Considering their relatively small size, windows lose a frightening amount of energy. In fact, per square metre nothing in your house loses more energy. This is because when compared to other building materials, glass is a poor insulator. In one example I looked at recently, for the same area of wall and window, the window will allow up to eight times more heat to escape.
Everyone knows that the standard for new windows these days I double glazing, but why? Well, it is mainly because of the space between the two panes of glass, but it essentially stops the two main types of heat transfer.
Convection. Convection refers to the motion of a fluid (be it a gas or a liquid) from a hot place to a cold place. If you pour some hot water into some cold water, the two mix, through convection, until they are both at the same temperature. Similarly, the air in your house, which is hot, tries to mix with the air outside your house which is cold. By being air tight, the window stops this heat transfer.
Conduction. Conduction refers to a materials ability to transfer heat through it. A poker in a fire is a good conductor. Touch the poker and it is hot, even the part which is not in the fire. Plastic is a bad conductor. That is why a plastic handle on the poker will reduce how hot it is significantly. Air which is trapped in the space between the panes, is a really bad.
So… Double glazing reduces heat loss by reducing convection, by being air tight, and conduction, by using air in space between the panes. Generally speaking, double glazing reduces heat loss through windows by 50%.
Next… gas filled double glazing. In gas-filled double glazing the cavity between the two panes of glass is filled with an inert gas (usually argon) which conducts less heat than air, therefore improving the window’s energy efficiency. Argon filled glass reduces heat loss by about another 5%
Low emissivity or low-e type glass is where an invisible layer of insulation is place on the outside face of the internal glass pane of the double glazing. This special material, allows light to pass through it while very little heat is allowed to pass out. When light hits the opaque surface much of its energy is turned to heat. As heat has a much longer wavelength than light, the specially applied coating on the glass prevents the heat energy from escaping. Low-E double glazing can reduce heat loss by a further 18%.
So if we add this all up, you’ll find that Low-E, argon filled, double glazing can reduce heat loss by about 73%. From a comfort point of view alone… a big winner.