Confirm whether the fog is inside the sealed unit
An insulated glass unit, or IGU, contains panes joined around a sealed edge. Moisture between those panes is evidence of a sealing problem. It can appear and disappear with conditions, so photograph the haze when it is visible and note which opening is affected.
Interior condensation may wipe away; outdoor dew may clear as the day warms. An older separate storm window also creates a space between two windows that is not the same as a factory-sealed IGU. Do not diagnose all of these situations as failed sealed glass. Fog does not measure how much insulating gas remains.
Choose the smallest repair that addresses the damage
Glass-only replacement may be possible in a serviceable vinyl, wood or aluminum window. The deciding issues are the glazing system, frame condition and availability of a compatible unit. A glass specialist should identify the required dimensions, thickness, coatings and any required safety glazing.
A replacement sash can address damage to the frame that directly holds the glass when a suitable sash is available. Full-window replacement becomes a different discussion if the surrounding frame is rotten, the opening leaks, operation is poor or the proposed design changes. A failed IGU alone does not establish that all those parts have failed.
Make the repair quote specific
Keep the manufacturer name, product labels, approximate installation date and photographs. Ask whether existing coverage applies, who handles the claim, and whether removal, installation and finishing are included. Compare the same glass specification and scope across quotations.
A service marketed as defogging may address appearance. Ask what it changes and what performance is actually warranted; clear glass is not evidence that the original factory-sealed performance has been restored. Avoid paying for a full replacement simply because the repair options were never assessed.
Technical references
- Andersen: Window condensation explained — Between-pane condensation indicates a sealed-glass problem; surface condensation differs.
- NRCan: Upgrading windows and exterior doors — Assess repair opportunities and the condition of existing windows.
- Pella: Window installation instructions — Product-specific replacement approaches.
Include a preferred date and time
Discuss your exterior project
Select the exterior work and note the current material and repair or replacement needs.
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