My suspicion is that the best insulation performance for this application would be aerogel in a rough vacuum. This article states:
Vacuum insulations are commonplace in various products (such as Thermos bottles). These systems generally require a high vacuum to be maintained indefinitely to achieve the desired performance. In the case of aerogels, however, it is only necessary to reduce the pressure enough to lengthen the mean free path of the gas relative to the mean pore diameter. This occurs for most aerogels at a pressure of about 50 Torr. This is a very modest vacuum that can be easily obtained and maintained (by sealing the aerogel in a light plastic bag).
I suspect that with a vacuum of 1 Torr you could get down to 0.001W/m-K or even below.
So perlite powder apparently has a thermal conductivity of 0.02W/m-K
My suspicion is that the best insulation performance for this application would be aerogel in a rough vacuum. This article states:
I suspect that with a vacuum of 1 Torr you could get down to 0.001W/m-K or even below.
So perlite powder apparently has a thermal conductivity of 0.02W/m-K