How it Works
The glass in your windows absorbs heat then radiates it again on the colder, outside, surface. Pilkington K Glass has a so-called low emissivity coating which is positioned on surface 3 (counting from the outside) of the Insulating Glass Unit (IGU).
The low emissivity coating is a poor radiator of heat. So the heat absorbed by the coated glass does not travel across the air gap to the outer pane and the cold outside world. Instead the coating reflects the heat back into the room.
Windows incorporating Pilkington K Glass allow heat from the sun through and keep heat inside, making it easier and more cost effective to keep buildings warm.