What Is an IGU? (And Why Your Glazier Keeps Saying It)

If a glazier has ever told you "the IGU needs to be replaced" and you nodded along while silently wondering what an IGU is, you're not alone. It's one of those industry terms that glass professionals use every day but rarely explain.

What IGU Stands For

IGU stands for insulated glass unit. It's the actual glass assembly inside your window frame -- not the frame itself, not the hardware, just the glass part.

An IGU is made of two panes of glass (sometimes three) separated by a spacer bar. The space between the panes is filled with argon or krypton gas, which insulates better than air. The whole assembly is sealed around the edges with butyl sealant and structural silicone.

Think of it like a sealed sandwich: glass, gas-filled gap, glass. That sealed gap is what gives modern windows their insulating ability.

How They Work

The gas fill between the panes slows heat transfer. In winter, the IGU keeps warm air inside and cold air out. In summer, the opposite. Low-E coatings on one of the glass surfaces reflect infrared heat for even more efficiency.

A standard double-pane IGU with argon fill and Low-E coating has roughly twice the insulating value of a single pane.

Why IGUs Fail

The seal around the perimeter is the weak point. Over time, UV exposure, temperature cycling, and age break down the sealant. Once the seal fails, argon leaks out and moist outside air leaks in. That moisture condenses between the panes -- the classic foggy window look.

Most IGUs last 15 to 25 years. South- and west-facing windows fail sooner. Cheap windows with lower-quality seals can fail in under 10 years.

Why This Saves You Money

When an IGU fails, a glazier replaces the glass unit -- not the entire window. Your frame, trim, hardware, and sash stay put. An IGU replacement typically costs $150 to $400 per window. A full window replacement runs $400 to $1,200+.

For a house with a dozen foggy windows, that's the difference between a $3,000 fix and a $10,000+ project. That's why your glazier keeps saying "IGU" instead of "window" -- they're telling you that you don't need to replace the whole thing.