Insulated Glazing Unit or Insulating Glass Unit (commonly referred to as IGU) is a set of two or more sheets of glass spaced apart and hermetically sealed to form a single glazed unit with a space between each sheet. Its most important function is to decrease heat transfer through windows.

IGU made of glass is called insulated glass, which refers to heat insulation, not sound or electricity. A common yet possibly less accurate term is Insulated Glass Unit or IGU, since the glass itself has little insulative properties. Predominantly, it is the gap between the glazing layers that provides the insulation. Broadly speaking, each IGU or each framed window section is known as a “lite”.

It is important that the gas remains as immobile as possible to prevent convection currents transferring heat across the insulating gap. This limits the thickness of the gap used and is a reason for adopting more layers, such as with triple glazing.

The space between the layers may be filled with air or an inert gas such as Argon or Krypton which would provide better insulating performance. Of course, a perfect vacuum would be best. (Argon has a thermal conductivity 67 % that of air.) Typically the spacer is filled with desiccant to prevent condensation and improve insulating performance. Less commonly, most of the air is removed, leaving a partial vacuum, which drastically reduces heat transfer through convection and conduction. This is called evacuated glazing. Similar techniques are also used in insulation products called vacuum insulated panels.

Often the insulating quality is used in reference to heat flow where the gap is the insulating medium. The gap is usually 12 mm to 20 mm thick. Within this range, the thickness does impact the insulating properties substantially, but smaller gaps have greater heat conduction through the air or other gas, and larger gaps allow more convection within the space leading to higher convective heat loss. A 16 mm air gap is often considered the optimum thickness for air although this depends on many factors such as the size of the window, the temperature difference between the two panes and whether it is vertical.

In general, the more effective a fill gas is at its optimum thickness, the thinner the optimum thickness is. For example, the optimum thickness for Krypton is lower than for Argon, and lower for Argon than for air. However, since it is difficult to determine whether the gas in an IGU has become mixed with air at manufacture time (or becomes mixed with air once installed), many designers prefer to use thicker gaps than would be optimum for the fill gas if it were pure. In some situations the insulation is in reference to noise mitigation. In these circumstances a large gap improves the noise insulation quality or Sound transmission class. Asymmetric double glazing, using different thickness of glass rather than the conventional 4-12-4 symmetrical systems, is more important than air gap thickness in improving the phonic insulation properties. This is often overlooked.


  • Reduction of air conditioning cost
  • Reduction of noise penetration
  • Prevention of dew condensation
  • Reduction of UV transmission