Condensation on the inside of a window in winter is normal – it is humidity in the room meeting cold glass, and it wipes off. Condensation between the panes does not wipe off, because it is not on a surface you can reach. It is inside the sealed unit, and it means the seal has failed.
What has actually happened
A sealed unit is two or three panes of glass bonded to a spacer bar around the perimeter, with dry air or argon in the cavity and a desiccant inside the spacer to absorb any residual moisture. When the perimeter seal fails, the cavity starts breathing. Every temperature swing pumps a little humid outdoor air in, the desiccant saturates, and moisture starts condensing on the inner faces of the glass.
Once you can see it, the desiccant is spent. It does not recover.
What it costs you beyond the view
The argon has gone, so the unit is performing closer to plain double glazing than to the low-E figure on the sticker. In an Alberta winter that is a real difference on the heating bill, and the glass will feel noticeably colder to stand beside.
The important part: you probably do not need a new window
This is the bit that saves people money. The frame, the hardware, the flashing and the fit are usually all fine. It is the glass unit that has failed – and the glass unit can be replaced on its own, in the existing sash, for a fraction of the cost of a new window.
We measure the unit, confirm the glazing type, spacer and thickness, order a matching low-E or argon-filled unit, and swap it in. Sealed unit replacement starts at around $280 supplied and fitted, against $700 and up for an insert replacement window.
When replacement really is the better answer
- The frame timber is soft, swollen or rotten at the sill
- The sash has dropped out of square and no longer closes onto full weatherstrip
- Several units in the same elevation have gone at once and the windows are near end of life anyway
- There is no flashing behind the cladding and water is getting into the wall – at which point the opening needs opening up regardless
We will tell you honestly which side of that line you are on, including when replacement is the cheaper answer over ten years. Send a photo and we can usually call it without a visit.
