Last updated: November 4, 2026
Every morning, you notice water droplets on your windows, damp windowsills and sometimes even blackened corners around the frames. Sound familiar? Your first instinct is probably to open a window and wipe away the moisture with a cloth. Yet the same problem returns morning after morning.
Why does condensation form on windows even in heated homes? Is the problem with the windows themselves, or could it be a sign of excessive indoor humidity? Window condensation can have several causes, so simply wiping it away is not enough. To tackle the problem in the long term, you first need to understand where the moisture comes from and why it collects on the glass.
What Causes Condensation on the Inside of Windows?
Condensation forms when warm, humid indoor air comes into contact with a sufficiently cold glass surface. When the air next to the glass cools below its dew point, some of the water vapour turns into liquid droplets.
For example, at a room temperature of 20°C and a relative humidity of 60%, the dew point is approximately 12°C. If the inside surface of the glass cools to this temperature or below, condensation may begin to form.
Everyday activities such as cooking, showering, drying laundry and even breathing add moisture to the air. When this moisture is not removed quickly enough, condensation on windows can become a recurring problem.
Can Window Condensation Lead to Mould Growth?
Yes. If condensation regularly collects on window frames, seals or surrounding surfaces, prolonged dampness can create favourable conditions for mould to grow.
Mould around windows is more than just a cosmetic issue. A 2007 meta-analysis of scientific studies found that people living in damp or mould-affected buildings may face an approximately 30–50% higher risk of respiratory symptoms and asthma.
That is why it is important not only to remove visible mould but also to identify why these surfaces keep becoming damp.
How Can You Reduce Window Condensation Quickly?
Start by measuring indoor humidity with a hygrometer. Maintaining optimal indoor humidity levels is important not only for comfort but also for preventing condensation. The US Environmental Protection Agency (EPA) recommends keeping relative humidity below 60%, ideally between 40% and 59%.
If your windows regularly suffer from condensation, try ventilating your rooms more frequently, using an extractor fan when cooking, ensuring moist air is removed after showering and avoiding drying laundry in poorly ventilated rooms. It is also important to maintain adequate heating and allow warm air to circulate freely around your windows.
However, these are only the first steps. If moisture continually builds up inside your home, occasional ventilation may not be enough.
How Can an MVHR System Help Prevent Window Condensation?
A properly designed and balanced Mechanical Ventilation with Heat Recovery (MVHR) system continuously extracts stale, moisture-laden indoor air and supplies filtered fresh air from outside, while recovering some of the heat from the outgoing air. If you would like to understand how an MVHR system works, it is worth exploring its operating principles and key benefits.
An MVHR system can therefore help regulate indoor humidity and reduce the likelihood of condensation, particularly during colder months. However, its effectiveness depends on airflow rates, outdoor conditions, the thermal performance of your windows and the sources of moisture within your home.
If your windows still suffer from condensation despite having an MVHR system, it is worth checking whether the system is providing sufficient ventilation.
Can Dirty MVHR Filters Contribute to Window Condensation?
Yes. Clogged MVHR filters increase resistance to airflow. If the system cannot compensate for this additional resistance, airflow rates may decrease, making it harder to remove excess moisture from your home.
A 2025 scientific study examining residential heat recovery ventilation units found that heavily contaminated filters reduced the supply airflow rate by approximately 37%.
Regular MVHR filter replacement is therefore an important part of maintaining proper ventilation system performance.
How Can You Stop Window Condensation in the Long Term?
Simply wiping condensation off your windows is not enough. You need to address the underlying cause, whether that is excessive indoor moisture or surfaces becoming too cold.
If your home has an MVHR system, start by checking the condition of the filters and the airflow rates. If the system is working correctly but condensation continues to form, it is worth assessing your windows for draughts, inadequate insulation and potential thermal bridges.
At Filter Express, you can find complete MVHR filter sets designed for a wide range of ventilation unit models. Make regular filter replacement part of your home maintenance routine to help keep your ventilation system operating as intended.