How does a vacuum reduce heat loss by conduction & convection?

Heat is conducted by air movement. As the hot air rises then the cold air above circulates to take it's place. Therefore the heat in a room with air heats up and circulates and then the hot air gives up it's heat to the colder surfaces of the windows etc so it is cooled and it then drops to take the place of the the now rising hot air. The same thing cannot happen in a vacuum as there is no air to heat up and therefore there is no circulation. This is why a double glazed window with a vacuum between surfaces would work better that a sealed unit of gl with a small amount of air trapped inside.