A Fogged Window Is an IGU Failure — Replace the Glass Pack, Not the Window
Claim: when condensation or fogging appears between the panes of a double- or triple-glazed window, the insulated glass unit (IGU / sealed unit) has failed — not the window frame. The correct repair is replacing the glass pack only (600 installed), not replacing the whole window (1,800+). This is the most common and most expensive misdiagnosis in residential window maintenance.
Mechanism
A double-pane window is two systems:
- The frame and sash — the structural surround, which has a typical life of 20–40+ years depending on material
- The IGU (insulated glass unit) — the glass sandwich (two or three panes separated by a gas-filled cavity, sealed at the perimeter with desiccant in the spacer bar), which has a typical life of 15–25 years
The IGU seal fails first. When the perimeter seal degrades — from UV exposure, thermal cycling, or poor original installation — humid air enters the cavity. The desiccant in the spacer bar absorbs the moisture until saturated, and then visible fogging or water droplets appear between the panes.
The frame did not fail. A glass shop can remove the failed IGU from the existing sash and install a new glass pack, restoring the window’s thermal and visual performance without touching the frame.
The decision test
- Fogging between panes only, frame closes flush and is structurally sound → IGU replacement only. Cost: 600 per unit installed.
- Fogging + frame is rotted, warped, or no longer seals on closure → full window replacement. The frame has failed; an IGU swap solves the wrong problem.
- Multiple IGUs failed simultaneously, combined repair approaches full-replacement cost → replace the whole window. At >50% of replacement cost, a like-for-like frame swap makes more sense than piecemeal glass replacements.
Scope
This decision rule covers fogged double- or triple-pane residential windows. It does not cover:
- Interior surface condensation (wipes clean from inside) — that is a humidity/ventilation problem, not an IGU failure
- Water appearing at the sill after rain — that is a perimeter leak or weep hole problem, not an IGU failure
- Single-pane windows (no IGU to fail)
- Commercial curtain-wall systems (different glass-pack replacement process)
Sources
- Argo Window Repair, 2026 cost guide: failed IGU replacement 550; full window replacement 1,500 — https://argowindowrepair.com/blog/window/window-repair-cost
- Magic Window / NorthShield, Canada 2026 window cost guide: replace when repair cost >50% of full replacement — https://www.magicwindow.ca/blog/window-replacement-cost-in-canada-2025
- Glass Doctor North Vancouver: “removes, replaces and reseals your window using top-tier panes” — the existing frame is kept — https://glassdoctor.com/north-vancouver/double-pane-windows
Idea Compass
North: Where this comes from
- windows (Home Systems) — the parent component note; this is the single most important diagnostic rule
- The Decision Lifecycle — reversibility × cost framing: IGU is reversible + low-cost → just do it
East: Tensions / failure
- Window-Perimeter-Leak-Is-a-Wall-Problem-Not-a-Window-Problem (Home Systems) — the other common misdiagnosis: rain-driven water ingress is not an IGU failure
- Interior-Window-Condensation-Is-a-Ventilation-Problem-First (Home Systems) — the third misdiagnosis: surface condensation is not an IGU failure
- The contractor upsell: window installers earn more from full replacement; confirm the frame is truly failed before agreeing to replace it
South: Where this leads
- vendor-roster (Home Systems) — a glass shop (not a window installer) is the right call for IGU-only replacement
- Energy rebate eligibility: a new IGU with low-E argon coating qualifies the opening for improved performance without the full replacement cost
West: What’s similar
- water-heater (Home Systems) — the same pattern: the tank’s anode (the sacrificial part) fails before the steel; replace the part, not the whole unit, until the vessel itself is compromised