Foggy windows are common in winter, but where the moisture is tells you whether it's a simple humidity issue or a failing window.
Condensation on the inside
Moisture on the room side of the glass means indoor humidity is high and meeting cold glass. It's most common in winter, in kitchens and bathrooms, and in tightly sealed homes. Running exhaust fans, using a dehumidifier, and improving ventilation reduce it.
Condensation between the panes
Fog or haze sealed between the panes of a double-glazed window means the seal has failed and the insulating gas has escaped. This can't be wiped away and signals the sealed unit (not necessarily the whole window) needs replacing.
Why humidity control matters
Persistent window condensation can drip and feed mould on sills and walls. Balancing indoor humidity — enough for comfort, not so much that it condenses — protects both your windows and your home.