Fogged IGU: replace the glass or the whole window?

Before comparing prices, have the quote identify the replacement IGU’s exact specifications. The thickness and edge dimensions need to match the sash, and spacer type, low-E coating, and gas fill can affect both fit and performance. A mismatch may lead to clearance problems, a different appearance, or more condensation than expected.

A glass shop can often measure and replace the unit without replacing the whole window, especially when the sash is sound. I’d also ask whether the seal warranty covers only the glass unit or includes labor, and whether the installer will replace damaged glazing stops or seals found during removal.

Whether an IGU-only replacement is practical often comes down to how the glass is retained. If the sash has removable stops or accessible glazing beads, the old unit can be removed, the opening cleaned, and a new IGU installed with proper setting blocks and sealant.

Some sashes are mechanically assembled or factory-sealed in ways that make disassembly difficult or likely to damage the frame. In those cases, a shop may recommend replacing the sash, or the entire window, instead of forcing an IGU swap.

For a DIY assessment or quote, check the retention method before assuming the job is manageable. The replacement IGU still needs accurate width, height, thickness, and coating details, but the way the glass is held in the sash largely determines whether the labor is practical.

An undamaged frame doesn’t automatically mean IGU-only is the better value. If the window is drafty from poor air sealing or doesn’t meet the home’s performance needs, include the potential improvement from a complete replacement when comparing costs. If it already seals and performs adequately, that benefit may not justify replacing the whole window.