Last updated: September 14, 2026
A mouldy smell in the bathroom, in the corner of a room or near an external wall is a warning sign that should not be ignored. Although your home may have an MVHR system, this alone does not guarantee that moisture problems will never occur.
The World Health Organization (WHO) states that persistent dampness and mould growth in buildings are associated with an increased risk of respiratory symptoms, allergies and asthma. Therefore, if you notice a distinctive mouldy smell, it is important not to mask it, but to identify the source of the moisture.
Why Is There a Mouldy Smell If You Have an MVHR System?
An MVHR system helps remove humid air from bathrooms, toilets and other areas while supplying fresh air to living spaces. However, this requires sufficient and properly balanced airflow.
The bathroom is one of the most humid areas of the home. Showering releases a significant amount of water vapour into the air, so inadequate ventilation can create favourable conditions for moisture to accumulate and mould to grow. The US Environmental Protection Agency (EPA) identifies improved ventilation as one of the ways to help prevent recurring mould problems in bathrooms.
Can Dirty Filters Cause Moisture Problems?
Indirectly, yes. Clogged MVHR filters increase resistance to airflow. If the system does not automatically compensate for this, the actual volume of supply or extract air may decrease.
A study of residential ventilation systems published at the CLIMA conference in 2022 found that clogged filters and other system components resulted in an approximately 17–30% reduction in supply airflow across two types of systems.
In a 2025 experimental study of an energy recovery ventilation (ERV) system, heavily contaminated filters reduced supply airflow by as much as 37% – from 142 to 89 m³/h.
This does not mean that a dirty filter directly “causes mould”. However, if it results in inadequate ventilation, moisture generated indoors may not be removed effectively.
What Should You Check First – the Filters or the Airflow?
Start with the simplest thing: the filters. Check when they were last replaced, whether they are heavily contaminated, and whether the MVHR controller is displaying a filter replacement or maintenance notification.
After replacing the filters, check whether the system is actually extracting air effectively from the bathroom and other humid areas. If the problem persists, the airflow, air valves, ductwork, system balance and operation of the MVHR unit itself should be checked.
What If the Filters Are Clean but the Mouldy Smell Remains?
In that case, the problem may not be related to the MVHR system at all. Mould needs moisture to grow, so you should look for the source. This could include water leaks, condensation, inadequate thermal insulation, thermal bridges or building materials and structures that remain persistently damp.
The WHO emphasises that the most important way to prevent mould is to prevent persistent dampness and microbial growth on surfaces and within building structures. The EPA recommends keeping indoor relative humidity below 60% and, where possible, within the 30–50% range.
Therefore, simply replacing the filters and expecting existing mould to disappear is not enough. The underlying cause of the moisture must also be addressed.
How Can You Reduce the Risk of the Problem Recurring?
Replacing filters is one of the simplest aspects of MVHR system maintenance. Clean filters that are suitable for your specific unit help the system maintain the intended airflow and protect its components from contaminants.
“Filter Express” generally recommends replacing MVHR filters every 3–6 months, although the actual interval depends on outdoor air pollution, where you live, nearby construction work and other environmental conditions. If your unit has a filter replacement indicator, you should also take its notifications into account.
Noticed a mouldy smell? Start by checking your MVHR filters and the ventilation system’s airflow, but don’t forget to look for the actual source of the moisture as well.
At “Filter Express”, you can choose complete MVHR filter sets to suit your specific unit model. Regular filter replacemen
