Condensation on windows does not mean just one thing: it all depends on where it forms. On the inner face of the glass, between the two panes of a sealed unit, or on the frame, it tells different stories, which is why locating it precisely matters.
Common causes
on the interior pane, high indoor humidity combined with a cold surface (thermal bridge, low-performance window); between the two panes, a broken sealed-unit seal that has let moisture in; on the frame, infiltration or a sealing defect. Cooking, showers, drying laundry and poor ventilation all raise indoor humidity.
Signs to watch for
persistent fogging on the glass, droplets or frost in winter, a permanent milky haze trapped between the two panes, mould around the frame, stained wood or drywall under the sill.
Why you should not ignore it
excessive indoor humidity promotes mould and signals a ventilation imbalance; a broken seal, on the other hand, makes the window lose its insulating performance (and cannot be repaired). Telling surface condensation apart from condensation between the panes points to two very different issues, one about the indoor environment, the other about the unit itself.
What a professional assessment reveals
Condensation between the panes reveals a broken seal; on the inner surface, it often points to excess humidity or a thermal bridge. An assessment, supported by thermography, tells these causes apart and locates the heat loss.
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