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Heat transfer by convection is often prevented by using fibers that have small air voids between the fibers crisscrossing so that air has a lot of trouble flowing. After doing some research I was not able to find how large those air gaps must be for convection to be significant. Specifically, how densely packed would a given substance have to be to avoid convective heat transfer? In my case this would be granular aerogel, but I suppose something like loosely packed asbestos would also be an item of interest.

The R values of aerogel and air are 10 and 0.29, respectively (if that is of any help).

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The space between the multiple glass planes of insulation windows is optimized for minimal energy losses by gas convection. Normally, a big distance would give a better insulation, but there is a practical limit for windows and within these constraints, the optimum value is before the gas or air does start to convect. In general, the distance is not greater than 1-3 cm in order to avoid this convection, it means that the same window with a gap of 5 cm or 7cm likely has more energy losses. This parameter depends on the filling gas (air, argon, xenon etc.) and the dimension of the window. Important: Asbestos is a very dangerous material, if it's not embedded in a harmless matrix like mortar, cement etc. and if it is not grinded/drilled/sawed. Asbestos should no more be used for standard insulation purposes in private homes.

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