How to prevent condensation, mould, and draughts around your windows

Windows are essential for maintaining a healthy and comfortable indoor climate. However, old, worn, or poorly sealed windows can create problems such as draughts, condensation, and mould. These issues not only make your home less enjoyable to live in but can also impact your health over time.

To prevent these problems, regular window care is crucial. Check your windows for signs of wear and tear, and repair or replace them as needed to ensure they function properly and contribute to a comfortable indoor environment.

Lucile Sarran, Senior Indoor Climate and Building Sustainability Specialist at VELUX, explains 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 glazing, because the 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 outer surface cold. So if you see condensation on the outside, there is no cause for concern.

Rooms with high humidity levels are most at risk for condensation. Naturally, the bathroom and kitchen are common areas where moisture builds up due to frequent exposure to steam and water vapour. Bedrooms can also be problematic, particularly if windows are kept closed while sleeping and there is no other form of ventilation.

Children’s and teenagers’ rooms are at risk as well, especially during the day if they spend long hours in the room with the doors and windows closed. Additionally, cold rooms are more prone to condensation, making it crucial to heat your home properly and consistently during the winter months.

For more information about maintaining a healthy indoor environment and improving air quality, read our article on indoor air quality.

The most effective way to minimize interior condensation, according to Lucile Sarran, is to ensure proper ventilation in the home. To prevent moisture buildup, focus on the following steps:

• Ventilate thoroughly: Air out your home 2-4 times daily, especially during winter.
• Wipe condensation: Regularly remove moisture from windows to prevent damage.
• Check for leaks: Inspect windows and repair any issues promptly.
• Maintain indoor temperature: Keep your home consistently above 18°C to avoid cold surfaces that encourage condensation.

For more insights into how natural ventilation can improve your indoor climate, read our article on natural ventilation.

With condensation on windows comes a risk of mould. Lucile Sarran from VELUX explains: "Just like with 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 in a window, you can also expect the mould to spread there."

Exposure to mould can cause symptoms such as respiratory problems, a blocked nose, and itchy eyes. For individuals with asthma or allergies, mould can significantly worsen symptoms, while children growing up in homes with mould are at an increased risk of developing asthma.

To protect both your home and your health, it is crucial to remove mould as soon as it appears. Preventing mould from becoming a recurring problem requires consistent ventilation. Air out your home 2-3 times a day using cross-ventilation, or, if you have mechanical ventilation, ensure it effectively replaces indoor air to prevent excess moisture and mould formation.

Mould can appear in different colours, but it is most commonly seen as black spots or white patches. It often forms around damaged seals or in areas with high moisture levels, such as near windows, on damp walls, or on shower curtains.

Less commonly, mould may also appear in other colours, such as grey, light pink, or green, depending on the environment and surface. Identifying and addressing mould early is essential to prevent it from spreading further.

Removing mould from windows and frames is relatively straightforward. For most cases, it can be done with basic cleaning methods. However, some situations may require more specific treatments or professional assistance. Here's how to tackle mould effectively:

• Surface mould: Most mould on windows and frames can be wiped away using a cloth or paper towel.
• Mould in grout or cracks: Use a mould removal product, such as Hysan or Rodalon. Spray the product directly onto the affected area, let it sit for a few minutes, and then wipe it clean with a cloth.

Important precautions when using chlorine bleach:

• Avoid using bleach on coloured walls or natural stone, as it can cause discoloration.
• Never mix chlorine with other cleaning products or chemicals, as this can create hazardous fumes.
• Be mindful of the environmental impact of chlorine-based products.

For larger infestations, it may be necessary to seek professional help. Specialist companies can diagnose and combat extensive mould problems effectively, while professional cleaning services ensure thorough treatment using the appropriate products.

Draughts from windows can make your home uncomfortable and increase energy costs by allowing heat to escape. This not only impacts your heating bill but also contributes to a larger CO2 footprint.

Fortunately, most draughts can be fixed once you identify the cause. Start by determining what is specifically causing the draught, and then apply the appropriate solution.

Sometimes, draughts can be caused by something as simple as improperly closed window fasteners.

To address this:

• Check that all window fasteners are properly closed.
• Ensure loose fasteners are tightened securely.

Before exploring more complex issues, confirm that all fasteners are correctly positioned to eliminate this as the cause of the draught.

A common cause of draughts is worn-out sealing strips around your windows. These strips are designed to be elastic, allowing them to adjust and close the gap between the window and the frame. Over time, however, they can become brittle or develop cracks, making replacement necessary.

How to check if your sealing strips need replacing:

1. Open the window and place a strip of paper in the window opening.
2. Close the window and try to pull the paper out.
   • If the paper is stuck or offers resistance, the sealing strips are still effective.
   • If the paper slides through easily, the strips need replacing.
    

If you feel a draught coming from the wall rather than the window itself, the problem may be caused by a damaged external seal between the window and the frame. When windows are installed, an external seal made of mortar, silicone, or sealing tape is applied to close the gap between the window and the wall. While this seal should not be completely airtight to allow moisture from the outer wall to ventilate, there must still be a functional sealing layer behind it, typically made of insulation or tarred material. If this layer is not airtight—or is missing entirely—it can result in draughts inside your home.

In some cases, the draught may come from the frame around the window. This often points to a lack of airtightness in the indoor trim or the seal between the window and the inner wall. Addressing these issues promptly is essential, as draughts caused by poor sealing not only affect the comfort of your home but can also lead to moisture damage, compromising the structure of your walls or windows.

Over time, windows can become slightly skewed as the weight of the window presses down on the hinges. This misalignment can create gaps, leading to draughts.

If the skewness is minor, the issue can often be resolved by installing a new and potentially thicker sealing strip to close the gap. However, if the window is significantly misaligned, replacing the hinges with new, more robust ones may be necessary to ensure the window closes tightly and eliminates air leaks.

In some cases, you may feel a draught of cold air near your windows, even though there are no visible leaks. This phenomenon, known as cold air drop, occurs when warm indoor air cools upon contact with a cold window surface and sinks to the floor.

Lucile Sarran, an indoor climate expert at VELUX, explains:

"If it's cold outside, a window with low energy efficiency also becomes cold, and air that comes into contact with the window is cooled down and quickly moves towards the floor. This creates an uncomfortable cold air drop that feels like a draught."

If other causes of draughts—such as loose fasteners, worn sealing strips, or damaged seals—have been ruled out, the issue is likely due to low energy efficiency in your windows. The solution is to upgrade to energy-efficient models, such as triple-glazed windows with insulating gas between the panes, to prevent cold air drops and improve indoor comfort.