What Is an Insulated Glass Unit or IGU?

Two sheets of glass leaning against a wall
  • Insulated Glass Units (IGUs) feature two panes of glass separated by an inert gas
  • The layer of gas diffuses heat transfer, which makes the window more energy efficient
  • IGUs are also known as “double glazing” of “double-pane” glass windows

Windows are a large source of heat loss in a home – but who want to live in the dark? Brighten up a home without sacrificing energy efficiency with insulated glass. Insulated glass units (IGUs) prevent heat loss through your glass doors and windows. The units consist of two panes of glass separated by an inert gas. The insulating layer provided by the gas between the windowpanes diffuses heat transfer. Most modern homes and buildings use insulated glass. The windows provide the same benefits of single-pane windows, with energy efficiency that helps homeowners save money.

Why Is It Called a Unit?


Insulated glass is often referred to as a unit since most of the parts are dependent on another for proper performance. Unlike single-pane glass, IGU glass panes are part of a sealed system that can’t be replaced individually. Insulated glass units go by different names, including: “double glazing” or “double-pane glass windows.”

Components of Insulated Glass Units


  • Glass – The glass in IGUs can be a range of thicknesses or type. Laminated or tempered glass may be used in areas where safety or strength is a priority. IGUs can also contain up to three panes of glass where extra heat or sound insulation is required. Thicker glass is more expensive but more efficient.
  • Spacer – IGUs utilize a spacer that separates the two glass panes where they meet at the edges / window frame. These spacers usually have some sort of desiccant to absorb moisture between the panes and prevent fogging. The width of the spacers depends on the gas used for insulation and window type. Generally, the wider the spacer, the more efficient (and expensive) the window.
  • Window Frame – Insulated glass is used in many different types of windows where efficiency is required: double hung windows, picture windows, casement windows and skylights use insulated glass to prevent heat loss.
  • Gas – The gas used between the glass panes varies with each manufacturer. In general, an inert gas such as argon, krypton or a mixture of both creates the insulating barrier between the indoors and outdoors.

Insulated Glass Unit Performance


Windows are a well-known problem when it comes to energy efficiency. By keeping the ambient indoor air temperature insulated from the outside, IGUs help lower your energy bill. The performance of insulated glass is determined by the thickness of the glass and the insulating space between each pane.

To improve the situation even further, window coatings can be combined with IGUs to create an efficient window that also reflects sunlight in warm climates or even absorbs and contains sunlight in colder climates. Coatings are applied on the interior of IGUs to reflect UV light back into homes to retain warmth, or on the outside to keep sunlight from heating up a home.

For maximum performance, multiple surfaces of the IGU can be coated, though this comes at a higher cost. In double-pane windows there are four surfaces that can be coated.

Insulated Glass Window Location


Not every window in a home may need the same coating. Windows facing south see the most sun exposure and may require different coatings than a north-facing window. Depending on budget, homeowners may find it more practical to invest in more efficient IGUs for south-facing windows.  

Insulated Glass Unit Service


Is your insulated glass fogging? Are you looking to replace single-pane windows and lower your energy bill? Your local Glass Doctor is ready to answer your questions and service your insulate glass units. Just call 833-974-0209 or schedule an appointment online.