Condensation is almost never a window problem — it is a ventilation and dew-point problem. Here is how to read the signs and fix them.
Condensation forms when warm humid indoor air meets a cold surface below the dew point. If the cold surface is the glass, the window is losing too much heat (old profile or single glazing). If the cold surface is the frame, check for thermal bridges in the installation — missing foam or bad flashing.
The usual fix is better ventilation: in airtight modern flats, passive ventilation via sash trickle vents or a controlled mechanical system brings humidity down from 65% to 45% and solves the problem overnight.
