Low E windows

FXSTerDecember 20, 2004

My wife and I just finished our house and although I didn't want them, our building supplier sold us Low E windows by Anderson. I don't have a problem with Anderson, just the Low E part. Anyway, we have a south facing wall with a lot of windows in it and figured that we would get a good amount of heat through them on sunny days, but so far it has not been what I expected. Could Low E glass be a problem if I wanted sunlight heat in my house?

Thank you for any ideas

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What kind of low E windows do you have? They reflect solar radiation to different degrees. Ideally if used properly they should block some unwanted radiation and help to prevent it from escaping form within the house but still allow solar energy in.

    Bookmark   December 20, 2004 at 4:13PM
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Andersen uses a "general purpose" low-e glass that has about a 50% solar transmission rate. By comparison, "northern glass" will have a transimission rate of about 80% or more, and "southern glass" will have a transmission rate of 30% or less. I have spoken to Andersen a number of times, and they have told me they have no intention of giving the customers any choice, and that you get what they offer. Other manufacturers will give you many choices of glass.

Even the best of transmitting low-e glass will be a bit lower than a regular thermal pane, but not a lot, and the gains at night from using the low-e are quite large, so I like low-e, if it is the right kind.

You are probably stuck, as the glass is not replaceable (per Andersen) in their windows, and your contractor probably won't spring for another brand and replace them all. I would, however, hit the contractor up on some compensation if you can show that he didn't give you what you ordered.

I did put Heat Mirror glass into my older Andersen windows, but it was at my own risk and labor, so that is not an option for you.

    Bookmark   December 20, 2004 at 5:55PM
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Thanks for the replies. I'm not sure what type of Low E windows they are, but I'll check into that tonight. It seems to me that it's kind of rude that Anderson "has no intention of giving the customers any choice". I just went with Anderson because of their reputation and also the supplier wouldn't sell anything else.
Again, thanks for the replies...I'll check into what type of Low E glass I have.

    Bookmark   December 21, 2004 at 8:17AM
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I disagree with 'booster' that Andersen only offers one glazing. We are spec'ing both Andersen L'ow-E' (their normal glazing) and Andersen L'ow-E Sun' glazing in different areas of our new home (the Low-E Sun to control/limit more of the solar heat gain in certain areas than the Low-E does).

As far as the question of is the low-e glazing limiting your solar heating: Yes, it does to some extent (the solar heat gain coefficient is less than regular glass), but my experience in using Andersen Low-E in a family room addition with a S facing wall of glass is that the room cooked in the winter - 85 degrees wasn't unusual! To sit in front of those windows on a sunny winter day would becme uncomfortable as you got hotter and hotter. Just my experience.

IMO, I think your builder did you a favor putting in the Andersen Low-E glazing - they'll conserve energy and save you $$ in the long run.

    Bookmark   January 4, 2005 at 5:02PM
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formulaross is correct in saying Andersen does offer an alternate glass in the "sun" version. I had forgotten about that one, as it was not an option in my discussion with Andersen. I was looking for more transmission, not less, and they will not provide anything that will transmit more than their "high performance" glass which is about 50% the transmission of a single pane and 60% that of a standard thermo-pane. By contrast, the Heat Mirror glass that I put into my Andersen windows has a transmission near 85% that of a regular thermo-pane, is 3 layers, 2 gaps, with an R7 rating. They are much better at solar heating than the regular Andersen windows were with High Performance glass, but not as good as the same windows were with the old Andersen welded glass (for solar gain). I have done all the heat gain/solar gain/day gain/night loss caclulations, and tested the results at home, and in my case I just about break even with the Heat Mirror over the old welded glass with a storm panel. I lose some heat gain during the day, I gain on less heat loss at night, the heat bill stays the same, but comfort is improved from more uniform temp. We are in the Minneapolis area.

Also, don't believe the statements about low-e saving money on cooling in the summer, unless you do some very specific things to make it happen, if you use high transmission or even the Andersen glass. The only heat that is stopped by low-e is reradiated heat from the ground, trees, buildings etc, which is usually not a huge amount unless you face a lot of hot and dark objects. Solar (depending on the transmission you have), goes right through, and then the low-e prevents it from reradiating out as it would through standard glass. The only way to prevent this is to reflect the sunlight directly back out the window before it can be absorbed by anything in the room. This can be done with the high reflectivity shades. Regular window shades and drapes do not work like they would with a regular window glass and can actually overheat and destroy a low-e window.

The folks that sell the windows will not tell you any of this and will quote generic (and incorrect) savings based on some lab conditions that don't happen in the real world. I got most of my information from 2 of the top University labs doing research on windows and efficiencies, the University of Minnesota in Minneapolis, and the University of California, Berkely.

I think Andersen makes a very good window, but is doing a disservice to a lot of folks, and the environment, by not offerring a variety of glasses, so the best glass for an application can be chosen. Their glass is made by Cardinal in Wisconsin, who I talked to, and would be able to make any variety that Andersen would want. Last time I checked a few years ago, folks like Marvin would put in anybodies' glass that you wanted for you application, I just wish Andersen would do the same.

There is a lot of info on glass and all the specs on the Net, mostly from the glass and window manufacturers. If you are so inclined, it is very intersting (to me anyway), and can give a lot of insight as to all that is available, and the ways it is used.

    Bookmark   January 4, 2005 at 5:49PM
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A few comments on energy efficiency and window performance.
Glass, by its nature, has very little insulating value. A sheet of double strength glass (3.0mm or 1/8"), alone, has an R-value of about .8. An R value measurement for a specific window might include both glass and frame (but, only when it was calculated using U value measurements). With a good solid sash and frame and a good storm window over a single pane window, you can get an R-value of maybe 1.5 max. Doesn't really matter what else you do with that window, that is about topped off.
A good dual pane IGU (Insulating Glass Unit) with a softcoat LowE coating and an argon gas infill can potentially get about an R-6. A triple pane IGU with a krypton gas infill can achieve an R-10.
R value is not the best measure of a windows performance, but since most folks are used to using R value, I used it here as well. U value is a much better measure for measuring window performance (whole house performance as well, but the fiberglass insulation folks have conditioned people to think in R valueÂ).
R-value is based on Resistance to heat flow. Heat always goes to cold. When you touch that really cold floor with your bare feet and you feel the cold spreading over your feet and up to your ankles and higher, it is not really that the cold is penetrating your body, rather what you are feeling is the heat being drawn from your body thru your feet to the cold floor. This is known as conduction.
Heat travels in three ways: conduction, radiation, and convection. All three of these factors affect your comfort level and all three of them affect, and are affected by, the performance of your walls and windows.
One really good measure of conduction is to take an aluminum bar and hold one end in your bare hand and one end in an open flame. Everyone reading this knows how long you have before you will drop the bar because the very sudden rise in temperature will certainly affect your ability to hold on.
Radiation or, radiant heat, is what you feel when the sun warms you as you are lying on the beach (except when you gain warmth from the sand around you, that would be conduction, again). Radiant heat is what you feel when you stand close to (but not touching) a steam radiator or a radiant heater. Radiant heaters use infrared radiation (IR) to warm you (or other objects), and IR is also the heat you feel directly from the sun.
Convection heat is warm air movement such as a forced air heating system. A convection current is set up as the warm air moves and is replaced by cooler air that is then warmed and the cycle continues.
A single window with very poor insulating properties will potentially be affected by all of those properties. Sealing the window will help against air leaks but that has little affect against the significant part of energy loss which is the glass itself.
As I mentioned earlier, windows are normally measured in U-values and not R-values.
U value measures how much heat is passed thru an object, or said another way, how much heat an object will pass; versus R-value which is resistance to heat flow. Although it doesn't sound like much of a difference, it does come into account much more when doing actual energy performance calculations.
A multiple pane IGU is designed to do two things. First, it maintains a sealed dead-air space between two lites of glass. This dead air space helps to moderate the differences between the outside air temperature and the inside air temperature. The outside lite will approach the temperature of the ambient air and the inner lite will attempt to reach the inside ambient temperature. The inner lite will not reach the interior temperature because that heat will be radiated toward the exterior lite. But, while the inner lite will not reach equilibrium with the inside air temperature, it will still be warmer than the outside lite for a net gain in room temperature and especially comfort. Additionally, the air within the IGU space will be warmer than the air outside the house because of the warmer interior lite. Again adding to the net gain of the entire unit.
One might comment that using single glazing with a storm window will accomplish the same. It MAY do so in the best of circumstances, but unfortunately the window MUST be a sealed airspace for optimum performance (otherwise heat will escape thru the leaks) and any airspace wider than about 7/8" begins to lose performance because of convection currents that form between the lites within the airspace.
The outer lite and the inner lite in an IGU have to be close enough to allow the inner lite to radiate heat to the outer lite. If they are far enough apart, not enough heat will transfer and the differences in the air temperature near the two lites will cause a convection current to form. This current will very quickly suck the heat out of the inner lite and negate much of the advantage of the multipane IGU.
You can also have convection currents in your house. A single pane window will be cold...this is why it was so common (and still is common) to put heat sources under windows on the exterior walls. The idea was (is) that heat from the heat source will rise and will warm up the glass enough to prevent much cold air from entering the room. What is really happening is that that heat is being sucked from the room, but if you can pump enough heat at the heat sink (window), then you can keep the room warm...you are simply overwhelming the amount of heat that nature can take out of the room.
Now, what if you could include something that would reflect the radiant energy back into the room? And that is the idea of LowE coatings. LowE coatings reflect IR energy (they also reflect UV energy as well, but that is for another post).
A softcoat LowE coating is normally applied on the interior of the exterior lite in a dual pane IGU. The exterior or the exterior lite is called surface #1 surface in an IGU, the interior of the exterior lite is called surface #2, the exterior of the interior lite is called surface #3, and finally, the surface you can actually touch inside your house is called surface #4. Hope that made sense!
The best location for the softcoat LowE coating is on surface #2 because the LowE will reflect IR energy. It does not care if it is generated inside or outside. In the summer, the suns IR and UV rays will hit the glass with the LowE coating and will be reflected by the coating, yet it will pass visible light. This keeps a significant amount of the heat (and the ultra violet) from the sun from entering your home.
In the winter, the LowE coating will again reflect the IR (or heat) back into the house, where it started. And the advantage of coating surface #2?
In the winter, the interior heat warms the interior lite. The interior lite then radiates that heat to the exterior lite (a process I mentioned earlier). BUT, there is now a difference...instead of the outer lite absorbing that heat and radiating it into the night, it reflects that heat back into the house. This does two things...first, it warms the interior lite and allows it to come much closer to the inner temperature of the room; second, the warmer interior lite also tends to warm the airspace which in turn helps to keep the interior lite warmer. See the pattern?
In summer, the IR energy is reflected by the exterior lite which allow the interior lite, and subsequently the airspace, to remain cooler and closer to the inside temperature of the house.
Does all this work? It does, and it works very well. This is not just conjecture, it has been tested thousands of times in lab environments and in the field. There are some very specific test and evaluation projects that prove without doubt that it works.
The U-factor measures how well an IGU prevents heat from escaping. U-factor is going to generally be between 0.10 and 1.20. As I have mentioned, the lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.
Solar Heat Gain Coefficient (SHGC) is the measurement that actually measures how well a product blocks heat caused by sunlight... SHGC is the actual measurement of solar radiation or IR energy, or solar heat (pretty much all the same thing), that passes thru a window either directly transmitted or absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits. Now is where we get the argument between the hardcoat and softcoat (or northern versus southern) LowE folks.
Hardcoat, or pyrolytic coating folks say that you should use a hardcoat LowE on the #3 surface. The hardcoat does not block IR energy as well as softcoat...and they admit that as well. But, they turn it around to say that hardcoat is better in cold environments than softcoat because since the hardcoat does not block IR as well as softcoat, the hardcoat will allow more of the suns heat into a room - passive solar.
And, they are actually correct as long as the sun is shining directly on the window. Of course during the other 20 hours of the day the window is losing a fairly substantial amount of energy because the hardcoat has a lower U value as well.
Hardcoat folks favor a higher number SHGC in the north because they believe that the additional heat that is transmitted thru the coating (which does not block heat as well as the softcoat does) is an advantage in absorbing the sunÂs energy in the winterÂand softcoat folks feel that because the higher SHGC number is related to increased heat flow thru the glass, the gain in sun-heat energy is not justified when compared to the losses due to the poorer U-valueÂand the fact that in the summer that same higher SHGC will allow more of the sunÂs heat into the home and raise cooling costs as well.
The hardcoat guys counter by saying that in the winter the sun is lower and shines directly on the window so the heat gain is significant and does offset the greater loss because of lower U-value, and in the summer the sun is higher and the house can be equipped with awnings or a soffit capable of shading the window in question.

Andersen uses a softcoat LowE from Cardinal Glass. Cardinal supplies about 70% of all residential LowE used in North America. Virtually all of the major window manufacturers use Cardinal's product.
Andersen uses this particular coating because it does have the best available energy numbers based on performance testing. You were correct in your original thought FXSTer, it is the coating that is blocking the sun's heat.
But, that also means that it is blocking the heat inside your home from escaping.

    Bookmark   January 26, 2005 at 6:08PM
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Regarding the comment about LowE coatings not saving energy in the summer, you might check out this link.

    Bookmark   January 26, 2005 at 6:11PM
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Cardinal is based in Eden Prairie, Minnesota, not Wisconsin, although there are a number of Cardinal plants in Wisconsin.
Marvin uses 100% Cardinal glass, as does Andersen...

    Bookmark   January 26, 2005 at 6:13PM
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0Oberon: Very interesting info, but I would have to ask a couple of questions, based on what I have been told. The IR transmission of the low-e coating does not change based on what direction it is going, I was told. If it lets IR in, it will let IR out. The same goes for short wavelength, reradiated, heat, if it will go one way, it will go both. Under these assumptions solar heat will enter and be trapped the same both summer and winter, so you need to have a balance. In actuality, there is really very little IR generated in a house, so there is very little loss from it going out through glass. I actually did all the calculations, and in Minnesota, the best window for year round perfomance is a non low-e triple pane on the south, and a heat mirror (basically triple pane) double coat low-e on the non sunny sides. Here you are best with the largest solar gain you can get, as long as you use a reflective blind in the summer. Other parts of the country would require a different combination of glasses to be optimized. The Andersen general purpose glass was not near the top in my calculations. Depending on how much sun he gets and how cold the nights are, FXSTer could come out better either way.

IMHO there is no free lunch, and there is no glass that is best for all applications. The laws of physics and heat transfer do not change from summer to winter, so the manufacturers who say they do are exaggerating to make there windows appear better than they are.

I think it was explained to me the best by John Carmody of the U of Minnesota who said that low-e passes solar (IR) to whatever % is built into the coating, up to a maximum attainable. It will always stop almost all short wave (reradiated) heat. IR comes from the sun or very hot objects, reradiated from everything else. The U value differences will come from the reradiated heat loss changes, the changes in conduction by using inert gas (not related to low-e) and how many and how big the air gaps are (again not low-e related. He was the one that described the calculation procedure to me.

Please let me know if you have other information on this, as I find it very interesting and universally misunderstood.

    Bookmark   January 27, 2005 at 1:13AM
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Excellent post Booster, I enjoyed reading it and you made some very good points.
BTW, as I tried to point out in my earlier post, much of this area is also controversial within the fenestration industry as well. There are some very knowledgeable folks who disagree with one another.
But, there really is no argument as to whether to use a LowE coating or not. There is virtually 100% agreement that a LowE coating is a good thing. The disagreement is what LowE coating to use depending on where you live.
A very good point that you made in your post.
You are 100% correct that there has to be a balance between incoming and outgoingthe question is defining what that means when related to window performance.
If window glass coating lets IR in, then it has to let IR out as well it doesnt matter whether it is coated or not, that is not the discussion. But, we have to be careful with definitions. IR is heat. It is heat transmitted in a certain way, but ultimately it is simply heat. LowE coating block IR whether from the outside, or from the inside.
It is absolutely important to remember that heat ALWAYS moves to cold.
Heat transfer across the cavity of an IGU is primarily affected by two factors:
Thermal radiation, from the warm lite to the cold lite and conduction through the air molecules (or inert gas as you said in your post) from the warm lite to the cold lite.
In a typical dual pane window without a LowE coating over 60% of the heat transfer thru the window is radiation. With a softcoat LowE coating, the percentage drops down to about 14%.
Imagine a typical dual pane window without LowE coating and the temperature outside at zero degrees F and the inside temperature at 70 degrees F (air temperature, not glass temp). Total heat loss for this configuration will be calculated at 34Btu/hr/ft^2. Of that, 21Btu/hr/ft^2 is related directly to radiation between the lites.
Now imagine the same window with a softcoat LowE coating. The radiative heat loss will drop to less than 3Btu/hr/ft^2.
The overall heat loss will drop from 34Btu/hr/ft^2 to 22Btu/hr/ft^2. If you add an inert gas infill, which will affect conduction losses, then the overall drop will be to 18Btu/hr/ft^2.
The temperature of the inner lite of glass without the LowE coating will be 45 degrees using that example. The temperature of the inner lite will be 53 degrees when using the LowE coating and with LowE coating and argon fill it will rise to 57 degrees.
Again, we are calculating for heat loss thru the window if the outside air temp is zero and the inside air temp is 70. If any of the conditions change, then the calculations have to be adjusted accordingly. What might be a changed condition?
If the sun is shining directly on the exterior window with the softcoat LowE coating, then that coating will be reflecting most of that IR energy. But, the lite will warm as it is doing so, so that the transfer of heat from inside the house to outside thru that window will be less, so that there will be a net energy gain, but not as much as the hardcoat LowE or potentially non-coated window.
If the window has a hardcoat LowE coating, and the sun is shining directly on the window, then there will be a net gain in energy performance because the IR radiation will pass thru the hardcoat and will help warm the inside of the house as well.
If there is no LowE coating at all, there will also be a net gain when the sun is shining directly on the window, because of the IR radiation as well.
The argument is, what happens when the sun is not shining directly on the window?
In that case, the hardcoat LowE and no LowE window will have a greater energy loss when compared to the softcoat coated window. The energy loss of the hardcoat will be much less than that of the non-coated unit, but will be more than the softcoat.
When doing energy calculations and including net solar gain for hardcoat or non-coated windows, the hardest part of the calculation is computing how much of the solar gain is actually maintained when the sun is not shining on the window.
There is no net energy gain in the winter using an uncoated dual pane window versus using a coated window hard or soft coat. The losses are substantially higher on the uncoated unit than are the potential gains because of the very substantial loss of heat from the house when the sun isnt shining on the window directly.
But, the calculated differences in loss or gain between hardcoat and softcoat coatings are more difficult to quantify, and that is why the disagreement within the industry.

As you said, there is no free lunch. The sun is certainly the major source of IR energy affecting your home, winter and summer, but IR is radiated from everything, not just very hot objects. In a sense, what you proposed in your post was a free lunch. In your proposal, you suggested a window that would let heat in to help warm the house, but then wouldnt let it back out later.which would be great, but unfortunately doesnt work in real life.
If the window passes heat one way, then it passes heat the other way as well.
Heat always radiates to cold. If one object is +10F and another object is -10F, then the heat contained in the warmer object will radiate to the cooler. Basically, there is no such thing as cold, there is only an absence of heat. Per the laws of thermodynamics, anytime you have two objects with a different level of heat, they are going to try to reach equilibrium with one another. The job of insulation is to try to keep that from happening, and it doesnt really matter how much heat they have to start with, just so that IR radiation is constant between objects of differing values.
If the furniture in your house is 68 degrees and your window is at 47 degrees, that window surface is drawing that heat from your furnishings in the form of IR radiation.
Virtually everyone from the North can remember at one time or another sitting next to a cold window and remember that it was not comfortable. That window was drawing the heat from you and making you chilled. You are warmer than the window and you are radiating IR to the cooler surface. Obviously, the closer the window is to room temperature, the less loss due to IR radiation and the more comfortable you will be.

Triple pane with dual LowE coating is very energy efficient, but without LowE then it is actually less energy efficient than a dual pane with a LowE coating.

Heat Mirror is a heck of a concept. It was developed originally at MIT in the 1970s, and the energy numbers are amazing! Unfortunately, due to a couple of issues, not everyone is happy with Heat Mirror in the field. I have heard of two primary reported problems associated with heat mirror. First, the polycarbonate film that they use in the center of the IGU has a different CoE (Coefficient of Expansion) than does the glass. This may lead to distortion of the film in severe cooling climates. Second, they have had problems with the IGU seal between the spacer and the film. Otherwise, it is a heck of a product. And while I suspect many people dont see either of those issues, they do appear to be somewhat well documented.

Hope all this makes sense. I also enjoy discussions like this, and it is obvious to me that you have done your homeworkthanks for the reply!


    Bookmark   January 27, 2005 at 12:25PM
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I just noticed that I said Heat Mirror has a polycarbonate film suspended between the glass. I should have said PET or Polyethylene Terephthalate* film. Sorry for any confusion.

*Try saying that three times fast!!

    Bookmark   January 28, 2005 at 8:16AM
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We are replacing windows in a kitchen that faces northwest. The Marvin sales rep has advised using low-e. I understand the energy efficiency- heat loss/retention benefits but am concerned that Low-E will look somewhat tinted and let less light in. Since this is a northern exposure I want as much light as possible. Is this true? Can one discern a difference in the clarity (ampt of light) in low-e windows versus regular windows in the same room ?

    Bookmark   March 21, 2005 at 7:03PM
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Some folks claim to be able to see a LowE coating. Some folks claim that the light in a room is different with a LowE coating.

Hardcoat LowE's can have a definite haze to them and have a potential to slightly discolor after a few years.

Softcoat LowE's are much more neutral and don't have either the haze nor the discoloration issue of hardcoats.

Marvin uses Cardinal's LowE products which are said to be the most color-neutral LowE's currently in use.

I will not say that you, as an individual, cannot see the LowE, but the vast (overwhelming) majority of people cannot tell if a window has a LowE coating or not.

    Bookmark   March 23, 2005 at 8:10AM
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Thanks for the response. I've got a print-out from Cardinal and it looks like the Low-E II w/argon reduces visible light transmittance from 82% to 72%. I don't know if that will bother me or not.

Do you know if the Low-E coating will reduce glare on my laptop screen (working in front of the window) or from the sun setting and shining in thru the glass?

    Bookmark   March 23, 2005 at 11:53AM
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A LowE coating will cut down a little bit on the glare, but not really enough that you will notice. LowE is designed to affect "light" transmission in the infrared and ultraviolet spectrums and to pass as much visible light as is possible at the same time.
You can buy a tinted LowE that is designed to cut glare as well as improve energy performance, but then you are into a definite change in the light in the room...visible light transmission in the 40% range.

    Bookmark   March 24, 2005 at 7:08AM
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Do you think that a Low-E window without a screen would let about the same ampt. of light in as a regular glass window with a screen? Also, do you think it will be very noticable to have Low-E windows next to a non-Low-E glass patio door? Thanks.

    Bookmark   March 24, 2005 at 7:38AM
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If you place a LowE door next to a clear door, you will see a noticeable color difference. In fact, if you place one manufacturers LowE next to another manufacturers product you may also notice a slight (or not so slight) color difference.

You have me on the other question though. I have never considered that particular question before and I have no idea...however, I think that that is a really good question and I am curious if anyone has ever tested for that...

    Bookmark   March 25, 2005 at 7:24PM
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you are my window guru, thanks

    Bookmark   April 26, 2005 at 8:24PM
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Thanks mtobias1.

    Bookmark   April 28, 2005 at 5:52PM
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Been gone a while,but still find this interesting. Only have a minute, but one statement has me really confused, as it goes against everything that I have been taught about low-e and in physics.

"If window glass coating lets IR in, then it has to let IR out as well it doesnt matter whether it is coated or not, that is not the discussion."

While this is technically correct, it does not apply to solar heat in vs room radiated heat out. The wave length of radiated heat is determined by the temperature of the object that generates it. Short wavelength from the high temp sun, much longer for the room temperatures in our houses. Low-e glass is made to pass some wavelengths and stop others. In high solar gain glass, it will pass the short wave length solar heat, but stop the longer wave length radiated heat from the room. This is the whole premise behind low-e glass and why it keeps your room warmer at night. Unless you are using something very hot that generates short wavelength heat, the low-e will stop it from leaving the room. That is why a low-e (high solar gain) window can actually increase the air conditioning cost when compared to clear glass, and why you need to use a solar shade to REFLECT the solar heat back out past the low-e.

Remember that the original question was why he seemed to not be getting as much solar heat as he did with clear glass after he put in his Andersen general purpose low-e windows. The fact remains that the 50% shading that the Andersen glass uses is causing the problem. My point then, and now, is that the glass is wrong for the application, Andersen doesn't offer glass that is right for the application, and nobody in the glass industry seems to want to give the customers any information other than "our glass is the best for everything".

In my opinion, for his application, he should have either high solar gain low-e glass with a solar shade for the air conditioning season. Or he could use clear, non low-e glass with an insulating curtain at night and get very similar overall energy useage.

    Bookmark   May 26, 2005 at 12:01AM
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Don't know if anyone is still following this thread, but I ran across this comparison/fact table in one of my archives left from when I was looking for glass for our house. This is for Heat Mirror, but it does give the numbers for plain two pane insultating glass also for comparison. If you look at the numbers for solar gain, u value, etc, you can quickly get a feel for what the tradeoffs are for the different styles of window. This info should be available from any window manufacturer, but you will have to ask for it. If they can't or won't provide it, I would look elsewhere.

    Bookmark   June 15, 2005 at 8:40PM
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This is a really great thread. I hate to see it end.
I recently read about something called Electrochromic windows. These seem to contradict the assumption that you cannot have it both ways. The website I found describing these says that they can be changed with the flip of a switch from dark to light and they can change the amount of heat they transfer in the same way.

    Bookmark   July 8, 2005 at 3:52PM
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Are you referring to the ones that change color/visibility when a voltage is put on them? They do look really neat, but I have never seen any thermal or other specs on them. Definitly would be wonderful if they did give the best of everything.

    Bookmark   July 13, 2005 at 11:41PM
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I didn't realize this thread was still active until a few days ago. I was also gone awhile and it took me a couple of days to get here, but I wanted to make sure that I was clear on this one. I have thoroughly enjoyed this debate from the beginning. This has been a fun thread.

Okay..."IR in, IR out" technically correct of course, but also my point being that heat passes thru a window in two ways. Conduction thru the edges of the frame and also thru the molecules of air (or whatever) between the glass lites. Heat also passes thru the glass as radiant energy. Radiant energy in the form of IR (well there is some heat in both UV and visible light, but primarily IR).

As you mentioned, there are differing frequencies and intensities, but ultimately as part of the IR spectrum.
LowE coatings are designed to limit the amount of radiant energy that passes thru the window. The high solar gain versions have much less affect on direct solar gain than do the low solar gain versions...which is self-explanitory, really.

As you said, we did tend to drift away from the basic question was will a LowE coating affect solar gain? Absolutely. All LowE coatings will affect solar gain to one degree or another. The question that should follow is will the drop in solar gain be balanced by the overall efficiency of the window with the LowE coating?

Andersen's does not offer the high solar gain product...why not? Do the folks at Andersen believe that the loss of solar gain is offset by the increase in window energy efficiency? I don't know, personally. But there are studies that suggest that possibility and I am sure the Andersen folks know of those studies.

There was one comment in your last post that is not correct, however: "That is why a low-e (high solar gain) window can actually increase the air conditioning cost when compared to clear glass, and why you need to use a solar shade to REFLECT the solar heat back out past the low-e."

You are correct that a high solar gain will allow more solar radiant energy into a room, but not when compared with clear glass. It will allow more heat into the room when compared with a low solar gain LowE, but under no circumstances will a window with a LowE coating pass more heat than clear glass. The entire point of LowE coatings, be they high or low solar gain, is to stop heat from passing thru the window.

LowE coatings were developed for use in northern or heating dominated environments to block heat from leaving a home in the winter. There was also a summer advantage as well, except in terms of direct solar radiant energy. In other words, when the sun is shining directly on an older version of a LowE coating, then the heat generated by the sun will pass into the room. Not all of it, but a large amount. Some people consider this an advantage in winter, of course.

The refinement of the coatings to include the ability to block large amounts of direct solar radiant energy is a relatively recent advancement, but, does that mean that the older coatingw were bad? Not at all, it simply means that the newer coatings performed better at stopping heat flow...but, not everyone considered that to be an advantage.

The hardcoat folks initially felt that they were at a real disadvantage when the newer, low solar gain, products came on the market. These products outperformed the hardcoat coatings in every measurable area, except one, and that one had been considered a weakness in LowE coatings previously by most folks in the industry. The hardcoat allowed more direct solar gain in the winter.

The hardcoat folks jumped on this "advantage" and began to use it to sell the "fact" that a high solar gain product is an advantage over a low solar gain product in the north in the winter. They really didn't have any studies at the time for comparison, they made the claims based entirely on "intuition" and "logic". Since you can feel the solar gain in winter thru their product, and you cannot feel it with the low solar gain product, then it is obvious that the solar gain is an advantage in winter.

The softcoat manufacturers saw that and thought "hey, our older softcoat LowE products also have higher solar gain numbers as well" (when compared with the newer low solar gain products). So the softcoat guys in a sense "reintroduced" their older products as high solar gain and marketed them for the same market share. So what was originally thought to be a flaw in the product (high solar gain) was now a major marketing device in some areas.
And the debate between high solar gain and low solar gain has been on going ever since...in the north.

In the central and southern regions there is no question but that the low solar gain products are a huge advantage over the high solar gain products and especially over clear glass.

Is having a high solar gain product an advantage in the north? There are a number of studies that show the benefit of low solar gain LowE in the south, there really is a very limited amount of data in the north to justify the use of high gain versus low gain...or to justify the use of low gain versus high gain, but there are some.

Two things to keep in mind. First, when using a high solar gain product, that when the sun is shining directly on the window, you will have a net gain in energy. Sometimes a huge net gain. This is true for clear glass as well, not just high radiant gain LowE.

When the sun is not shining directly on the window, you will have a net loss in energy. This is less with the LowE product than with clear glass, but it still exists. The question is whether the net solar gain exceeds the net of the loss when you don't have the sun shining.

Several factors weigh into the equation...first, how long did the sun illuminate the window? How many solar days can you expect per winter? Even, what happens in the summer when you don't want the net solar gain?

A house can be designed to take these factors into account. For example desiging with long overhangs that allow the lower winter sun to directly impact the window and yet block the direct summer sun are not at all uncommon. As you mentioned, solar shades are another option...in summer. And insulated drapes for the times that you don't have net solar gain in the winter will help...at night for example.

Booster, anyone who lives in the Energy Star northern region who takes a very active interest in their house can achieve an energy advantage using high solar gain LowE over both low solar gain, and especially over clear glass. Again IF they are a person who actively participates in the energy performance of their house and they live in the Energy Star "North" region.

Take two identical houses with the exact specifications and people who use set back thermostats, insulated drapes, all the "saving energy" bells and whistles...except that one house has high solar gain windows and one house has low solar gain windows...the high solar gain can achieve an approximate 12% energy savings over clear glass (everything else being equal)as a yearly average, and the low solar gain product can achieve an approximate 9% savings over clear glass (dual pane, but no LowE) in the same circumstances. Advantage - high solar radiant gain.

However, the same set of circumstances in the other three Energy Star regions, no matter what you do to improve your home's energy performance (drapes, blinds, thermostat settings, etc), a home with low solar gain LowE will outperform a house with a high gain LowE and REALLY outperform a home with clear glass. The farther you go south, the greater the advantage of the low solar gain product.

Now this comparison was based on someone who takes an active role in keeping their home energy efficient. What about the person who installs it and forgets about it?

In that case, in every geographical Energy Star region in the US, the low solar gain product will outperform the high solar gain product in overall efficiency and performance.

IF the homeowner wants to have maximum efficiency in their home's energy performance and does not want to use any special precautions or methods to achieve it, then the low solar product will out perform the high solar gain product in every area of the US. This is based on a year-round average of energy used.

Both of these products will significantly out perform clear glass. The WORST advantage of a LowE product over clear glass is in the deep south where the high solar gain LowE will outperform clear by about 7%. The best performance difference is also in the deep south where the low solar gain product will outperform clear glass by about 35% (or better in some studies - up to 50% in some southwest regions).

Wow, way too long again...well, if anyone reads this far...I hope it made sense!


    Bookmark   July 22, 2005 at 9:01AM
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Oberon: I also find this very interesting, and a lot of what you have just said is what I was trying to convey. That is, that if you are looking for solar gain as the original poster was (I assume he lives in the north as I do), the mid to low solar gain windows are not necessarily the best choice for the application. The problem is that folks like Andersen will not tell you that, and just say that the "general purpose" glass is always best. As you said, if the house is designed for passive solar, it all gets more important, and it is of value for the customer to ask/demand all the necessary information from the manufacturer.

The whole air conditioning cost thing with low-e may need a bit of clarification. I will list a few things that I based the statements on:

1.High solar gain low-e (hardcoat)will let in most of the solar gain, summer or winter.

2. As you said, clear glass will let in even more of the solar gain, summer or winter.

3. The objects in the room are going to generate heat at a frequency (wavelength) based on their temperature (100 degrees F,-long wave radiation)

4. The hardcoat low-e will not allow the reradiated heat (3 above) to pass, and will reflect it back into the room, so very little heat leaves the room.

5. The clear glass will allow the reradiated heat to pass, thus allowing some heat to leave the room.

6. Using an insulating curtain will not help the low-e window because the temperature of the curtain will still be too low for its radiated heat to pass the through the low-e glass. In fact, most low-e window manufacturers say NOT to use an insulating curtain over their products as it can overheat the glass.

7. Using the insulating curtain on clear glass does help a lot as the heat trapped by the curtain can be reradiated through the clear glass and be sent back outside.

8. You CAN reflect the solar energy back through a low-e window with reflective solar shades or even highly reflective miniblinds. The same is true for clear glass.

9. The conductive heat transfer rate is much lower in the summer than winter because the temperature difference is in the 20 to 30 degree range instead of the 100 degrees difference it can be in the winter.

So, when does this make high solar gain low-e cost more on your AC bill? If you have quite a bit of glass that gets considerable sun, and you don't have reflective blinds, you will probably cost a bit more to AC than with clear glass. If you put the insulating curtain on the clear glass it will win hands down. You can't put the insulating curtain on the low-e, so you can't get the advantage. Put a reflective blind on the low-e, and then it will probably be as good or better for the AC bill as the clear with a curtain.

I have only one first hand comparison that I have seen, but it was quite vivid. A friend had a townhouse (big buck one at that) here in Minnesota that had a south facing, two story, vaulted wall, that had a big fireplace up the center with almost all the rest glass. It was originally built with clear thermal panes. and was very hot in the summer in that room, easily topping 100 degrees, if the AC was off. They added white insulating curtains, and were very happy with the results in both the summer and winter, as all the glass also made it cold in the winter at night, and the curtains helped a bunch. In an update/remodel that was done, the windows were replaced with high solar gain low-e. The insulating curtains had to come down per the window manufacturers recommendation. The summer AC cooling cost actually went up a little from when they had the clear glass windows (using KW not $)without the curtains, and was much higher than the the clear glass with the insulating curtains. The room got unbeliveably hot very quickly. It was more comfortable in the winter, however,. They added solar shades to the inside of the windows to reflect the solar energy and the AC bills came down to very close to what it was with the clear glass and curtains. The heating costs of the low-e, with no curtain or blind in the winter, was a touch higher, but very similar to the cost of the clear glass with the curtain on it. Of course the view was much better with no curtains covering it all at night.

I guess I would have to say that the major reason low-e could increase your AC bill compared to clear glass is that you can't put an insulating curtain on them per the manufacturers, and if you could, they wouldn't help in the summer anyway. Secondarily, depending on location and layout, just the trapping of heat by the low-e could also raise you AC load.

I totally agree that other places than the frozen north have totally different problems to deal with. Ours are tough when you think that we may see 100+ degrees in the summer and -40 in the winter.

    Bookmark   July 22, 2005 at 6:30PM
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I've been doing quite a bit of research on windows and this thread has helped a lot. Ultimately, it seems that final decisions about the types of windows to select are primarily dependent upon the climate and the orientation of the windows.

My specific situation is:

1. New construction in central Texas (about 3,000 SF)
2. About 80% of my windows face north (based on square footage)
3. 50% of the east facing windows are protected by a trellis
4. There are no south facing windows
5. There are only two 2'x4' clerestory awning windows facing west

Given these conditions, I would think I can select windows (at least for the north side) with fairly high VT and SGHC numbers. I haven't yet figured out if that means they will be less expensive.

Am I off base? Any suggestions on what type of frame would be the best?

    Bookmark   July 24, 2005 at 8:35PM
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Facing north you could probably use any windows you want, and as was said U factor being important. The big thing you gain with high visibility and solar gain is that they are much better to look through if you have a nice view!

    Bookmark   July 25, 2005 at 5:21PM
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I think I have gotten a good, basic education from reading this thread (or trying to read most of the posts' information). Have not gone to the other links to sample that info and hope it is as clearly developed as your postings.
Questions after background:
We are (hopefully) planning to build new home in Dallas-Ft. Worth area on an acre corner lot with many small-diameter oak trees--maybe 20 ft tall max--these oaks will lose leaves in fall/winter but provide some decent shade in spring/summer. WE will probably have no option but to front house facing the west because of how other house on that side is oriented.
We will probably have some rooms on second floor but probably not have trees that are higher than the roof line for overall shade.
Have only seen the lot couple of times and this description is guess-timate.Will meet builder who owns the lot and his designer tomorrow for initial discussion of possible designs and costs. There are other lots in this small subdivision but others seem to have drawbacks like lack of trees or steep sloping lot.
I wondered
1)how much/if any/we should try to get N/S orientation for house footprint--which I think might be possible on an acre corner lot that is basically square and there might be few restrictions besides square footage in development
2) will trees that cannot shade the entire roof like umbrella in summer provide noticible energy savings?
3) will shade that the trees do provide mean that top and bottom floor windows on same sides of house need different energy values?
4) should I trust the designer and builder to know what they are doing and not be asking these questions at this meeting?

It is always interesting to read/listen to knowledgeable people discuss their fields of expertise--especially when they can have a rational exchange with different points of view--you are all very lucid and organized--
thanks from someone who does not know that much about building except that she does not know
Certainly hope that your all visit the boards and offer your insight again

    Bookmark   August 4, 2005 at 2:17AM
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Wow - I couldn't read half this stuff without my brain going dead on me. I just had a house built with "low-e" because of the brutal Nebraska sun in west and southwest windows. Anderson 400 series. Mainly I was concerned about summer heat.
Of course the builder put the heat sources in front of the windows in the floor.
I have 2 questions.
Will that heat blowing affect the life of the low-e? (Not there's much I can do about it now)
What kind of window treatments should I use or not use to maintain the low-e factor?

    Bookmark   October 7, 2005 at 8:15AM
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Hi Marys,

The LowE coating will not be affected at all by placing the heat source directly below the window. There are actually several good reasons for placing the heat source in that location and it is not an issue at all.

Your windows have a low solar gain LowE which means that they should do a very nice job of preventing summer heat from entering your home. Any window treatment should be fine with this product since there won't be enough heat passing thru the window to make a big difference.

LowE is a "passive" improvement in your home's energy performance. You really don't have to worry about it at all.


    Bookmark   October 9, 2005 at 8:40PM
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I confess that I have *not* read all the postings here but I've read enough to know that there is some mis-information present. I won't start a flame war by being more specific and I don't claim to be an expert, *but* getting back to the original question: low-E glass is not recommended by many people for the south facing windows of a passive house. Maximizing the SHGC (which includes affects from all spectral regions of sunlight, visible and IR) is what you want to do on the south side.

    Bookmark   April 19, 2006 at 10:38AM
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Leeelson, maybe you should read all the postings. Your statement WAS the expert line of reasoning 20 years ago, but Oberon has explained perfectly what's going on today.

Oberon, I'm sure some of us would like know which company you work for, because it's obvious you're well-informed. I don't know how you can tell us per the terms of service, but is it the "C" or "P" glass company? "H", "M", "A", or "P" window company?

    Bookmark   May 21, 2006 at 5:12AM
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I admit to being surprised to seeing this thread again resurrected...

Kevin, while I understand that "C" is probably Cardinal and "P" can be either Pilkington or PPG. I can even guess that "A" is Andersen, "M" is Marvin, "P" is Pella; I am at something of a loss concerning "H"...I know of several window companies that begin with H, but none of them really fits into the Andersen/Pella/Marvin triumvirate.

Still, of the three "glass-but-not-window-companies" mentioned, I might also add Guardian, AFG, Visteon; all companies that actually make their own glass...and also companies like Old Castle, Arch Aluminum, Viracon, Binswanger; all companies deeply involved in the window industry even though they don't specifically produce glass.

Honestly, while I do work in the industry, I don't work for a window company nor an IG company. Beyond that, I really must stay reasonably anonymous since the folks that I do work for actually pay me to say the same sort of stuff there that I say here. You might also notice that I don't comment on specific window companies for much the same reason - potential conflict of interest.

Anyway, thanks for the kind words and have a great day.

    Bookmark   May 22, 2006 at 8:10PM
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"H" is for Hurd because they were the only hard coat supplier I could buy from easily.

I can definitely feel the difference between hard and soft coat Low E on my face during a sunny day.

OK, so do you know a manufacturer that can offer a fiberglass frame, triple glazing, and both hard and soft coat Low E? (This is my dream window for my mountain cabin at 10,500 ft., 10,000 heating degree days). Inert gas is pretty much not possible for this altitude. Heat mirror is also out because of the difficulty of glass replacement. And/or let me know if it shouldn't be my dream window.

It wouldn't be a recommendation, just sharing knowledge. (sheesh, I don't like the weird definition of "advertising" on this site)

    Bookmark   May 24, 2006 at 5:00AM
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Hi Kevin,

A couple of thoughts...

Are you considering hardcoat and softcoat in the same window??? I read your last post that way, but I certainly could have misunderstood.

There are actually two (well, three now) types of softcoats available. Basically, they fall into the high solar gain and low solar gain categories.

Hardcoats are really "mid-E" rather than LowE in that their emissivity numbers aren't as good as the softcoats.

The high solar gain softcoats have almost the same solar energy gain, but also they are better at keeping non-solar-heat inside your home when the warmth is trying to escape.

I suspect that when you felt the difference between the hardcoat and softcoat products you may have been comparing a low solar gain rather than high solar gain. But, you are correct that the hardcoat will allow slightly more solar heat to pass thru the window when will even the high solar gain softcoat - and again, the trade-off is that the hardcoat will let more heat travel in the other direction as well.

Your idea of triple pane will work very nicely at blocking inside heat from escaping, but if you are planning to coat two surfaces then you might have issues with additional blocking of solar gain as well. Something to keep in mind.

Triple panes can come in two different varieties.

First, you can have a triple pane with wider airspace between each lite - almost like two double panes joined together.

Second, you can have a triple pane that is like a double pane with an extra lite dropped between the inner and outer lites.

The first version gives you a fat IGU which doesn't work in some folk's sash arrangement.

The second version gives you a relatively thinner IGU that will fit into a narrower sash and is probably more adaptable to various types of windows.

What about energy performance?

Both versions work very well when they have a LowE coating on two interior surfaces. The narrower spaced IGU performs exceedingly well when krypton gas is used in the airspace - krypton actually has its best performance numbers in an airspace that is in the 1/4" range.

But, drop the LowE and krypton gas and the narrow space IGU has a number of problems because of the narrow space and will not have as good performance as a dual pane with a LowE coating.

The wider version of a triple pane - clear glass only - will have performance numbers almost as good as a dual pane with LowE and argon. Add LowE coatings to two surfaces and add argon - which has an optimum performance in a 7/16" airspace and again you have some darn good performance numbers.

In your case, as you have already noted, you can't have either argon or krypton gas infill. You are simply too high. Your windows will probably come with either a capillary tube or a breather tube to allow the IG to equalize to the air pressure at your elevation.

Typical "breather tubes" are aluminum, short in length, provide a relatively large opening for equalizing pressure, and they are "sealed" once the window has been installed.

Typical capillary tubes, on the other hand, are stainless steel, longer (18" possible), very narrow opening, and are left open to the air after the unit is installed.

Although specific recommendations and usage are very much manufacturer dictated, as a general rule capillary tubes are used in dry, high altitude, environments to equalize pressure on windows installed in that environment. Again, they are not sealed when installed; they remain open to the air...this allows the window to react to changes in barometric pressure which can be considerable at altitude.

Don't get the idea that you can see the window "breathe" if it has a capillary tube however. It actually takes a very long time for air pressure inside the window and outside the window to equalize. This slow air exchange helps keep the window from excessive flexing.

    Bookmark   May 24, 2006 at 7:56PM
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Thanks again, I didn't know about soft coat, high SGHC. I didn't mean to imply the two different coatings on the same window. I just need high SGHC on the south facing windows, with regular Low E on all other elevations, and I want the same window frame on all of them.

So I think I'll forget about triple pane, and pursue nighttime/vacant time window insulation for the non-south windows.

    Bookmark   May 25, 2006 at 9:53AM
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Our builder provided Atrium Low E windows for our new house in Colorado (U factor 0.34, SHGC 0.33, VT 0.48). The house has lots of windows on the south/west side, including windows in the vaulted wall of 2 story ceiling. Northeast side has lots of big windows too.

Is there any guidelines in choosing window coverings? I'm trying to find something blocking the sun on the south/west side, such as a polywood shutter or roller shade on the lower windows, and possibly using some sort of film over the windows high up on the vaulted wall. Window gurus here, please comment on this, is it appropriate to apply some tinted film over the low e windows on the high ceiling? If so which kind works best to block the summer sun? On the north side I'm thinking about using drapes since we don't want the shutters to block the view. How to achieve the best energy savings, both in summer and winter, by selecting right window treatment on top of having the low e windows?

Thank you very much!

    Bookmark   July 25, 2006 at 12:09AM
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