You have had it removed. The tenant reported it, you sent someone round, it was cleaned off, and everyone moved on. Then three months later it is back in exactly the same spot. Sometimes it is worse than before.
If this sounds familiar, you are not alone. Recurring mould is one of the most common and frustrating issues landlords face. But the reason it keeps returning is almost always the same, and it is not the one most people assume.
The Mistake That Leads to Repeat Problems
The most common response to mould in a rental property is to remove what is visible and leave it at that. A surface clean, a coat of paint over the affected area, job done. It looks fine. The tenant is happy, until it comes back.
Here is what is actually happening. Mould does not die when a surface dries out. It goes dormant. It sits in the material, waiting. The moment moisture returns, it reactivates and begins to grow again, often faster than before because the conditions that caused it were never changed.
This is confirmed by IICRC S520, the international standard for mould remediation, and by the German Environment Agency’s Mould Action Guide, which is widely regarded as one of the most thorough evidence-based references in the field. Both are clear on one point: any mould removal must begin with identifying and eliminating the source of moisture. Remove the mould without addressing the cause, and the mould will return.
Mould Needs Three Things to Grow
Understanding why mould keeps coming back starts with understanding what it actually needs. Mould requires three things: a food source, air, and moisture. In any property, the first two are always present. Plasterboard, timber, paint, dust and organic matter all provide food. Air is unavoidable.
That means moisture is the one variable that can be controlled. And yet it is the one that most interventions fail to address properly.
What landlords and tenants often do not realise is that surfaces do not need to look wet for mould to grow. Research cited in the German Environment Agency guidance shows that mould can begin to develop when the relative humidity at a surface stays above 64% for a sustained period. Surfaces do not have to be visibly damp. They just have to be consistently cool and slightly humid.
The Hidden Mechanism: Cold Surfaces and Condensation
This is where it gets important for landlords to understand a little building physics, because it explains why mould appears in very specific places and why it keeps coming back to the same spots.
When warm, moist indoor air meets a cooler surface, the air cannot hold its moisture any longer and it deposits it on that surface as condensation. The cooler the surface, the higher the relative humidity will be right at that point, even if the rest of the room feels fine.
This is why you so often see mould in corners, on external walls behind furniture, around window reveals, and at the base of external walls. These are the coldest surfaces in most properties, and they are frequently areas of thermal bridging. A thermal bridge is a point in the building fabric where heat escapes more easily, causing that area to be noticeably colder than the surrounding surface.
Furniture pushed against a cold external wall makes things worse. Warm air does not circulate behind the furniture, so the wall stays cold. Humidity builds up. Mould follows. The tenant reports it. The cycle begins.
It Is Rarely Just One Thing
One of the most useful things in the German Environment Agency guidance is its description of how mould infestations are often the result of several overlapping factors, none of which alone would have caused the problem. An older property with modest insulation, a room that is not heated consistently, a tenant who dries laundry indoors, and a window reveal with a slight draught all combine to create the right conditions.
This matters because it changes how you approach the problem. If you only address one factor and not the others, the mould is likely to return. A ventilation fan in a bathroom will not solve a mould problem that is being driven by a cold external corner and insufficient heating in a bedroom.
A thorough investigation needs to look at all of the contributing factors together.
What Good Practice Actually Looks Like
The IICRC S520 standard and the UK Government’s own guidance on damp and mould are aligned on this. The correct sequence is: investigate first, establish the cause or causes, address those causes, and then remove the mould properly.
Skipping the investigation stage is where the money gets wasted. A landlord who spends on repeated cleaning without fixing the underlying conditions is not managing the problem, they are paying for it to come back.
An investigation by a qualified specialist should look at surface temperatures and humidity readings, not just visible mould. It should consider the building fabric, the ventilation, and how the property is used. In some cases, the cause is structural. In others, it is a combination of an inadequate heating system and a poorly positioned piece of furniture. In others still, there is a slow leak that nobody knew about.
The GOV.UK guidance published for landlords and housing providers is equally clear: understanding the root cause is not optional. It is the only way to resolve the issue in a lasting way.
What This Means in Practice for Landlords
If mould has come back more than once in a property, the starting point is not another clean. It is an honest assessment of whether the cause has actually been identified and dealt with.
Some questions worth asking. Has the ventilation ever been properly assessed in that room? Is the heating in that part of the property adequate and consistent? Are there any thermal bridges, such as uninsulated corners or cold spots around window frames? Has the building fabric been checked for slow moisture ingress?
These are not complicated questions, but they require someone who knows what they are looking at to answer them properly. A properly scoped investigation is almost always cheaper than several rounds of remedial work that do not solve the problem.
A Final Thought
The good news is that once the actual cause is understood and addressed, mould problems in rental properties can be resolved properly and stay resolved. The properties that see recurring mould are almost always those where the investigation stage was skipped. Getting that part right is the most valuable thing a landlord can do.
Intra Air are indoor air quality specialists working with landlords and letting agents across Derby, Nottingham, Leicester, Birmingham and the wider Midlands. EMPO members receive £50 off any investigation or mould removal service. Find out more at https://empo.co.uk/suppliers/intra-air/ or call 01332 480537.