Interior condensation is warm, humid indoor air meeting a cold glass surface — like a cold drink sweating in summer. It's a humidity-and-temperature problem, not always a window defect. Old single-pane and failed units run very cold, so they collect moisture first. Lowering indoor humidity and upgrading to warmer-surface glass both reduce it.

Inside the glass (room side): indoor humidity is too high for the glass temperature; manageable. Outside (exterior surface): usually a GOOD sign — the window insulates so well the outer glass stays cool enough to catch morning dew; harmless and temporary. Between the panes: the sealed unit has failed and moisture is trapped; this needs repair or replacement.

They greatly reduce it but can't override physics. New units with warm-edge spacers and Low-E coatings keep the interior glass much warmer, so condensation forms far less often. But a home that generates lots of moisture (cooking, showers, drying laundry indoors) with no ventilation can still see some on the coldest days. Windows plus reasonable humidity control is the real fix.

The colder it is outside, the lower indoor humidity should be. A common guideline is about 30–40% relative humidity in mild winter weather, dropping toward 25–30% in deep cold snaps. Persistent fog on good windows usually means the indoor air is too humid — a bathroom fan, range hood, or HRV usually solves it.

Two usual reasons: those windows are in higher-moisture rooms (kitchens, bathrooms, bedrooms), or they're in spots with poor air circulation — behind heavy curtains, blocked by furniture, or far from a heat register. Cold air pooling against the glass plus trapped moisture equals fog.

Mould means moisture has sat on that surface repeatedly. Clean the existing growth, then tackle the source: lower indoor humidity, improve airflow around the window, and address any cold drafty unit producing condensation. Recurring mould around windows is one of the clearest signals that aging windows should be replaced.

Usually not. Room-side condensation on a new window points to indoor humidity, not a fault, and exterior dew is a sign of good insulation. The one case that IS a defect is moisture sealed between the panes, which shouldn't happen on a new unit and would be a warranty matter.

Frost is condensation that froze because the glass surface dropped below freezing — a hallmark of single-pane or failed older windows combined with high indoor humidity. Warmer multi-pane glass rarely gets cold enough on the room side to frost, which is why upgrading usually eliminates it.