Argon-filled or Low E storm windows?

I'm reluctant to rip out 65-year-old wooden windows in my Chicago rehab, but I need the energy efficiency. Anyone have any experience with this relatively new product? I'm thinking I can save money and my antique windows, and get the efficiency I'm looking for. Please tell me whether this makes sense?
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Why is argon gas a better inslulator than air? I'd like to know. Because it sounds fancy and is more expensive?
The idea is to keep the gas from moving around in your window, which is impossible, without putting some fibers in it.
I'v not seen an explantion of why some windows are filled with noble, or just esoteric sounding gases.
Rich
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On Thu, 9 Mar 2006 19:23:40 -0500, RichK wrote:

Simple chemistry and thermal transfer coefficients. Argon has better insulating properties than air does.
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Do you happen to have the values handy, or are you just repeating sales pitch?
Since gas "carries heat" from one surface to another, rather than conducts like metal, the more important factor is keeping gas from moving. Air or argon between two glass suraces has the same ability to move.
Seem little is gained by using fancy gas in place of air, but it sounds much better in the advert.
Rich
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The thermal conductivity (k) of air is 0.024, and argon is 0.016. Window manufacturers must provide performance results of certified tests on all windows. If you go though a catalog from a manufacturer and compare the heat loss for windows that are identical except for the argon fill and LowE, you will see quite a difference between the two products. These are tests of the entire window assembly, not just a calculation.
Dennis
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RichK wrote:

I agree. Its what makes gas 'gas'. That it moves around so freely. I can assume Argon has lower specific heat than average air. Which means it holds less heat. Which means it transfers less heat as it moves. Of course there is no breeze between the panes of the glass except that created by convection!? Which would be tied to the heat the gas can hold. So it probably performs better. And is probably easier to maintin than a vacuum.
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Heat is transferred by conduction and convection (in this case). Perhaps argon is better on both counts, but it would be nice if the whole mechanism was translated into R values, so you'd know what it costs for an additional R unit.
In addition, I wonder how long the gas stays trapped between the two sheets of glass. And how do you know when it escapes.
It all adds up to a bit of hocus-pocus, for a price.
Rich
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Rich, the "whole mechanism" *is* translated into R values. The U factor (and other data) is marked on a sticker placed on every window sold today. U factor is the inverse of R value. If you buy a plain window where U = .55, that is an R value of 1/.55 = 1.8. A good LowE, argon filled window is about U = .32, so the R value is 3.125. Those stated values are for the entire window assembly tested as a unit, so it already includes convection effects of the fill gas, as well as conduction of the gases, frames, air leakage, etc.
Occasionally you used to see a manufacturer trying to sneak by with an advertised "CoG" U factor. That means Center of Glass, and the U factor will be deceptively good, since it does not include the heat losses through the frames. I haven't seen a CoG claim in awhile, however, and at any rate, that is not the value that must be printed on the sticker.
Dennis
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Thanks Dennis for a very thorough explanation. You can tell I have not shopped for windows in a while.
Well, I did get some quotes, but the folks involved had no idea what they were talking about :-)
Rich

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No problem, Rich. You also seem concerned about cost (who isn't?). LowE argon is no longer a pricey option. I am nearly done with an addition to my home, and bought Andersen windows for the new construction as well as replacing some older ones in the original part of the house. The LowE argon option adds less than 10% to the cost of a window. My Andersens were in the $150 range with that option.
Even lower cost windwows like Home Depot's American Craftsman line come standard with LowE argon in my area, at extremely attractice prices.
Dennis
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The gas should stay for the life of the window. If it leaks, there will be condensation between the panes as humid air replaces the argon. You will likely only notice the condensation as the weather gets cold.
Mike
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Because Argon molecules are very light, and therefore do not move heat well, but at the same time are big enough that it's possible to trap them.
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