Insulated glass windows are required primarily because of the high propensity for heat transfer - heat is lost through the windows. Insulated glazing helps to insulate the building and therefore reduce energy costs. Using double glazing also helps to reduce noise.
There are different types of insulated glass:
This is when 2 sheets of glass are put together with an air gap between them to form one single glazing unit. This arrangement is commonly known as double glazing. Sometimes 3 sheets of glass are used - known as triple glazing - which increases thermal properties even more. This would be used in very cold climates.
The glass itself is not the insulating factor - it is the air in the gap. The air gap is usually between 12mm and 20 mm thick, with 16mm generally accepted as being the optimum. However, it depends on the size and design of the glass.
As an option, inert gases can be used instead of air to increase thermal properties. Argon is generally used. Krypton is expensive but is used when the air gap needs to be smaller.
Coatings can be applied to the glass to increase the thermal qualities further. This can have the effect of making the glass look filtered. Depending on the kind of coating, different kinds of light can be filtered out. Some coatings can be applied to single pane glazing to increase the thermal properties.
Low E stands for low emissive glass. The advantage of this glass is that it reduces the transfer of either heat or cold. This means the building stays warmer in the winter but cooler in the Summer.
The reason for this is that it reduces the amount of radiation able to be transferred through the glass. In the winter, the heat inside the building (long wave radiation) is prevented from escaping because a special coating reflects it back into the building. In the summer, the heat from the sun, short wave radiation, is also reflected back from the building.
There are 2 types of Low E glass. The first, hard coating, is a coating applied at high temperatures directly on to the glass when it is manufactured.
The second is soft coating which is applied in a vacuum chamber. The coating consists of multiple thin layers of silver and metal oxides. This is more expensive but has much better performance.