What causes condensation on the inside of my windows?
Condensation forms when moist air meets a surface cold enough to bring it below its dew point. Windows are usually the coldest surface in a room, so they show it first. The moisture is not coming through the glass; it is already in the room air.
Ordinary living produces a surprising amount. Cooking, showering, drying laundry indoors, and simply breathing all release water vapour. Drying a load of washing on a radiator can release several litres into the air.
The three levers are ventilation, heating and moisture generation, and the effective approach usually combines them.
Ventilation is the most important and the most neglected, particularly since homes have become better sealed. Trickle vents in window frames exist for this and are often closed. Extractor fans in kitchens and bathrooms should run during and for a while after use, and opening windows briefly and widely clears moist air faster and loses less heat than leaving one slightly open all day.
Heating helps because warmer air holds more moisture and warmer surfaces are less likely to reach dew point. Consistent low background heat manages condensation better than short intense bursts, which produce large temperature swings.
Reducing generation matters: covering pans, drying laundry outside or in a vented tumble dryer, and closing the bathroom door while showering.
Condensation between the panes of a double-glazed unit is a different problem entirely. It means the seal has failed and the unit is no longer sealed; no amount of ventilation will fix it and the unit needs replacing.
Persistent condensation matters beyond nuisance, since sustained damp on frames and walls leads to mould.