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Since a vacuum is the absence of matter, heat conduction cannot take place through a vacuum, and therefore a vacuum is a perfect insulator.
(
Bolding mine)
This statement by Percy seems right - heat conduction is
by definition the transmission of heat across matter. Since a vacuum contains no matter there can be zero conduction across it and so it is perfect insulator
with regard to heat conduction.
On the other hand a vacuum is clearly
not a perfect insulator with regard to electromagnetic radiation (the clue is that big yellow thing you see in the sky when you look up on a sunny day
).
A perfect insulator resists the transfer of any form of energy. And even if we were to limit the statement to "Since a vacuum is the absence of matter, heat conduction cannot take place through a vacuum, and therefore a vacuum is a perfect thermal insulator" infrared radiation is a transfer of energy and, therefore, heat.
As I said above, you need to change this to "a vacuum is a perfect thermal insulator
with regard to conduction".
Disclaimer - I haven't done any physics for 26 years so I may well be wrong !
Oops! Wrong Planet