physics (only one)

Point 1:
Double-glazed window have two panes and thus are thicker than single-glazed windows. The thicker the material, the more insulation effect in heat loss/transfer caused by conduction.
Point 2:
Between the panes, if the gap is filled with air or inert gas, it will provide more insulation effect in heat transfer/loss caused by convection.
Point 3:
If the gap is made partially or fully vaccum, it will provide very good insulation effect in heat transfer/loss caused by radiation, convection and conduction.

I hope I can help you.