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We've bought a huge Georgian B&B in the UK. It has vast single-glazed windows. The house is not listed however we want to retain as much fabric as possible. Therefore we are looking at secondary glazing rather than double glazing for the windows.

Given the style of some windows I've some flexibility in how far I separate the original and secondary panes. Is there an optimum distance, or some algorithm for this?

Notes:

Historically I recall that a wide separation can set up circulation within the gap, thus transferring the heat; however, current hearsay is that wide is better. They can't both be right.

For some windows we are employing a carpenter to remake the windows to take double-glazed units - so I do know that can be an option. However, with the expense it's one I'll use selectively.

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1/4 to 3/8 inch. Smaller spaces than this means fewer collision between gas molecules so heat transfers faster. Larger spaces than this allow convection cells to form. For residential use, lean toward 1/4" Large buildings often use large windows. They often have thicker glass with larger air spaces for structural reasons, but in big buildings the problem even in winter is getting rid of heat.

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    Thanks, though are you sure about this for secondary glazing as opposed to double? In my case I can't see a separation less than 1" is physically possible, and most installations I've seen are more like double that. Providing a link to a reference would be fantastic.
    – Cheeseminer
    Feb 3, 2015 at 11:33
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    I have to object. The explanation of fewer collisions of gas molecules sounds terribly wrong. See en.wikipedia.org/wiki/Mean_free_path that the mean free path of molecules in air at ambient pressure is about 68 nm. So whether the distance of the windows is quarter an inch or two inches does not really matter wrt collisions of molecules and the respective heat transfer. The formation of convection cells could be an issue (as hinted by the OP) but I have no clue about that.
    – Ghanima
    Apr 5, 2015 at 22:15
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    @Cheeseminer secondary glazing benefits from the same effects as double glazing - it just tends to have a larger gap for practical reasons. I'm not sure whether there is an optimum gap for secondary glazing beyond "as close as possible".
    – Flyto
    Apr 6, 2015 at 9:10
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    Some references would make this a better answer, as it stands now, it's just one person's opinion.
    – Johnny
    May 6, 2015 at 18:42
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    I agree, but I find that while I pick up trivia, I don't always remember the pointer to where I saw it. Nor did I check out that reference. That said, the largest separation I've heard of (again, no reference, sorry) is 5/8" and the smallest 3/16" I figure that bad data is at least somewhat useful in that it may point someone in the right direction.
    – Sherwood Botsford
    May 8, 2015 at 3:41
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Wide is better for sound, also for sound use different thickness of glass.

For thermal, it can depend on the time of day and what way the windows face, as windows gain heat as well as let heat out.

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Seems we need an answer from a double glazing manufacturer, since secondary glazing can be done with mylar spacers, silicone sealers and suitable framing so that spacing in secondary glazing installations could be done to whatever specifications you choose.

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