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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).