Most windows since the 1980s are insulated glass units (IGUs) — two (or three) panes of glass sealed together with an insulating gas (argon) and a spacer bar between them. The seal keeps the gas in and moisture out. When that seal fails, the unit loses its insulating value and moisture gets trapped between the panes. The whole sealed unit — not the frame — then needs replacing.
$400–1,000+ per window; whole-house replacement is five figuresAddress soonWindows
Why it matters
A failed seal means higher energy bills, cold drafts, and permanent fogging you can't clean off (it's between the glass). One or two blown units is a minor budget item and an easy negotiation chip; failed seals across the whole house signal the windows are near end of life — a real five-figure replacement. Single-pane windows (older homes) are a comfort and energy issue in the Canadian climate even before any seal question.
How to spot it
Look between the panes, not at the surface: haze, cloudiness, condensation, or a milky film that won't wipe away = a blown seal. Then date the unit: most IGUs carry a manufacture date stamped on the metal spacer bar visible between the panes, or etched in a corner of the glass (often a number/letter code or a month–year). That date tells you how old the glass is, regardless of how new the house or frames look — a key, easily-missed clue. Also check for rotted/soft wood sills, painted-shut or stuck sashes, and original aluminum frames (thermally poor, often sweat and ice up in winter).
What it costs
Roughly $400–1,000+ per window to replace the sealed unit or the window; a whole-house replacement runs well into five figures depending on count, size, and frame type.
What to do
Budget soon / negotiate. Count the foggy units, note the spacer-bar date stamp where you can read it, and get a glazier or window company to quote. A few blown seals = a small ask in your offer; many + an old date stamp = factor a replacement into your number.
Education and triage, not a home inspection. Casaroo flags fogging and dates the glass from what you can see — a glazier confirms whether it's a sealed-unit swap or a full window replacement.
Common questions
What is Failed window seals & sealed-unit age?
Most windows since the 1980s are insulated glass units (IGUs) — two (or three) panes of glass sealed together with an insulating gas (argon) and a spacer bar between them. The seal keeps the gas in and moisture out. When that seal fails, the unit loses its insulating value and moisture gets trapped between the panes. The whole sealed unit — not the frame — then needs replacing.
Why does it matter for home buyers?
A failed seal means higher energy bills, cold drafts, and permanent fogging you can't clean off (it's between the glass). One or two blown units is a minor budget item and an easy negotiation chip; failed seals across the whole house signal the windows are near end of life — a real five-figure replacement. Single-pane windows (older homes) are a comfort and energy issue in the Canadian climate even before any seal question.
How can I spot it?
Look between the panes, not at the surface: haze, cloudiness, condensation, or a milky film that won't wipe away = a blown seal. Then date the unit: most IGUs carry a manufacture date stamped on the metal spacer bar visible between the panes, or etched in a corner of the glass (often a number/letter code or a month–year). That date tells you how old the glass is, regardless of how new the house or frames look — a key, easily-missed clue. Also check for rotted/soft wood sills, painted-shut or stuck sashes, and original aluminum frames (thermally poor, often sweat and ice up in winter).
How much does it cost to fix?
Roughly $400–1,000+ per window to replace the sealed unit or the window; a whole-house replacement runs well into five figures depending on count, size, and frame type.
Last reviewed 2026-06-27.
Casaroo reviews each guide against current pricing, code, and insurer practice. Cost ranges are 2026
estimates that vary by region, size, and access — confirm specifics with a licensed professional.
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