in a titanium company.

Clearly, STL must own stock in a titanium company.

No, but my brother-in-law is sitting on about 300 acres in Tennessee (old family farm) with rich deposits of titanium oxide, however the oxide is in a form that is not yet cost effective to mine and process with the current technology. However, energy, metals, and mining companies are still willing to pay him and his family large amounts of money just for a LEASE on the mineral rights, should technology change quickly or the value of the metal increases substantially. The lease expires soon and he is trying to determine whether processing technology has advanced to the point where it would make the lease more valuable, before he just automatically signs an extension of the old lease. He is a chemist by training and education, so that should help him in his decision-making.

I believe the coating is an oxide of Titanium, but don't quote me please....

I think it should be left on if you want to stop textiles and photos from fading, it probably stops IR from leaving the house too, but as to it helping or hindering the house getting cooler I can't say exactly, "swings and roundabouts"?

Ti Coating is a UV inhibitor. But again, window glass does not transmit UV radiation. The color changes you refer to are based on Photo sensitivity of the material not the UV sensitivity.

The Ti will not limit IR transmission if the amount allows the glass to be completely transparent. Occlusion of the glass (darkening, or other form of translucency) will limit the IR transmission.

You can effect the transmission in about the 2400nm range by messing with the OH content, but with soft glass you are already at virtual 0% transmission anyway.

But again, window glass does not transmit UV radiation. The color changes you refer to are based on Photo sensitivity of the material not the UV sensitivity.

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Not so. Windows specifically treated to block UV are available to help keep curtains, draperies, and other window treatments from fading. Untreated glass transmits UV in varying degrees which are dependent on specific wavelength. I have tested various kinds of glass myself to see whether it blocked long- and short-wave UV. Some do, some don't. I might also point out that fluorescent lamps emit some UV which can cause certain pigments to fade which do not otherwise fade under regular incandescent lamps.

The question is not easy, as there are several parameters to be considered.

First, although solar radiation is minimal in winter, the presence of sunlight may be important in the maintenance of a comfortable temperature inside a room. Keep in mind that solar radiation in winter may be in the order of a few hundreds of watts per square meter. The convenience of shielding the sunlight depends naturally on actual exposure of the window to sunlight. For south-oriented windows it may be worth removing the film. For other directions, the difference in having the film or not may be hardly appreciable. Other parameters to be considered are the actual daily period of solar exposure and the area of the windows.

Additionally, by a theoretical point of view, there is a basic difference in having the film applied in the internal or external side of the glass. If the layer is placed internally, its effect shall be to reduce the emissivity of the glass on the surface facing the house room. The radiative dissipation from hot room to cold glass will be minimised, and the thermal dispersion will be reduced. On the contrary, if the layer is placed outside, being the natural emissivity of the glass high (in the IR region), the cold glass will absorb a higher thermal radiation from the environment, cooling the room further.

We could also consider that the layer on the internal side makes the glass hotter during daylight and colder at night (vive versa the layer on outside), but at this point the problem becomes very complex, also because convective effects cannot be neglected in all the reasoning.

The best solution, after having performed all necessary trade-offs and calculations, by assuming a wide set of environmental parameters, is: make your wife happy , and you both will be bursting with health. The advice of your wife have priority over the principles of thermodynamics.

Titanium Oxide is used in glass to produce Low-E Glass. It will reflect light in the spectrum associated with heat back in or out of the room. It will do nothing for the heat you wife is going to give you if you don't abide by her wish to keep it on. Just think of it as one less chore you will have to do for her spring and fall.

I'm with you Brother!!

Yea, and try cleaning all the adhesive off the glass and you will roo the day you started the job.

...now where has Kanga gone to?

Having just given up and read this, the solution is obvious. Just take out the windows each year and turn them around. Maybe there is a market here for windows that pivot 180^o about a mid-axis.

Store products in a dry place, protected from exposure to direct sunlight and rainwater

A quote from Kris's link - is it me? Looks like we need STL's blinds after all.

Consider yourself usurped!!

Did you actualy read the text on that site you posted? It was advising that the glass treated with its coating were kept out of direct sunlight. Not much Bl00dy use as windows then, are they? Where's John Stalker and his dog when you need 'em. I'm sure he'd have a solution.

Obviously the main object is humour - but it's just close enough to being believable to be worth elaborating. Unless the temperature of the glass and film varies across the thickness of the pane, the principle of reciprocity can be applied - equal absorption of transmitted light in either direction, and similarly equal emissions. If it's a film on a double glazed unit it would indeed be better on the inside of the outside pane in winter and probably vice versa in summer (but then two would be better than one, and you wouldn't be considering stripping it anyway - right?).

On further reflection (pun intended), I wish to add a corollary to my posting in #11. The answer still is "it depends", but in addition to the notion of net or average IR transmission being to the outside in wintertime without the film, let me say that IF there are suitable curtains, drapes or blinds on the windows, and they are used at night, they would effectively prevent the thermal loss by radiant IR at night, possibly tipping the average towards a heat gain, if there are many sunny days, and so it might make removing the film worthwhile, if only for the pleasant effect of sitting in the warmth of the IR from sunlight on a cold winter's day!

Someone also asked how is it that IR can be absorbed at a shorter wavelength and re-transmitted at a longer wavelength. This is true in general for any electromagnetic energy that is actually absorbed and not just reflected. It is the principle behind the functioning of fluorescent lighting, as the lamp actually creates Ultra-violet radiation, which strikes the coating inside the glass. This coating absorbs the UV, which excites the electrons in the material of the coating. The coating then "fluoresces" and emits visible (white) light as the excited electrons drop back into lower energy states and give up their excess energy. This also how "the Greenhouse effect works" as the interior of a Greenhouse (and the Earth as well) absorbs IR from sunlight, heating solid surfaces, which excites their electrons, but then re-emits IR at a slightly lower frequency or longer wavelength, more of which will be reflected by the glass back into the room. This is because longer wavelengths have a higher critical angle (the angle of incidence at which energy would pass through rather than be reflected).

Similarly, radio waves of lower frequency and longer wavelength (HF) reflect off the ionosphere and come back to Earth (skip), while higher frequencies with shorter wavelengths (VHF/UHF and higher) pass through even at the lowest angles, making them good only for local (ground wave) or satellite communications.

Please explain in more exact detail as it reads at this moment like hype, sorry....

Your explanation with regards to a fluorescent light is great, but what has this to do with furnishings in a room converting energy from one level to another???

Please explain with a better example that actually relates to the problem you are trying to tell us about, I am fascinated.

If by chance you accidentally "over extended" yourself in this area, just submit a retraction, nobody will hound you for that.....we just want to know one way or the other.....

Thanks in advance.

How's this for a solution?

1. Insist to your wife that she is wrong. But, you will take scientific measurments just to prove her wrong.
2. Take out something that looks like a sextant. (It does not have to work.) Aim it out the window every hour for six consecutive hours. Write a series of numbers on a large piece of paper.
3. Put a thermometer on a stand about one foot from the window. Again it does not have to actually work.
4. Drag this out for a week.
5. Then announce that she was right, apologize to her, and have a long and happy marriage.

......LOVE IT!!!!

There are so many complications to this issue (and too many other "fish to fry"), that I will take the easy way out and say my wife is right!

Well, the answer seems to be in how the question is worded!

Hmmm...

Change "living room" to "office" and "wife" to "co-worker" and I bet we get a whole lot of different answers!

ROFLMAO at people who said "wife is always right". This may be true, but not really an answer to the challenge.

There isn't enough heat to be derived from window films to make up for a cold wife.

Pick a fight you can win.

Pick a fight you can win.

I did! That is why I said, "Change 'wife' to 'co-worker'."

ROFL

No aliases. You never wanna forget the nature of your opponent.

By the way, if a man speaks when his wife can't him, is he still wrong?

By the way, if a man speaks when his wife can't him, is he still wrong?

Eu,

Speaking of pedanticism (ROfloorL), is that hum, like possibly a hymn? I don't know the verb him!

And, yes, he is wrong no matter what, if he knows what is good for him (or is it hum?)

I've got your back ER!

And Ken adds, pedantically:

How does a wife "him". I'm unfamiliar with that word as a verb.

Perhaps eu meant to write "If a man sings and his wife can't hymn, is he still wrong?" Of course even then, the use of hymn as a verb is a bit unusual.

Maybe "Humms"?

Ken adds:

Ken, try to keep up. STL already wrote words to this effect quite a while back.

I still got to many other priorities, and the actual difference probably is not worth the effort and energy to make the change.

Respect Our Forum, Loyal Management, And Others? Yes I do.

I'd leave it on. The same reflective properties that kept the heat out in the summertime will act to keep the heat in in the wintertime, making your furnace run less to keep the house warm.

I'd leave it on. The same reflective properties that kept the heat out in the summertime will act to keep the heat in in the wintertime, making your furnace run less to keep the house warm.

The READER'S DIGEST condensed version!

(I wanted to block light coming into the room but keep the brightness.)

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The brightness of what? If the brightness refers to light coming into the room through the window, then you're hosed, Bud, because you just "blocked the light coming into the room" with your film.

If the brightness comes from some other source, then the clause "block light coming into the room" has nothing all to do with the clause "keep the brightness." These two clauses are diametrically at odds in this sentence. Or quite possibly unrelated even though they are joined with the conjunction "but." You might as well say "The Moon is smaller than the Earth but it is farther away."

Brightness might also be interpreted here as an aesthetic quality of the room having nothing at all to do with light per se.

What, exactly, do you mean here? How is this to be interpreted?

I don't mean to be pedantic but, generally speaking, if you ask an ambiguously-phrased question, then expect to get an ambiguously-phrased answer. Or more than one, most likely.

I don't mean to be pedantic but, generally speaking, if you ask an ambiguously-phrased question, then expect to get an ambiguously-phrased answer. Or more than one, most likely.

Eu, I agree. The questioner seems to be technically-challenged. Don't know how this one ever got past the CR4 editor.

I understood/guessed/surmised/assumed him to mean he wanted to block the heat of the sun (light), but allow in the visible light (brightness).

I think most people interpreted it this way as well.

Too bad we have to interpret the technical meaning in a "Challenge Question". This was just a poor choice of words in my opinion.

Somehow the question (as posed) calls to mind this cartoon: