Why is vacuum a good insulator?

Answer 1

A vacuum is a good insulator because it prevents the transfer of heat by eliminating the medium (matter).
A perfect vacuum is the absence of matter. Heat is transferred by conduction, convection and radiation. Radiation is the only form of heat transfer that does not need a medium. Furthermore, radiation is generally the least efficient method of heat transfer.

Answer 2

There are four methods by which heat transfers from one area to another. They are, evaporation, convection, radiation and conduction.

With a vacuum, only radiation can transfer heat across the vacuum and because the other three forms of heat transfer are negated that makes a vacuum a good insulator.

Then again, you may be referring to electricity. For electricity, negative and positive ions in a liquid or electrons in a solid, need a medium to travel through. Electrons in a solid for example need to jump from atom to atom as they move or conduct through the solid. If you have a vacuum it makes it very difficult for electrons and or ions to jump across the vacuum. They will fly across a vacuum if the voltage is high enough though.

What's more is, you can help electrons to fly across a vacuum if you heat up the cathode (negative electrode) to a very high temperature, but still, you also need a high voltage to pull them across the space of the vacuum after they fly off the cathode via heating. And so, a vacuum is a very good insulator for electricity at room temperature and moderate voltages.