Many here have stated that with double pane glass units with a space greater than say 3/4″ a convection currents develop reducing the overall efficency of the unit.
Is there any scientific data to back this up? And yes I went to buildingscience.com and found nothing.
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This might seem like a lame answer, but yes, there are data. The window manufacturers use such data to design their windows so they can achieve the highest insulating value. The balance is between a smaller airspace which has greater conduction versus a wider airspace which has greater convection. The optimal spacing varies with the overall size of the panes. Besides lab testing there are mathematical models that predict the behavior with surprising accuracy.
Here's the lame part: I don't know where you would look to find the data published, though.
http://www.google.com/search?hl=en&q=double+pane+window+convection+currents
Joe H
The balance is between a smaller airspace which has greater conductive [losses] versus a wider airspace which has greater convective [losses].
Wayne nailed it. Perhaps one reason you're having a hard time finding "data" to support this is that it is a basic, well-understood component of thermodynamics (among those who study it, at least!). So, since it's so well understood, few would find any reason to "prove" it (sort of reinventing the wheel there).
I recall studying the double-pane window in textbook problems while in college. It should come as no big surprise that the optimal spacing was generally around 1/2" to 3/4".
Edited 11/27/2007 5:28 pm ET by Ragnar17