Window Condensation: Causes and How to Ventilate Properly

In the morning the pane is misted up and droplets collect on the frame – condensation on windows is a common nuisance in winter. Whether it is harmless or a sign of a problem depends on where the water forms. We explain the causes and show you how to ventilate properly.

Why does condensation form?

Warm air can hold more moisture than cold air. When moist indoor air meets a cold surface, it cools down and the moisture settles as water. The temperature at which this happens is called the dew point. The higher the humidity in the room and the colder the surface, the more likely condensation becomes.

Three cases – three meanings

Condensation on the inside of the pane

This is the most common case. It is usually caused by excessive humidity – from cooking, showering, drying laundry or lots of plants – combined with a cold glass surface. Old windows are particularly affected, as is the edge of the glass if it has an aluminium spacer. Modern windows with triple glazing and a warm edge spacer have much warmer inside surfaces.

Condensation on the outside of the pane

If the pane mists up on the outside in the morning, that is a good sign: the glazing insulates so well that the outer pane receives hardly any heat from inside and cools down considerably at night. This mainly happens in autumn and disappears as soon as the sun warms the glass.

Condensation between the panes

If moisture forms in the cavity between the panes, the edge seal is leaking. The insulated glass unit has then lost its insulating effect and should be replaced.

How to ventilate properly

• Open wide instead of tilting: open the windows wide several times a day rather than leaving them tilted all the time. Tilted windows cool down the reveals and waste heating energy.
• Cross-ventilate: open windows on opposite sides at the same time – the air is exchanged within a few minutes.
• After showering and cooking: air out the moisture straight away and keep the doors closed while doing so.
• Measure humidity: a hygrometer shows whether you are within the ideal range of around 40 to 60 per cent.
• Heat evenly: do not let rooms cool down too much, including rooms you rarely use.

New windows need a ventilation plan

Modern windows are very airtight. This saves energy, but it also means that moisture no longer escapes unnoticed through gaps. When you replace your windows, plan how fresh air will get in – for example with trickle vents in the window frame, which ensure a controlled minimum air exchange, or with a mechanical ventilation system with heat recovery. In Germany, for example, the DIN 1946-6 standard requires a ventilation concept when more than a third of the windows in a house are replaced.

If your windows keep misting up despite proper ventilation, they are probably getting on in years. Read our articles 7 signs you need new windows and Double or triple glazing.