An insulated glass unit, commonly abbreviated as IGU, is the sealed assembly at the heart of every energy-efficient window. A basic IGU consists of two or more panes of glass separated by a perimeter spacer bar, hermetically sealed around the edge, with the cavity between the panes filled with dry air or an inert gas. IGUs first appeared commercially in the 1950s and now constitute the overwhelming majority of windows sold in temperate and cold climates.
Double Glazing and Cavity Design
The fundamental principle of an IGU is that the still gas trapped in the cavity between the panes conducts heat far less effectively than a solid glass pane. The wider the gap, the lower the conductive heat transfer. However, this effect has a limit. As the gap widens beyond roughly 16 millimetres, convection currents begin to form within the cavity, circulating the gas and increasing heat transfer rather than reducing it.
The optimal gap width for an air-filled cavity is approximately 12 to 16 millimetres. Within this range, conduction is minimised and convection currents have not yet become significant. For an argon-filled cavity, the optimal gap is slightly wider, around 12 to 16 millimetres, because argon atoms are heavier than air molecules and move more slowly, suppressing convection.
Gas Fills: Argon and Krypton
Argon is the most commonly used cavity fill gas. Argon is a noble gas that constitutes approximately 0.93% of the Earth's atmosphere, making it abundant and inexpensive, typically costing only a few dollars per IGU. Its thermal conductivity is approximately 34% lower than that of air, which reduces the U-value of a double-glazed IGU from roughly 2.8 W/m2K with air to approximately 1.6 W/m2K. For an argon fill to be effective, the concentration must be at least 90%. Even with perfect initial filling, argon will diffuse through the edge seal over time at a rate of roughly 0.5 to 1% per year, meaning a well-made IGU should retain above 90% argon for 20 years or more.
Krypton is a heavier and less thermally conductive noble gas, with thermal conductivity approximately half that of argon. Krypton is considerably more expensive, roughly 100 times the cost of argon, because it constitutes only about one part per million of the atmosphere. Krypton is most effective in narrow cavities, approximately 8 to 10 millimetres, making it particularly useful for slim-profile triple glazing and for historic window retrofits where cavity depth is limited. A triple-glazed IGU with two Low-E coatings and krypton fill in 8 millimetre cavities can achieve a remarkable centre-of-glass U-value of 0.5 W/m2K or better.
Warm-Edge Spacer Technology
The spacer is a critical component that determines the IGU's edge thermal performance and long-term durability. Traditional aluminium box spacers act as a thermal bridge, conducting heat around the perimeter of the IGU and causing the edge to be several degrees cooler than the centre. This cold edge encourages condensation formation and reduces the overall U-value of the window.
Warm-edge spacers replace metal with materials of lower thermal conductivity. Popular systems include Superspacer, a silicone foam spacer with an integral desiccant; TPS, or Thermoplastic Spacer, a flexible, warm-applied butyl-based material; and Swisspacer, a hybrid spacer using a thin stainless steel profile with a polypropylene thermal break. Warm-edge spacers can improve the overall window U-value by 0.1 to 0.2 W/m2K compared to aluminium, give warmer edge temperatures, and reduce condensation risk.
Desiccant and Seal Durability
Every IGU spacer contains desiccant, usually a molecular sieve material, to absorb residual moisture trapped in the cavity during manufacturing and any moisture that diffuses through the seal over time. Without desiccant, this moisture would condense on the cold inner glass surface, producing the familiar foggy or frosted appearance of a failed IGU. The first indicator that an IGU has reached the end of its service life is typically persistent condensation between the panes that cannot be wiped away from either side.
The edge seal is typically a dual-seal system. The primary seal, usually polyisobutylene or butyl, provides the moisture and gas barrier. The secondary seal, typically silicone, polysulphide, or polyurethane, provides structural adhesion. A properly manufactured dual-seal IGU has a service life of 20 to 30 years under normal exposure conditions.
U-Values Across IGU Types
The thermal performance of IGUs has improved dramatically over the decades. A double-glazed IGU with clear glass and an air-filled cavity achieves a centre-of-glass U-value around 2.8 W/m2K. Adding one Low-E coating and argon fill brings the U-value to approximately 1.1 to 1.4 W/m2K. Triple glazing with two Low-E coatings and argon fill yields a U-value of 0.6 to 0.8 W/m2K. The most advanced triple-glazed IGUs with two triple-silver Low-E coatings and krypton fill can reach centre-of-glass U-values of 0.4 to 0.5 W/m2K, which approaches the performance of a well-insulated wall.
Condensation Resistance
The condensation resistance of an IGU is described by the Condensation Resistance Factor, or CRF, or the Temperature Index I value in European practice, which indicates the ratio of the temperature difference between the inside glass surface and outside air to the total indoor-outdoor temperature difference. A higher number means warmer interior glass and less condensation. In the United States, the NFRC condensation resistance rating ranges from 1 to 100, with higher numbers being better. A rating above 50 is generally adequate for residential use in cold climates, while 70 or above is preferred for humid environments such as indoor pools and spas.