To the best of my understanding double/tripple paned windows are just layers of glass with air in-between them. Air is a relatively bad conductor of heat, so as the air is trapped in the glass and can not form convection currents, or blow away it is a good form of insulation. In the same way as a jumper is good at keeping you warm, not because the wool is a good insulator (although it is not bad) it is the air that is trapped between the fibres.