Guide: Avoid condensation, mould and draughts around windows

Your windows play a central role in creating a healthy and comfortable indoor climate in your home. But if your windows are old, worn or poorly sealed, problems can arise that negatively affect the indoor climate. Draughts, condensation and mould make a home less comfortable to live in and can also cause health problems in the long term. Therefore, it is important that you ensure that you maintain your windows and repair, or replace, worn windows when the need arises.

Lucile Sarran, Senior Indoor Climate and Building Sustainability Specialist at VELUX, works every day to optimise homeowners' indoor climates and explains here why condensation occurs on windows:

"Condensation that forms on the inside of the window usually occurs on a cold day when there is high humidity indoors due to cooking, showers, many guests in the house or insufficient ventilation. The water vapour in the air condenses when it comes into contact with the cold pane or window frame. This happens less often with newer, very energy-efficient windows, for example with triple-glazed windows, because the window pane and window frame are better insulated and therefore do not get as cold."

Lucile Sarran explains that new energy-efficient windows are actually so well insulated that condensation can instead occur on the outside of the window. "If condensation forms on the outside of the window, it just means that the window is well insulated - that it keeps the heat in and the outside surface cold. So if you see condensation on the outside, there's no need to worry."

The risk of condensation is greatest in rooms that are often humid. The bathroom and kitchen are of course among the rooms that are often damp, but bedrooms can also be. Especially if you sleep with the windows closed and don't have other forms of ventilation. Children's and teenagers' rooms can also be exposed during the day if they spend a lot of time in the room with closed doors and windows. There is a greater risk of condensation in cold rooms, so it is important to heat your home properly and evenly in winter.

According to Lucile Sarran, the most important method to minimise interior condensation is to ensure effective ventilation in the home. "The most important thing to do to avoid moisture buildup is to ventilate thoroughly. This is especially important in winter. Wipe condensation off the windows and check for leaks. Also make sure the indoor temperature doesn't get too low." It is recommended to maintain a consistent temperature in the home of above 18 degrees.

Frequent condensation can have a number of negative consequences for the health of the home. If there is frequent condensation on the window frame or between the window frame and the pane, it increases the risk of mould, which can cause health problems. Water and mould can also make wooden window frames porous.

Condensation on windows brings a risk of mould. Lucile Sarran from VELUX explains: "Just like condensation, low temperatures and high humidity create perfect conditions for mould. Mould grows on damp surfaces that can absorb moisture - for example wood. If there is a leak on a window, you can also expect mould to spread there."

If you are exposed to mould, symptoms such as difficulty breathing, a stuffy nose, and itchy eyes may occur. People with asthma or allergies may find that their symptoms become significantly worse if they are exposed to mould. Children who grow up in homes with mould are also at greater risk of developing asthma.

To avoid health problems for both the home and its inhabitants, it is important to remove mould if it occurs. And to avoid the problem of mould becoming permanent, it is important to prevent it by ventilating 2-3 times a day by opening windows, or, if there is mechanical ventilation in your home, ensuring that the air in the house is replaced often enough to avoid mould formation.

Mould can have different colours, but you can most often see it in the form of black spots or white stains. Mould can grow especially around damaged joints and in environments with a lot of moisture, for example near windows, on damp walls or on shower curtains. Mould is also found in other colours such as gray, pink, or green, but it is more rare.

It is relatively easy to remove mould from windows and window frames, where it can be wiped away with a cloth or paper towel. If there is mould in joints and cracks, it can be more difficult to combat it. However, it will often be possible to remove mould with a fungicide such as diluted bleach, which is sprayed on the affected area and wiped off with a cloth.

Chlorine is also effective against mould, but be aware that chlorine can have a strong bleaching effect, so you should not use chlorine on surfaces such as coloured walls or natural stone. Also be aware that chlorine contains a number of environmentally harmful substances, and that you should not mix chlorine with other cleaning agents or chemicals, as this can create dangerous fumes.

If there is a very large mould infestation in your home, it may be necessary to seek professional help. There are companies that specialise in diagnosing and combating mould in homes, and you can also find cleaning companies that can treat the mould infestation with the right products.

Draughts from windows can create an uncomfortable indoor climate, and leaks around the windows also cause heat to leak out, so you end up with a higher heating bill and a higher CO2 footprint. If you have noticed that your windows are draughty, fortunately there is often something you can do about it. You must first determine what specifically is causing draughts from your windows, and then you can find the optimal solution to the problem.

Draughts from your windows can be caused by the very simple fact that you forgot to fully close your window latches. Before you start looking for more complicated reasons why your windows are draughty, you should make sure that you have closed all window latches properly. And if any of your mechanical fastenings are loose, make sure to tighten them.

If your windows are draughty, it is often because the sealing strips around the windows are worn. The sealing strips must be elastic so that they adapt and close the gap between the window and the frame. Over the years, these seals may become soft or cracks may develop in them, and then it will be necessary to replace them.

You can easily check whether your seals needs replacing by performing a so-called "paper test". Open the window, place a strip of paper in the window opening, and then close the window again. Then try to pull on the paper strip. If the paper is completely stuck or offers resistance, your sealing strips are still tight. If the paper strip slips through, replace the strips.

Replacing sealing strips is relatively simple, so if you're up for it, it's a task you can most likely do yourself. If you are not comfortable replacing the sealing strips yourself, you can contact a window fitter or joiner who will be able to help with it.

If you notice that the draught is not coming in from the window itself, but rather along the wall, the problem may be caused by the exterior join between the window and the window frame. When a window is installed in a wall, a joint of mortar, silicone or joint tape is laid on the outside. The external join itself should not be completely tight because moisture from the outer wall must be able to be vented away. But behind the joint there must be a sealing layer, which usually consists of insulation or tarred work. If this layer is leaky, or in the worst case, completely missing, draughts will occur on the inside.

If you can feel that the draught is coming from the frame around the window, it may be due to a leak in the interior molding or joint between the window and the interior wall. Regardless of whether the draught is caused by the external or internal seal around the window, it is important to rectify the problem as soon as possible, as this type of leak can also cause moisture damage.

Over a number of years, your windows may hang slightly crooked because the weight of the windows presses down on the hinges. If that happens, it can also cause draughts. If the skew is only very slight, the draught may be repaired with a new and possibly thicker sealing strip. However, if the window is very crooked, new and stronger hinges may be needed for the window to close tightly.

In some cases, you may get the feeling that cold air is seeping in from outside, even though there are actually no leaks around the windows. This phenomenon is called cold fallout. Lucile Sarran, indoor climate expert at VELUX, explains the phenomenon as follows:

"If it's cold outside, a window with low energy efficiency will also be cold, and air that comes into contact with the window will be cooled and move quickly towards the floor. It creates an unpleasant chill that feels like a draught."

If you have determined that the draught you are experiencing is not due to loose window latches, worn seals, damaged joins or crooked windows, the problem is likely due to your windows having low energy-efficiency and therefore creating this cold fallout effect. The problem of cold draughts can be solved by installing new, energy-efficient windows, for example modern energy-efficient windows with triple-pane windows and insulating gas between the panes.

• Frequent ventilation is the best method to avoid condensation and mould.
• Maintaining, repairing or replacing windows is important to avoid draughts.
• Less condensation forms on new energy-efficient windows.
• We have a selection of automated roof windows that help you ventilate your home in an easy way.

Read more about the care and maintenance of VELUX roof windows