Why your windows are crying

Do you recognise this? You get up in the morning, look out of the window and see that it is ‘sweating’. The water runs down the pane as if it had run a marathon. But why? There is an invisible player behind it: the dew point.

Imagine the air as a large sponge. When the air is warm, this sponge can store a lot of water – the moisture remains invisible in the air. But when the air cools down, the sponge ‘shrinks’ and its space for the water becomes smaller. At some point, there is no more space left and the excess water has to get out. This is exactly when the water vapour in the air turns into visible droplets – on leaves, windows or as mist in the air. This moment, when the air can no longer hold enough water, is called the dew point. Physically, this means that the air temperature has reached the so-called saturation temperature, at which the relative humidity is 100 % and the water vapour condenses into liquid.

The same thing happens with your window: if the cold pane is colder than the dew point of the air, the air says: ‘Sorry, I’ve run out of room for my water!’ – and the pane gets the full load. The same thing happens when you see dew on the grass or fog in the air in the morning.

You can calculate the approximate dew point in °C using the current air temperature and relative humidity in %:

Dew point = air temperature – (100 – humidity) x 0,125

The more humid the air, the closer the dew point is to the current temperature. Imagine that the air can carry less and less water the colder it gets.

An estimate of the dew point:

• 40 % humidity: Current temperature – 13°C (approximately)
• 50 % humidity: Current temperature – 10°C
• 60 % humidity: Current temperature – 7°C
• 70 % humidity: Current temperature – 5°C
• 80 % humidity: Current temperature – 3°C
• 90 % humidity: Dew point is almost equal to the current temperature
• 100 % humidity: Dew point and temperature are identical – fog forms!

An example

-The room temperature is 20°C and the humidity is 60%.

When the air cools down from 20°C to 13°C at night, it starts to ‘sweat’. According to the rule of thumb, the dew point is 7°C below the temperature.

Another example: humid sauna air

The temperature is 25°C with a humidity of 80%

According to the rule of thumb, the dew point here is 3°C below the temperature. So:

25°C – 3°C = 22°C

This means that if the temperature drops only minimally to 22°C (for example at night), water droplets form on the leaves or even on cold surfaces. And this explains why the air feels so unpleasantly ‘sticky’ on hot, humid days – because the dew point is almost the same as the air temperature. Your sweat doesn’t evaporate because the air itself is already full of water. The only thing that helps is cold water – or simply grumbling.

What does this mean for you?

• Avoid mould in your home: If the walls or windows in your home are colder than the dew point of the air, it will be damp – and where there is damp, unfortunately, there will be mould. That’s why it’s important that the surface temperature of your windows (and walls) remains above the dew point. How can you do this? For example, through better thermal insulation or regular ventilation.
• Understanding heat waves: When the dew point is 20°C or higher on a summer’s day – for example, at 60% humidity and 29°C – the air feels incredibly humid. The sweat simply doesn’t evaporate. And you feel like you’re in a tropical sauna.

And this is also how you understand the dew point …

The next time you stand at a window and ask yourself why it is ‘crying’, you will know: The air simply has no more room for its moisture because the cold pane cools the air.

The rule of thumb: the higher the humidity, the closer the dew point is to the current temperature. At 60 % humidity it is about 7°C below the temperature, at 80 % only 3°C.

When using a heat pump for cooling in summer, the dew point becomes an important factor, especially in terms of condensation and humidity control. Here are the key points you should know:

1. Dew point and condensation

When a heat pump operates in cooling mode, it cools the air in the room. If the surface temperature of the cooling elements (like cooling coils or pipes) falls below the dew point of the room air, the moisture in the air condenses. This moisture appears as water droplets on the cold surfaces.

2. Water formation and drainage

This condensation process is similar to what happens with an air conditioner. The water must be drained away, which is why heat pumps used for cooling are usually equipped with a condensate drainage system. Without proper drainage, water puddles may form, or mold could develop due to excess moisture.

3. Indoor humidity

When a heat pump cools a room, the air temperature drops, but at the same time, the relative humidity increases because cooler air can hold less moisture. However, since condensation occurs, moisture is also removed from the air, resulting in an overall reduction in humidity. This can create a comfortable indoor climate, but it could also make the air too dry if too much moisture is removed.

4. Risk of moisture problems

If the cooling surfaces (like pipes, walls, or cooling panels) become colder than the dew point, condensation may also occur on walls, ceilings, or windows. This is especially common in hot, humid weather, where the contrast between the cool surface and warm, moist air is high. If not managed properly, it can lead to mold growth, as condensation water remains on the surfaces.

5. Dew point monitoring

Modern heat pumps are often equipped with dew point sensors. These sensors ensure that the surfaces do not become too cold, which prevents unwanted condensation on walls, ceilings, or pipes. This technology helps maintain a balance between effective cooling and moisture control.

6. Energy efficiency

If the heat pump cools the air too much (below the dew point), additional energy is required to drain the condensation water. This process can reduce the energy efficiency of the heat pump when used for cooling. By controlling the humidity and surface temperature, energy consumption can be optimized.

7. Practical solutions

• Avoid excessively low cooling temperatures: Do not set the target temperature too low, as this increases the risk of the surface temperature dropping below the dew point.

• Dehumidification instead of cooling: Many modern heat pumps offer a dehumidification mode, which reduces humidity without drastically lowering the temperature.

• Use dew point sensors: Heat pumps with dew point sensors can automatically adjust cooling to avoid condensation and moisture issues.

• Ensure proper condensate drainage: Make sure that any condensed water is reliably drained to prevent water damage, puddles, or mold growth.

Conclusion

When using a heat pump for cooling in summer, the dew point becomes critical because the cold surfaces cause moisture to condense from the air. This condensation must be controlled and drained to avoid mold, puddles, and damage to walls and ceilings. With proper temperature control, dew point sensors, and a working condensate drainage system, these issues can be avoided.