Double glasing work by trapping a poor conduting material, a gas, between two pains of glass. This means that the trapped gas next to the outer pane cannot sink into the room and cool the air quickly. The overall effect is that the thermal conductivity of the glass/gas sandwich is less than that of the glass alone. The more panes of glass the better the insulation so triple glazing is better than double. Also the gas choice is improtant. Ususlly the gas is dry air, but better insulation is obtained with argon which conducts heat less well them air without the sealing problems and expecnce of helium. Argon filled units will be more expensive then air fill though!.