When the air at the coldest points in a house reaches its “dew point”, the water vapour in the air turns into condensation. These points are usually the single glazed windows and/or the aluminium frames. They allow the cold from inside to penetrate your home, as they do not act as “thermal breaks”.
Double glazing reduces condensation by forming a thermal barrier between your home interior and the cold outdoors.However, if you do not have a thermal break in the frame as well you will still get condensation. This is why timber frames work so well – they act as a natural insulator. The dew point is shown in the chart below. This chart is based on the temperatures at the centre of the glass, but the edges are always colder, and condensation usually begins there.