Why window panes fog up
On cold mornings panes are often fogged on the inside, in summer sometimes on the outside. Both have the same cause: the glass surface is colder than the dew point of the adjacent air. The air cools directly at the glass, reaches saturation, and the excess water vapour condenses as fine droplets.
For indoor air at 20 °C and 50% relative humidity the dew point is about 9.3 °C, at 60% already about 12.0 °C. Every pane, frame and glass edge that is colder will get wet.
How cold does a pane get?
The inner surface temperature of glazing depends on its U-value. For windows, an internal surface resistance of 0.13 m²·K/W is usually assumed. Roughly calculated, the following values result for the centre of the pane at 20 °C inside and 0 °C outside:
Single glazing, U ≈ 5.8 W/(m²·K): about 4.9 °C — fogs up at almost any normal indoor humidity.
Older double glazing, U ≈ 2.8 W/(m²·K): about 12.7 °C — usually stays dry at 50%, borderline at 60%.
Modern low-emissivity glazing, U ≈ 1.1 W/(m²·K): about 17.1 °C — hardly fogs up in the centre of the pane.
If the outdoor temperature drops to −10 °C, the older double glazing only reaches about 9.1 °C and fogs up already at 50% humidity. These values are approximations for the centre of the glass. The glass edge, where the spacer between the panes sits, is usually considerably colder. That is why condensation often forms first as a strip along the bottom edge.
Fogging inside, outside or between the panes
Fogged on the inside: the indoor air is too humid for this glazing. In winter this is a common signal that more airing is needed — especially in the bedroom in the morning, after showering and after cooking.
Fogged on the outside: this happens mainly with very well-insulated windows. The outer pane receives hardly any heat from the room and cools below the dew point of the outdoor air at night. This is not a defect but a sign of good thermal insulation.
Fogged between the panes: then the edge seal is usually leaking and humid air has entered the space between the panes. The glazing has then partly lost its insulating effect and should be checked by a professional.
Is window condensation dangerous?
Panes that fog up occasionally are not a problem. It becomes critical when water collects regularly and runs onto the window sill, seals or window reveal. Mould can form there, and wooden or plaster surfaces suffer lasting damage. Fogged windows are also a sign that humidity is high at other cold spots too — such as external corners or behind furniture. More in the article Mould on exterior walls.
What you can do
1. Wipe off condensation in the morning with a cloth or squeegee so it does not soak into the frame and reveal.
2. Burst-ventilate regularly instead of keeping windows permanently tilted. Tilted windows cool the reveal. More in the article Ventilating correctly in winter.
3. Do not block the radiator under the window with curtains, covers or wide window sills, so that warm air can rise along the pane.
4. Check humidity with a hygrometer. Germany's Environment Agency (UBA) recommends 40 to 60%.
5. In the long term: replace old single or early double glazing with modern low-emissivity glazing. You can estimate the cost with the renovation cost calculator.
Know your own dew point
With the dew point calculator you immediately see at which surface temperature your indoor air condenses. Measure the temperature at the glass edge with an infrared thermometer and compare it with the dew point: if it is lower, the pane will fog up — and you know by how many degrees or percentage points of humidity you need to adjust.