Why do I have black mould?
Black mould needs two things to grow: a damp surface and a source of moisture. In most London homes it is caused by condensation — warm, moist indoor air meeting a cold surface and depositing water there. Give that damp surface a few days and mould appears. This is why mould concentrates in the coldest, least-ventilated spots: around windows, in corners, and behind furniture pushed against external walls.
Understanding this is the key to getting rid of it. As long as the surface stays cold and the air stays moist, cleaning the mould only removes the evidence — it returns within weeks. A black mould investigation identifies why the conditions exist so they can be removed at source.
Where mould grows, and why there
Mould is a diagnostic clue in itself. Around windows, it points to cold glass and reveals and to overnight humidity. In corners, it points to thermal bridges keeping those surfaces cold. Behind wardrobes, it shows where furniture stops air circulating against a cold external wall. In the bedroom, it reflects overnight humidity from sleeping combined with limited ventilation. Reading these patterns correctly is how we distinguish a condensation problem from penetrating or rising damp.
Why anti-mould paint and cleaning fail
Anti-mould paint, bleach and a wipe-down feel like solutions, but they only tackle the visible growth. If the wall is still cold and the air still moist, the mould has everything it needs to return. Generic 'damp treatments' designed for rising or penetrating damp achieve nothing against condensation mould, because they never address warm moist air meeting a cold surface. The only lasting fix is to change the conditions.
How Retrofit IQ investigates black mould
We measure the conditions behind the mould. Thermal imaging locates cold surfaces and thermal bridges. Humidity, temperature and moisture readings, with dew-point analysis, show where and when condensation forms. We review ventilation, and where relevant test airtightness. This lets us confirm the true mechanism — almost always condensation — and rule in or out penetrating and rising damp. You receive a diagnosis and a plan that addresses surfaces, moisture and ventilation together.
Part of a full Building Performance Investigation
Mould is where cold surfaces, humidity and poor ventilation meet — all whole-building issues. A black mould investigation is most valuable as part of a full Building Performance Investigation, which ties them together into one prioritised plan. It is the most reliable way to stop mould coming back for good.
Furniture, storage and air circulation
One of the most common patterns we find is mould on the wall behind large furniture — wardrobes, beds, sofas — placed against a cold external wall. The furniture stops air circulating, so the surface behind it stays colder and damper than the rest of the room, and mould flourishes unseen until it is discovered. Simply pulling furniture a few centimetres from cold external walls, and avoiding packing cupboards against them, allows air to move and keeps surfaces warmer. It is a useful immediate measure, though the lasting fix is to warm the wall and manage the room's moisture.
The role of heating patterns
How you heat a home affects whether it grows mould. Short bursts of intense heat followed by long cold periods let surfaces cool below the dew point between heating cycles, encouraging condensation. A gentle, more consistent background of warmth keeps surfaces above the dew point and is often more effective against mould than sporadic blasts — and, in a reasonably efficient home, need not be expensive. Our investigation considers your heating and occupancy patterns alongside the fabric, because the way a home is lived in is part of the picture.
Mould in bathrooms and kitchens
Bathrooms and kitchens generate the most moisture, so they are frequent mould hotspots — on ceilings, around windows, in grout and in corners. The key here is extract that actually clears the moisture: a fan that runs long enough after a shower or while cooking, vented properly to the outside. Where extract is weak, poorly sited or simply not used, moisture spreads to cooler rooms and condenses there too. We assess the extract provision in these wet rooms as part of diagnosing a whole-home mould problem.
When to seek a measured investigation
If mould keeps returning despite cleaning, appears in several rooms, comes with a musty smell or high humidity, or affects someone with a respiratory condition, it is time for a measured investigation rather than another round of anti-mould spray. Recurring mould is a sign that the underlying conditions — cold surfaces, excess moisture and inadequate ventilation — have never been addressed. Diagnosing and correcting those conditions is the only route to a home that stays free of mould, and it is exactly what our investigation is designed to deliver.
What you receive
The health case for acting on mould
Black mould is not merely unsightly. Living with persistent mould is recognised as a risk to respiratory health, and it can be particularly serious for children, older people and anyone with asthma or a weakened immune system. The spores and the damp conditions that sustain them affect indoor air quality throughout the home, not just where the mould is visible. This is why we treat a mould problem as a matter of health as well as building performance — and why removing the conditions that cause it, rather than repeatedly cleaning the surface, is so important.
Why bedrooms are so often the worst affected
Bedrooms concentrate every factor that causes mould. Overnight, sleeping bodies add moisture to the air for hours; the room is usually closed and unventilated; and heating is often turned down, so surfaces are colder. By morning, the humid air has condensed on the coldest surfaces — around windows, in corners, and behind wardrobes on external walls — leaving exactly the damp conditions mould needs. Understanding this pattern is central to solving bedroom mould: it points squarely at cold surfaces, overnight humidity and inadequate ventilation, all of which we measure.
What proper mould removal should involve
Wiping mould away with bleach removes the visible growth but leaves the conditions untouched, so it returns. Genuine removal is a two-part process: safely cleaning the existing mould, and then changing the conditions so it cannot regrow — warming the cold surfaces, controlling the moisture and improving ventilation. Anti-mould paint and sprays are, at best, a temporary cosmetic measure over a surface that is still cold and damp. Our investigation identifies which conditions to change, so that once the mould is cleaned, it stays gone.
Preventing mould from coming back
Long-term prevention comes down to breaking the cycle of cold surfaces plus moist air. That usually means a combination of measures: raising surface temperatures through targeted insulation and thermal-bridge control, reducing moisture at source, and providing effective ventilation to carry moist air away. Pulling furniture slightly away from cold external walls to let air circulate helps too. The exact recipe depends on your home, which is why measurement matters — a prevention plan built on data rather than guesswork is one that actually holds.
- A clear diagnosis of the true cause of your mould
- Thermal images of the cold surfaces involved
- Humidity, moisture and dew-point findings
- A ventilation assessment
- A prioritised plan — surfaces, moisture and ventilation together
A real-world investigation example
A tenant's child bedroom kept growing black mould in the corner and behind the bed, and repeated cleaning made no lasting difference. Thermal imaging showed a cold external corner — a thermal bridge — sitting well below the rest of the wall, while humidity readings showed the closed, occupied bedroom running humid overnight. Dew-point analysis confirmed the corner surface dropped below the dew point every night, giving the mould exactly the damp surface it needs, and the bed pushed against that wall sealed in the cold, damp air. Warming the corner, moving the bed slightly off the wall and improving ventilation removed the conditions, and the mould did not return.
Common homeowner mistakes
The defining mistake is treating mould as a cleaning problem — bleach, sprays and anti-mould paint — when the cold surface and moist air that cause it are untouched, so it returns within weeks. Another is having a damp-proof course fitted for a problem that is condensation, not rising damp. Homeowners also seal the room up to keep it warm, cutting the ventilation that would carry moisture away and making the mould worse. Packing furniture tight against cold external walls, with no air circulation, is a reliable way to grow hidden mould.