An Insulated Glass Unit (IGU)—often called a “glass pack”—is the complete glass assembly at the heart of a modern window. It consists of two (or three) panes of glass separated by aluminum or foam spacer bars, sealed with butyl and silicone, and filled with argon or krypton gas. The entire assembly is manufactured as a single unit and can be replaced without touching the window frame or sash.
Components of an IGU
- Glass panes — can be clear, Low-E coated, tempered, or laminated
- Spacer bar — maintains consistent gap between panes; contains desiccant
- Primary seal — PIB butyl rubber bonding spacer to glass
- Secondary seal — silicone or polysulfide for structural bonding
- Insulating gas — argon (most common) or krypton for premium units
When to Replace an IGU
IGU replacement is the correct repair when the seal has failed (fogging between panes) but the frame and sash are still in good condition. A glazier removes the glazing compound or rubber gasket, slides out the old IGU, and installs a new one cut to the same dimensions. This typically costs $150–$350 per unit—far less than full window replacement at $400–$900+.
IGU Ratings
IGUs are rated by U-factor (heat loss) and Solar Heat Gain Coefficient (SHGC). An Energy Star qualified IGU for the North-Central US (Ohio) should have a U-factor ≤ 0.27 and SHGC ≤ 0.40.
