Foggy Windows: Understanding Failed Sealed Units and Replacement Options

Condensation between double or triple-pane glass indicates seal failure — understanding what causes it and what the repair options are helps homeowners make informed decisions.

Window and Door Replacement

Condensation between double or triple-pane glass indicates seal failure — understanding what causes it and what the repair options are helps homeowners make informed decisions.

Fogged or hazy glass between panes means the sealed unit's inert gas fill (argon or krypton) has escaped and been replaced by ambient air. The moisture in the ambient air condenses on the cold inner glass surfaces, creating the milky or foggy appearance visible from the interior.

Energy Efficiency and Style

Seal failure causes: thermal cycling stresses the spacer and seal at the glass edge with every temperature change. Over 15 to 25 years of seasonal cycling in Ontario's climate, this cumulative stress eventually allows the seal to fail. Windows on south and west elevations — which experience the most temperature differential — typically fail earlier than north-facing units.

The failed sealed unit must be replaced — the fog cannot be cleared by drilling holes, injection treatments, or other remediation methods that circulate desiccant materials. These treatments may temporarily reduce the visible fog but don't restore the window's thermal performance or long-term condition.

Getting a Free Estimate

Replacement options: in a wood or aluminum-frame window where the frame is structurally sound, replacing only the glass unit (the sealed panel of glass itself) rather than the full window assembly is a cost-effective option. Many window suppliers offer glass-only replacement at 30 to 50% of the cost of a full window unit.

Assessment before glass-only replacement: verify that the window frame is structurally sound and in good condition before ordering replacement glass. If the frame shows rotting wood, failed weatherstripping, or operational issues, the incremental cost of a full window replacement may be justified rather than investing in new glass in an aging frame.

Devon Moore, Operations Lead Co-Founder & Operations Lead — D&D Exterior Finishing

Devon Moore is the co-founder and Operations Lead at D&D Exterior Finishing, specializing in siding, roofing, windows, and exterior renovations across Waterloo Region.