GBlist: Re: Argon in windows

[cneurath <cneurath@tds.net>]

Mitch Toews presumably of Loewen Windows came close to explaining how 
Argon filling reduces heat flow through a sealed double pane window.

Thermotech Windows of Ottawa http://www.thermotechwindows.com/ had a 
rep come to our Sustainable Energy Fair last time and he had a great 
demonstration which partially showed how argon works.  He had three 
mylar balloons (which are much more impermeable than rubber balloons) 
filled with 1) air, 2) argon, and 3) krypton.  When he held them 
aloft and dropped them the air filled balloon dropped slowly, the 
argon filled balloon dropped quickly and the krypton filled balloon 
dropped like a "lead balloon".  Both the argon and especially the 
krypton filled balloons felt much heavier than air when holding them. 
So, the density of argon compared to air is key.

Next, you have to know that heat transfer through the gas of a double 
pane window is mainly by convection.  Just like in a rolling boiling 
pot of water, the hot water rises from the bottom and forms eddies as 
it cools and then rolls back down.  This transfers heat from the hot 
bottom of the pot to the cooler air/water interface at the top. 
Similar convective eddies are formed between the warm inside pane of 
glass and the cold outside pane of glass.  Denser gasses don't 
circulate as easily (and perhaps argon is more viscous than air too?) 
so the eddies don't move as quickly and they don't transfer as much 
heat.  If you could keep the gas stationary while applying a heat 
gradient I doubt argon would have a significantly different R value 
from air.  So, it is the damping of fluid motion that is important.

This gets further involved when you look into the optimum spacing 
between panes of glass.  I think I remember that for argon the 
optimum spacing is about 1/2 inch for minimum heat transfer in a 
vertical unit.  For krypton the optimum spacing is something like 1/4 
inch.  When a manufacturer wants to minimize the thickness of a 
window unit, especially when they get into three or more panes, they 
may choose krypton over argon to avoid making the unit an unwieldy 
thickness.  Krypton gas costs much more than argon though, so this 
becomes one more tradeoff.

Mitch Toews claimed that the "dryness" of argon gas had something to 
do with it's effectiveness.  I doubt it.  Whatever gas is put between 
sealed panes has to be very dry, whether it is air, argon, or 
krypton.  Normally, a desicant is added to the spacers to absorb any 
residual water vapor once the unit is sealed.  For metal spacers, 
this is placed inside the hollow sections.  For high efficiency 
silicone rubber spacers, the desicant is mixed in with the rubber 
compound.

As to the issue of whether the argon or krypton leaks out of the 
window after a number of years ... my guess is that if the argon or 
krypton leaked out it would be replaced with ambient air. Then the 
window would fog up the same as when a sealed, air-filled window 
leaks.  It is the water vapor getting inside that is the tip-off to a 
broken seal.  If there is no fogging, then presumably you've still 
got all the argon you started with.  None of my argon filled triple 
pane windows have fogged up yet, but the oldest are only about 7 
years old so that's not saying much.

Oh, and now that I've looked up Kate Offringa's web link I see that 
site answers most of these questions too.  But I didn't see any 
window maker sponsors from Canada which has the most energy efficient 
windows in North America as far as I can tell.  It's too bad there is 
such an information barrier between the Canadian and U.S. window 
markets.

Kate's posting:

------------------------------

Date: Tue, 23 Jan 2001 09:23:46 -0500
From: Koffringa@ase.org
Subject: RE: GBlist: Argon in windows

Visit the Efficient Windows Collaborative web site for the answer to this
and lots of other windows questions.

Here's a link to the gas fills page.

http://www.efficientwindows.org/gasfills.html

Kate Offringa
Program Manager
Efficient Windows Collaborative
Alliance to Save Energy
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