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Condensation between the window panes in your Guelph home: what a failed seal means and what to do

Fog, haze or mineral streaks between the two panes of a window mean the sealed glass unit has failed and moisture has got inside; Natural Resources Canada's advice for that case is to replace the insulating glass unit. It cannot be wiped away and it does not clear on its own, but on most Guelph windows from the 1990s and 2000s the frame is fine and only the glass needs replacing.

Last updated October 24, 2026 · Reviewed by the Guelph Windows team

Why is there condensation between my window panes?

Condensation between the panes means the seal around the edge of the insulating glass unit has broken. A modern window's glass is a sealed sandwich: two or three panes separated by a spacer, sealed at the edge, with the gap filled with dry air or an inert gas such as argon. Natural Resources Canada notes that condensation can form between the glass panes if the seal is broken. Once it has, humid air cycles in and out with every temperature swing, the argon escapes, and water vapour condenses on the inside faces of the glass where no cloth can reach it.

The early sign is a faint fog in the morning that clears by afternoon. Later the fog stays, and later still you see white or grey mineral streaks and a permanent milky band, because each cycle of condensation leaves dissolved minerals behind when it dries. By that stage the glass is also doing less to insulate the room, since the gas fill that gave it its rating is gone.

Is it a failed seal or just condensation?

Run your finger across the fog. If it wipes off, it is on a surface you can reach and the seal is fine. Three kinds of condensation look similar from across the room:

Inside surface, room side. Normal in cold weather. NRCan explains it occurs when moist indoor air meets a cooler glass surface, and the risk rises as it gets colder outside or indoor humidity increases. The fixes are lower indoor humidity, better air movement at the glass (open the blinds, do not block the register under the window), and keeping the HRV running if the house has one. It is not a window defect, though a better window reduces it.
Outside surface. Dew on the exterior, usually around sunrise after a cool, still, humid night. NRCan notes this is more likely on high-performance windows because the low-e coating keeps heat in and the outer pane stays cold. It is a sign the window is working, not failing.
Between the panes. Cannot be wiped from either side. Seal failure. The only remedy is a new sealed unit.

Why Guelph's south end is seeing this now

Kortright Hills, Westminster Woods, Pineridge and Clairfields were built from the 1990s through the 2000s with builder-grade vinyl windows and double-pane sealed units, and a sealed unit has a working life. The edge seal is a flexible compound that is stressed every day by the glass expanding in the sun and contracting at night. Guelph's climate gives it the full range: hot south-facing glass in July, deep cold in January, and dozens of freeze-thaw swings in between. South- and west-facing units fail first because they see the largest daily temperature swings, which is why one side of a Westminster Woods house often fogs while the north side stays clear. Units that were pressure-washed hard, had the exterior caulking fail, or sit in a frame that has bowed slightly also go early because the seal is being worked harder than it was designed for.

Older houses in the Ward and around Exhibition Park have a different problem. Original single-pane wood sashes have no seal to fail; what they get is surface condensation and, in winter, frost. That is a draught and humidity question rather than a glass failure, and often a repair of the sash and weatherstripping rather than a replacement.

Replace the glass or replace the window?

Replacing only the sealed glass unit inside the existing frame is the right answer when the frame is sound. That is most vinyl windows of this age. The installer measures the unit, orders a new sealed unit to match, and swaps it in the frame, usually without disturbing the interior trim or the exterior caulking line. It is the core of the window repair service here, and it is a fraction of the disruption of a full replacement.

A full window replacement makes more sense when:

The vinyl frame has warped, bowed or discoloured, or the welded corners have cracked. A new unit in a bad frame fails again.
The hardware, balances or locks are obsolete and the sash no longer operates properly.
Several units in the same elevation have failed or are failing. If four of six on the south wall have fogged, the other two are not far behind, and one replacement visit is cleaner than six glass swaps over four years.
You want to move to triple-pane and the existing frame cannot take the thicker unit.
Air is leaking around the frame itself, not through the glass. That points to the installation, not the sealed unit.

Neither a glass-only swap nor a like-for-like replacement in the existing opening appears on the City of Guelph's published list of permit-triggering renovation work; installing new or enlarging existing windows does, so the permit question only arises if you change the opening.

A new sealed unit can be specified with a low-e coating and gas fill that outperform the original, so a glass-only repair is also a chance to improve the room. If you are considering triple-pane throughout, the triple-pane and energy upgrades page explains what changes.

Can you wait, and what happens if you do?

A fogged unit is not dangerous and it is not letting water into the wall; the moisture is trapped between the panes. What you lose is insulation and the view. The room gets a little colder near the glass, the fog gets permanent, and the mineral deposits etch the inside of the glass so that even if the unit were dried out it would never be clear again. If the failure is on a window you rarely look through, waiting until a second one goes is reasonable. If it is the kitchen window over the sink, you will want it done.

What this means for your quote

Note which windows are affected, which direction they face, roughly how old the house is, and whether the frames still operate smoothly. The installer we book for Guelph will tell you unit by unit whether it is a glass swap or a full replacement, and will not quote a full replacement where a sealed unit will do. The Guelph window replacement cost guide sets glass-only work against full replacement so you can see the difference before anyone visits, and you can describe the fogged windows here to start.

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