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I have two panes of glass, about a 3/8" thick (.95 cm). Space efficiency and aesthetics ignored, how far apart should they be spaced to maximize heat insulation, assuming the gap contains regular air (and not Argon). I read that it's a balancing act between convection and conduction, and that store bought double panes windows are usually in the 1/4" - 1/2" range (0.635 cm - 1.27 cm). Other sources suggest a gap up to 2" (5 cm) is ideal, with the reasoning that a larger gap is always better up until it's wide enough to allow convection. Assuming it's exactly freezing outside and room temperature inside, how to I figure out what gap is best?

Thanks.

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Back in my graduate M.E. heat transfer days (the 1980's) we worked that out for typical window geometries. It turned out that the 1 1/2 to 2" separation for classic interior windows/storm windows was about right to minimize the convection currents for most circumstance, particularly during the heating season. The ARGON in sealed assemblies has a whole different dynamic, and the conductivity of the Argon itself becomes an important part of the calculation and observed result.

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There are many factors, but intuitively the gap should be adequate to allow a decent thermal gradient for the air.

Air can establish steady convection, rising on the room side pane and descending along the colder outside pane in a much narrower plume than that of a two-inch gap, even with the damping effect of slow movement on the two sides of the pane.

That by itself is not a major issue. but if the current circulating around gets turbulent causing the mixing of the hot and cold streams, that will compromise the steep thermal gradient, mixing hot and cold. That has to do with the height and width of the pane and the size of the gap.

Actual full-scale tests or a good simulator can be a great help.

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