Nicola Rivers, indoor climate specialist at BESA member, Zehnder Group UK, delves into the growing problem of mould in homes and its impact on the health of thousands of building occupants. She analyses the findings of detailed research carried out by her company into the issue, why many landlords think the problem is insurmountable and what our industry can do to help.
No building is immune. Condensation is an often-overlooked issue that silently develops inside buildings, leaving behind a trail of potentially severe consequences. While it might seem harmless, if left unmanaged, it can have a significant impact on a building’s structural integrity and the health of occupants.
Mould outbreaks are caused by excess moisture build up in the property and a lack of adequate air flow to ventilate that moisture effectively. This causes condensation to form on cold surfaces, turning water vapour into liquid.
Older buildings without sufficient insulation are most vulnerable, as colder walls can encourage moisture to condense. However, even newer builds, which have been designed with improved insultation to meet Net Zero objectives, can experience issues if airflow is restricted or compromised.
In our efforts to build more energy efficient, airtight homes, we often overlook the effects this can have on indoor air quality. Without ventilation, excess moisture and stale air become trapped inside the building creating an unhealthy environment – and effective ventilation is key to solving this problem.
The Scale of the Crisis
Zehnder's research found that 99.2% of UK social landlords report having mould in their housing stock and almost a third (32%) of tenants reveal their homes have mould present in 5-6 rooms.
More than 1 in 10 landlords claim that mould affects over half of their housing remit and believe that the main root cause of the mould is simply down to the weather (41%).
The study polled both social housing landlords and tenants in London to find out to what extent they are suffering with mould and condensation in their homes.
The study posed questions designed to produce a better understanding of the mould problems landlords and tenants face, how widespread the problems are, how it is currently being dealt with, what help they seek as well as any barriers to change. Other root causes are believed by landlords to be old buildings (36%), humidity (34%), condensation (28%), poor ventilation (23%), recent insulation of the house (22%) and general damp (17%).
While they have tried multiple strategies to solve the problems, landlords admitted that while some of these methods worked for a time, the mould came back (51%) or the approach didn’t work at all and the mould is still there (16%).
When asked how the returning mould makes them feel, over two thirds of Landlords admitted to feeling frustrated and angry (64%), over half feel worried and scared (57%) and a third feel embarrassed (30%).
In the Zehnder study, over half of tenants (54%) admitted to turning off the extract fans in their bathrooms and kitchen, which is often the only form of ventilation in the property. Yet the reasons for doing so are rooted in a lack of understanding, with 36% claiming heat loss from the fan, running costs (34%), noise (33%) and nearly a fifth (18%) claiming they just don’t need them.
The task of solving the problem appears insurmountable as 46% of tenants living in social housing claim to have mould present in 3-4 of their rooms. Almost a third (32%) have mould present in 5-6 rooms, 12% have it in 1-2 rooms and 1 in 10 tenants reveal their properties are riddled with mould throughout (7-8 rooms).
Wet rooms only (bathroom / shower room) (40%)
Main Bedroom (32%)
Dining Room (30%)
Children's bedrooms (28%)
Living Room (25%)
Kitchen (25%)
Understandably an overwhelming 98% of tenants worry about the mould that is in their home with their health (21%) and children being exposed to it (21%) being their top concerns, along with the mould affecting allergies (21%) and having to constantly clean (21%). One in five spend money repainting and worry that their possessions are being ruined, while 19% worry it looks horrible. 16% are even worried that the walls or ceiling will fall down.
Barriers to solving the problem
A third of landlords see technology (37%), cost (36%) and lack of knowledge / understanding (31%) as a key barrier to solving their mould problems. Just under a third claim local authority (29%) and lack of support (28%) as a key barrier.
Almost a third (27%) claim the property occupants are a barrier and 98% of landlords think the problems with mould are associated with a lack of education for occupants in how to live healthier in their home (e.g. drying washing inside, turning off the fans, keeping windows closed).
But tenants aren’t helping matters by not speaking to their landlords and turning off what ventilation they have that could help the situation. Over a quarter of tenants (28%) have not sought external help in solving the mould outbreaks in their home and 83% of tenants have not spoken to their landlords about the mould issues.
Managing condensation in homes, particularly in social and rented housing, requires a multi-faceted approach and the right balance of heating and ventilation. Upgraded insulation must be considered in partnership with an effective ventilation system to help regulate indoor humidity levels and promote a healthy indoor environment, minimising the conditions conducive to condensation. Without it, we risk building airtight boxes that turn bad in time.
While condensation on its own shouldn’t be cause for concern, it can lead to costly issues – both for the property and occupant health. For the building, persistent condensation on walls and ceilings in colder weather can damage interior finishes and has the potential to compromise a building’s structural integrity. Prolonged moisture accumulation can then lead to the decay of building materials. Over time, this deterioration can weaken structural components, resulting in costly repairs and jeopardising the safety of the house.
Condensation-induced moisture also undermines the effectiveness of insulation materials, reducing their thermal performance and in turn defeating the point of insulating for thermal efficiency. This constant battle between warm indoor air and cold external surfaces leads to increased energy demands for heating, driving up energy costs and putting strain on building systems.
But most alarmingly, the conditions created by condensation provide the perfect environment for mould and mildew to develop - creating conditions for dampness and toxic black mould to takeover, which poses serious health risks to humans. The spores released into the air can lead to a variety of respiratory problems and allergic reactions among occupants.
A Call to Action
While a third of landlords in our survey are approaching this widespread issue in the right way by installing better ventilation or calling in an expert (29%), others are following generalist advice that won’t solve the root cause of the problem, either temporarily fixing it, or not fixing it at all. These include applying anti-fungicidal wash (28%), using an anti-mould spray (25%) or bleaching the area (24%).
A quarter of landlords we surveyed claimed they asked the occupants to change their living habits, yet large numbers are still just washing it off with soapy water (21%) or painting over it (17%), so it isn’t surprising the mould comes back.
The mould epidemic is a stark reminder of the need for a more holistic approach to social housing. It’s about more than fixing a patch of damp or installing a fan - it’s about creating homes that are safe, healthy, and fit for the future.
As an industry, we need to help landlords navigate this epidemic and arm them with information and tools available to make their properties healthier – we are part of a bigger and quite complex puzzle where education, accessibility and collaboration is key.
BESA’s Indoor Air Quality group, of which Zehnder UK is a member, has produced a series of guides on addressing indoor air quality, damp and mould, and the crucial role of building ventilation, which are freely available on its website here. The group includes members from other industry bodies and professional institutions as well as contractors and manufacturers - seeking to achieve the collaboration so vital to finding solutions to this growing problem.
In particular, its Guide to Mould and Damp was published in 2023 and is now being used by social and private sector landlords as a starting place for developing strategies for dealing with a problem that threatens to overwhelm them. Targeted IAQ training is also available through the BESA Academy.
“There are no silver bullets to addressing IAQ – and no ‘magic box’ air cleaning solutions,” said Adam Taylor, who chairs the IAQ Group. “Improving IAQ is a building engineering issue that calls for solutions that can be applied throughout entire buildings.
“Individual products do play a big part in this, of course, but they must be part of a joined-up, whole building solution based around fully designed, professionally installed and commissioned systems that are then maintained throughout their lifetime and supported by ongoing measuring and monitoring of airborne contaminants.
“Just as net zero cannot be delivered by single technologies, such as heat pumps, so air quality in buildings requires integrated solutions,” added Taylor, who is also CEO of ARM Environments.
BESA is also staging two sponsored walks to mark this year’s national Clean Air Day in support of the Ella Roberta Foundation, which is dedicated to raising awareness of air pollution and its impact on human health.
The Association’s Next Gen network, of young engineers, and its specialist Indoor Air Quality group will lead the walks, which are sponsored by Nuaire and S&P Ventilation.
The first event will take place on June 19 in London to highlight the impact of air pollution in an urban setting; and the second in the Peak District the next day to emphasise the health & wellbeing benefits of clean air.
If you would like to take part in either (or both) walks and support the Ella Roberta Foundation visit the website to register.
BESA is also a supporter of the annual World Ventilation Day, which takes place on November 8.