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Double glazed unit with K Glass showing sun on the outside of the glass emitting shortwave radiation which can pass through the glass but the long-wave radiation from radiators on the inside of the glass is reflected back into the room.

K Glass

Pilkington K Glass, which usually forms the inner pane of a double glazing unit, allows less heat to escape through your windows than ordinary glass. A special enery saving coating lets the suns rays through but reflects heat from fires and radiators back into your home. Heat which ordinary double glazing allows to escape.