Condensation

Condensation on windows and walls can lead to mould, rotting woodwork and poor air quality.

During the winter most homes experience some condensation on the windows. But is it a problem?

No condensation is a good thing, but might also be a sign that your home is draughty. Draughts help to keep humidity levels low but could also be wasting heat and adding to your energy bills. Check our page on airtightness and ventilation for more info.
Slight condensation during cold weather is actually fine in most cases. Occasional, light condensation probably won’t cause mould or rot.
Regular and/or heavy condensation could be a problem. If your windows regularly experience condensation, and especially if it drips down onto the frame and sill, over time mould and rot can set in. Read on for advice on dealing with this.

Humidity

This is a measure of how much water vapour there is in the air. Importantly, warm air can hold more water vapour than cold air.

The image below is psychrometric chart which shows the relationship between absolute humidity and relative humidity at different temperatures.

Absolute humidity is the amount of water vapour in the air, measured in grams of water per cubic metre of air. This is shown on the right-hand axis on the chart.
Relative humidity is the amount of water vapour in the air as a percentage of the maximum possible at a particular temperature. 100% relative humidity means the air is fully saturated and can’t hold any more water. This is given by the family of curves on the chart.

The dashed lines above indicate conditions of 25°C and relative humidity of 71%. The corresponding absolute humidity is 14 grams per cubic metre.

The dew point is the temperature at which the air reaches 100% relative humidity for a given absolute humidity. This is an important concept for us to understand because the dew point is the temperature at which a surface will attract condensation.

For the case above with air of 25°C and 71% relative humidity, we draw a horizontal line to the 100% relative humidity curve, then down to the temperature axis to find that the dew point is 17°C. This means any window or wall that’s below 17°C will experience condensation.

A thin layer of air immediately next to the surface will be the same temperature as the surface. If the surface is colder than the dew point, this thin layer of air will need to exceed 100% relative humidity to hold the moisture. This is impossible so the excess humidity is forced to condense on the surface.

How to prevent condensation

If you followed the explanation above, you’ll see that there are two factors which affect whether condensation will occur. To reduce condensation we need to increase surface temperatures or decrease the humidity.

Increasing surface temperatures

If we can keep all surfaces above the dew point temperature we won’t get any condensation.

The reason windows are the first place condensation forms is because they’re usually the coldest surfaces in the room. Upgrading your windows to double glazing will keep the inside face warmer, reducing condensation.

Cool patches on walls called thermal bridges can also attract condensation. Improving insulation can help to remedy these.

Mould around window caused by condensation on a thermal bridge

Areas around windows are common examples of thermal bridges which can lead to condensation and mould as seen here. Adding a thin layer of insulation around the window reveal will address the thermal bridge and stop the condensation forming.

Reducing room humidity

If you don’t have the budget for new windows, or if you’re still having condensation issues with double-glazing, you’ll need to reduce the humidity in the room.

Referring back to the chart, you’ll see that as relative humidity decreases so does the dew point:

For conditions of 25°C and 71% relative humidity (red arrows) the dew point is 17°C.
If we kept the temperature at 25°C but reduced the relative humidity to 50% (blue arrows) the dew point would reduce to 13°C, making condensation less likely.

The average outdoor humidity in the UK is about 70-80%. If our homes were perfectly ventilated, with windows open year-round, the indoor relative humidity would be about 40-50%. This is lower than outdoors because the house is heated and heat decreases the relative humidity.

However, we don’t keep windows wide open in the winter because this would be wasteful. And human activity adds moisture to the air. Breathing, washing, cooking and drying clothes all create water vapour which boosts the indoor humidity. This all means the indoor humidity can rise to well above the outdoor level.

Hygrometer for measuring humidity and preventing condensation

If you’re having issues with condensation it’s well worth investing in one or more “hygrometers” to track the humidity level in your house. The ideal indoor humidity range is 40-60%. Much higher than this can be uncomfortable and can lead to condensation.

The simplest way to reduce humidity is by increasing ventilation:

Open windows or use an extractor fan while cooking
Ensure bathrooms have extractor fans and turn these on while showering
Open trickle vents on windows
Leave internal doors open to encourage airflow around the house

Other lifestyle habits to reduce humidity include:

Avoid hanging wet washing indoors to dry (hang outside if possible or use a tumble dryer)
Wipe down wet surfaces after showering using a squeegee
If using a tumble dryer, ensure the vent pipe is properly installed so the moist air is routed outside the home

Dehumidifiers

If you’ve tried all of these measures and still have persistently high humidity you may want to consider investing in a dehumidier. These circulate air while extracting moisture. They use a fair amount of electricity but the cost might be worth it if it increases the air quality in your home.

Dehumidifiers actively remove moisture from the air. Don’t be tempted by cheap mini dehumidifiers. Look for ones with controls that allow you to select a target humidity. The good ones have around 20 litres drying capacity (per 24 hours), a large reservoir and good energy efficiency. This Honeywell one has a Class A energy efficiency and large 2.5 litre reservoir.