Insulating glass (IG) is made up of two or more glass window panes that are separated by a space. This space helps reduce the movement of heat through a part of a building's structure. A window with insulating glass is often called double glazing, triple glazing, or quadruple glazing, depending on how many glass panes are used in its construction.

Insulating glass units (IGUs) are usually made with glass that is 3 to 10 mm thick (1⁄8 to 3⁄8 inch). Thicker glass is used for specific purposes. Laminated or tempered glass may also be included in some units. Most units use glass of the same thickness on both panes, but different thicknesses may be used in special cases, such as for sound reduction or security.

The space between the glass panes is the main part that helps with insulation. This space can be filled with air, but argon gas is often used because it provides better insulation. In some cases, other gases or a vacuum may be used instead.

History

The earliest known use of double glazing may have been in Siberia, where Henry Seebohm observed it in 1877. In the Yeniseysk area, extremely cold winter temperatures often drop below -50 °C, showing how the idea of using two panes of glass for insulation may have begun.

Fitting a second pane of glass to improve insulation started in Scotland, Germany, and Switzerland in the 1870s. Insulating glass is based on older designs like double-hung windows and storm windows. Traditional double-hung windows used a single pane of glass to separate the inside and outside of a building.

• In the summer, a screen was placed on the outside of the double-hung window to keep out animals.
• In the winter, the screen was removed and replaced with a storm window, which created two layers between the inside and outside, improving insulation. Storm windows could be opened using folding metal arms, but screens were usually not used during winter because insects are inactive.

Traditional storm windows and screens require a lot of time and effort to install and remove. Storm windows must be taken down in the spring and stored, then reinstalled in the fall. The heavy frames and glass make it hard to replace storm windows on upper floors of tall buildings. However, modern versions of these windows can have a detachable glass pane in the bottom that can be replaced with a screen when needed, avoiding the need to change the whole window.

Insulated glazing (IG) creates a compact unit with layers of air and glass, eliminating the need for storm windows. Screens can be installed year-round with IG and removed from inside the building, avoiding the need to climb outside. IG can be added to traditional double-hung windows, but this requires modifying the wooden frames because IG units are thicker.

Modern windows with IG often replace older double-hung units and include improvements like better sealing between the upper and lower parts of the window and spring-operated balancing systems. These systems remove the need for heavy weights inside the wall, improving insulation and reducing air leaks. IG also helps keep homes cool in summer and warm in winter. The spring mechanisms allow the top of the window to swing inward, making it easier to clean the outside from inside.

The insulating glazing unit, made of two glass panes sealed together, was patented in the United States by Thomas Stetson in 1865. It became a commercial product in the 1930s, with several patents filed and a product named Thermopane introduced by the Libbey-Owens-Ford Glass Company in 1944. The Thermopane brand, registered in 1941, became a common term for insulated glass units. Unlike modern units, Thermopane used a glass seal to join the panes and had less space between them than today’s units.

Construction

Single pane glass is a poor insulator (R-value of about 1, RSI below 0.2), so it provides little insulation. Glass coatings, such as reflective or colored layers, are often used to reduce heat from sunlight. Other coatings reflect infrared light, which helps control heat.

Low emissivity glass (low E glass) is a common choice for insulating glass units (IGUs). Low E glass is made by applying a metallic coating to a glass pane. These coatings are usually placed on the second or third surface of the glass and help reflect infrared light while blocking parts of the ultraviolet and visible light spectrum. This reduces heat entering the IGU, affecting its thermal performance (R-value) and Solar Heat Gain Coefficient (SHGC). There are two types of low E coatings: hard coatings and soft coatings. Hard coatings are made with tin oxide applied to hot glass, making them durable and less expensive. Soft coatings are applied through a vacuum process and offer better performance but are more fragile and require protection from oxidation. They are often shielded by an inert gas fill.

Glass panes in an IGU are separated by a "spacer." A spacer, which may be a warm edge type, keeps the glass panes apart and seals the space between them. Early spacers were made of steel or aluminum because they were durable and inexpensive. However, these materials conduct heat, which reduces the IGU's insulation and can cause condensation or ice to form at the bottom of the unit. To improve insulation, spacers are now often made from materials that conduct less heat, such as structural foam. Some aluminum spacers include a thermal barrier to reduce condensation and improve insulation, as measured by the U-value.

Spacers may also help reduce noise in areas with high external sound. They often contain desiccant, a material that removes moisture from the space between glass panes, preventing condensation. New technologies have improved spacers by enhancing their thermal performance and durability, such as using aluminum with a thermal barrier or foam.

A common way to improve insulation is to replace air in the space between glass panes with a gas that conducts heat poorly. Gas heat transfer depends on its viscosity and specific heat. Monatomic gases like argon, krypton, and xenon are often used because they do not transfer heat through rotation, making them more efficient than polyatomic gases. Argon has 67% of air's thermal conductivity, and krypton has about half of argon's. Argon is widely used because it is affordable, while krypton and xenon are more expensive. Noble gases are non-toxic, clear, odorless, and chemically inert. Some manufacturers use sulfur hexafluoride for insulation, as it has lower conductivity than argon and is stable and inexpensive. However, sulfur hexafluoride is a strong greenhouse gas and is restricted in Europe under the F-Gas directive. It is banned for most uses except high-voltage switchgear.

In practice, the effectiveness of a fill gas depends on its thickness. For example, krypton works best at a thinner gap than argon, which works better than air. However, it is hard to confirm if the gas remains pure after installation, so designers often use thicker gaps than ideal. Argon is the most commonly used gas in insulated glazing because of its cost-effectiveness. Krypton is used in special cases, such as thin double-glazed units or high-performance triple-glazed units. Xenon is rarely used due to its high cost.

Vacuum technology is also used in some non-transparent insulation products called vacuum insulated panels.

Insulated glass units are often made to order on factory production lines, though standard units are also available. Manufacturers need details such as the width, height, glass thickness, and type of glass for each pane. On the production line, spacers of specific thicknesses are cut and assembled into the required dimensions. Desiccant is added to remove moisture. Glass panes are cut and cleaned to ensure clarity.

An adhesive called polyisobutylene is applied to the spacer’s surface, and the glass panes are pressed against it. If the unit is gas-filled, two holes are drilled into the spacer to remove air and replace it with the desired gas. The holes are then sealed. A newer method uses an online gas filler, eliminating the need to drill holes. The primary sealant (polyisobutylene) prevents gas from escaping and stops water vapor from entering. A secondary sealant, such as polysulfide or silicone, is applied to the edges to allow movement of the primary sealant. Desiccant removes moisture from the air space, preventing condensation on the inside during cold weather. Some manufacturers combine the spacer and desiccant into a single step for efficiency.

Performance

The best insulation in a standard IGU depends on how thick the space between the panes is. A larger space improves insulation up to a point, but if the gap becomes too large, warm air can move inside the unit, reducing insulation. Most sealed units reach their best insulation when the space between panes is 16–19 mm (0.63–0.75 in) at the center of the IGU.

The thickness of the IGU must balance good insulation with the ability of the framing system to support the unit. Many residential and commercial glazing systems can handle the ideal thickness for double-paned units. However, using triple glazing to reduce heat loss creates problems because the added thickness and weight make the units too heavy and hard to install in most residential or commercial systems, especially in moving parts like windows or doors.

Vacuum insulated glass (VIG), or evacuated glazing, avoids heat loss from convection by removing nearly all the air between the panes, creating a near-vacuum. These units are sealed with a special type of glass that melts at lower temperatures to join the parts. This seal can weaken if the temperature difference across the unit becomes too large. Some manufacturers suggest a maximum temperature difference of 35 °C. Small pillars are needed to support the glass and resist atmospheric pressure. Early VIG designs from the 1990s had limited insulation, with R-values up to 4.7 h·°F·ft²/BTU (0.83 m²·K/W), similar to high-quality double-glazed units. Newer VIG units claim R-values as high as 14 h·°F·ft²/BTU (2.5 m²·K/W), better than triple-glazed units. However, the pillars inside VIG units block clear views, making them unsuitable for most windows or refrigerated displays. VIG windows also lose heat at the edges, reducing their overall effectiveness.

Insulation effectiveness is measured using R-values or RSI-values. Higher values mean better resistance to heat transfer. A standard IGU with clear, uncoated glass and air between the panes has an RSI-value of 0.35 K·m/W. In US units, each change in an IGU’s design typically increases its R-value by 1. Adding argon gas improves efficiency to about R-3. Using low emissivity glass on surface #2 adds another R-value. Well-designed triple-glazed units with low emissivity coatings and argon gas can reach R-values as high as R-24. VIG units can achieve R-values up to R-15 (center of glass). Combining VIG with another glass pane and a warm edge spacer can increase R-values to R-18 or higher, depending on coatings. Double VIG units with warm edge spacers may reach R-25 or more, depending on coatings and other factors.

Adding more glass layers improves insulation. While double glazing is most common, triple and even quadruple glazing are used in very cold areas like Alaska or Scandinavia. Some systems use five or six panes, with insulation levels similar to walls.

In some cases, insulation refers to noise reduction. A larger air space between panes improves noise insulation. Using asymmetric double glazing, where the two panes have different thicknesses, can also improve sound blocking. If standard air spaces are used, sulfur hexafluoride gas can replace or supplement inert gases to enhance noise reduction.

Other materials and designs affect sound. Laminated glass with varying interlayer and glass thicknesses is often used for sound dampening. Adding a structural aluminum thermal barrier spacer in the IGU can reduce noise transmission by blocking exterior sounds.

Reviewing the components of the glazing system, including the air space material, helps improve overall sound insulation.

Transmittance measures how much visible light passes through the glass, expressed as a fraction. Some light is absorbed or reflected. Certain types of light, like radio waves, can be blocked by low emissivity glass or metalized coatings, which may reduce Wi-Fi and cell phone signals.

Longevity

The lifespan of an insulating glass unit (IGU) depends on the quality of materials used, the size of the gap between the inner and outer glass panes, temperature differences, the quality of installation, the direction the window faces, and the geographic location. It also depends on how the unit is treated over time. IGUs usually last between 10 and 25 years. Windows that face the equator often last less than 12 years. Most IGUs come with a warranty that lasts between 10 and 20 years, depending on the manufacturer. If an IGU is altered, such as by adding window insulation film, the manufacturer may cancel the warranty.

The Insulating Glass Manufacturers Alliance (IGMA) studied how commercial insulating glass units fail over a 25-year period. In a standard IGU, condensation forms between the glass layers when the seal around the edges breaks and the desiccant (a material that absorbs moisture) becomes full. This condensation can only be removed by replacing the entire IGU. Seal failure and replacement are major costs when owning IGUs.

Large temperature differences between the inner and outer panes can cause stress in the spacer adhesives, leading to failure. Units with a small gap between panes are more likely to fail because the stress is greater. In rare cases, changes in atmospheric pressure combined with wet weather can cause water to fill the gap between the panes.

The flexible seals that prevent air and water from entering around the window can degrade, tear, or become damaged over time. Replacing these seals is often very difficult or impossible because many IG windows use extruded channel frames without screws or plates to hold the seals in place. Instead, the edge seals are installed by pressing a one-way flexible lip into a slot on the frame. This design makes it hard to remove and replace the seals.

In Canada, since 1990, some companies have offered services to repair failed IGUs. These companies drill holes in the glass or spacer to allow air to flow into the unit. This method can reduce visible condensation but does not clean the inside of the glass or remove stains caused by long-term moisture exposure. These companies may offer warranties lasting 5 to 20 years. This repair method reduces the window’s insulating value but can be considered an environmentally friendly option if the window is still in good condition. If the IGU had gas fill (such as argon or krypton), the gas will escape over time, lowering the R-value (insulating value) of the window.

Since 2004, similar repair services have been available for failed double-glazed units in the UK. In Ireland, one company has offered this service since 2010.

Large temperature differences across the surface of glass panes can cause cracks. This often happens when part of the glass is shaded and another part is heated by sunlight. Tinted glass increases heat and thermal stress, while annealing (a process that reduces internal stress in glass) helps prevent cracks. Thermal expansion occurs when warm materials expand but are blocked by cooler materials, creating pressure and stress. Cracks often start at the cool, shaded edges of the glass where small grooves or notches cause stress to build up. Glass thickness does not directly affect thermal cracking because both the stress and strength of the glass increase with thickness. Annealed and tempered glass are usually more resistant to cracking.

Efficiency rating

The heat moving through a window can be calculated based on the materials used, such as the sash, frame, and sill, as well as the size and type of glass. This calculation can be shown in kilowatts (kW) or more usefully in kilowatt hours per year (kWh pa), which helps compare energy costs over a year for a specific location.

In double-glazed windows, heat moves through the glass by radiation, through the glass by conduction, and through the air gap between the panes by convection. Heat also moves through the frame and around the edges of the window due to air movement. The amount of heat moving depends on the weather throughout the year. For example, sunlight entering a window in winter can help warm a building, but in summer, it may increase the need for air conditioning. Using curtains at night in winter or sun shades during the day in summer can reduce unwanted heat movement.

To compare different types of windows, the British Fenestration Rating Council created a "Window Energy Rating" (WER). This rating ranges from A (best) to lower letters like B, C, etc. The rating considers how much heat escapes through the window (U value), how much heat enters from sunlight (g value), and how much air leaks around the window (L value). For example, an A-rated window gains about the same amount of heat from sunlight as it loses through other methods in a typical year, though most of this heat gain happens during summer when it might not be needed. This rating shows better thermal performance than a typical wall.

Window rating programs and certifications include:
• NFRC (National Fenestration Rating Council)
• Passive House (Passive Haus)
• EnergyStar
• Living Building Challenge
• AFRC (Australia Fenestration Rating Council)
• BFRC (British Fenestration Rating Council)
• AMMA (American Architectural Manufacturers Association)
• ASTM
• NAMI