What is an insulated glass unit (IGU)?
An insulated glass unit (IGU) is a sealed assembly of two or more glass panes separated by spacers and filled with air or inert gas to reduce heat transfer through windows.
An IGU consists of multiple glass panes bonded together with a hermetic seal around the perimeter, with metal or foam spacers holding them apart and maintaining a consistent gap. This sealed airspace, often filled with argon or krypton gas, acts as insulation by slowing the movement of heat between indoor and outdoor environments. The spacer itself contains a desiccant material to absorb moisture inside the cavity.
IGUs come in two main configurations: double-pane units (two sheets of glass with one air gap) and triple-pane units (three sheets with two gaps), with triple-pane offering better thermal resistance but at greater weight and cost. Both types reduce condensation, improve acoustic dampening, and lower heating and cooling costs compared to single-pane windows.
Seal failure is the primary concern with IGUs. When the perimeter seal degrades over time due to temperature fluctuations, moisture exposure, or manufacturing defect, moisture infiltrates the space between panes and condenses, creating the characteristic fogging or cloudiness. Once the seal fails, the insulating properties decline, and the window cannot be repaired. Replacement of the entire IGU or window unit becomes necessary to restore performance and appearance.