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I want to know more about optimising an air gap between two surfaces, such as window panes or a building's walls.

I was taught that as a general rule, a 6cm air gap is the optimum. Any larger and convection counters the effectiveness. But something tells me this is hardly an accurate figure!

Can anyone improve on this rule of thumb? What are the most significant variables which would alter the optimum separation distance?

Examples of what I would want to apply this to are:

  • A building's window panes of different sizes: glass, perspex or combinations
  • Oven / furnace surfaces of different sizes: between two ceramic refractory walls, or refractory walls and steel. (Temperature differentials and absolutes of perhaps up to 1000 degrees C)

Perhaps there are some approximated formulas to calculate optimum separation distance? And if the effective thermal conductivity of the air changes, a formula for that too.

Thanks!

For interest to other readers, this question is related.

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I would like to comment just on one aspect of the question. We should have in mind that there are ways to decrease convection, say, between two walls. For example, I heard about the following invention many years ago: a number of inclined thin plates are installed between two walls in such a way that their higher sides are at the hotter wall, like in the following schematic (the hotter wall is on the left), but the plates are attached to both walls and the spacing is less between neighboring plates: |\| |\| |\| |\| As a result, convection is decreased.

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