If black mould keeps coming back, the problem is rarely the mould itself.

Black mould keeps coming back. In most UK properties, particularly solid wall housing stock, recurring surface mould is a symptom of cold wall condensation.

Cleaning the surface may remove visible growth. Anti-mould paint may slow regrowth temporarily. But if the internal surface temperature of the wall remains too low, condensation will continue to form — and, inevitably, the black mould keeps coming back.

Black mould keeps coming back most commonly during autumn and winter. Warm indoor air naturally carries moisture. When that moisture meets a cold external wall, and the surface temperature falls below dew point, condensation forms. If surface relative humidity remains above roughly 70–75% for sustained periods, mould growth becomes likely. Removing surface staining does not change this moisture behaviour.

Condensation on solid brick walls is especially common in pre-1930 properties where there is no cavity insulation. Solid masonry conducts heat efficiently and often presents cold internal surfaces during colder weather, particularly on north-facing elevations or exposed coastal properties. Bedrooms, corners and areas behind furniture are typical locations where mould returns and keeps coming back, especially black mould on external walls.

It is important to understand that anti-mould coatings do not increase wall temperature. They do not alter dew point behaviour and they do not improve insulation performance. They act as surface treatments. In fact, even when black mould keeps coming back, these coatings can’t change the underlying condensation mechanism if the wall remains cold.

Equally, fully waterproof or non-breathable systems can introduce risk if not carefully designed. Solid masonry buildings generally perform best when moisture can move through the structure in a controlled way. Introducing impermeable layers without appropriate assessment can increase interstitial moisture risk or displace condensation to colder junctions. Any intervention must consider hygrothermal behaviour, not just surface appearance.

A long-term solution to recurring black mould therefore requires addressing cold wall condensation at source. That means increasing internal surface temperature, maintaining appropriate vapour permeability, reducing thermal bridging, and considering ventilation and heating patterns within the property. No system can override occupant behaviour entirely, but improving surface temperature significantly reduces the likelihood of sustained high surface humidity. If you find mould keeps coming back, especially black mould, focusing on these factors is crucial.

Thin internal insulation can form part of a retrofit strategy where space is limited and full internal wall lining systems are impractical. By improving the thermal behaviour of the wall surface while remaining vapour permeable, condensation risk can be reduced without significant loss of room dimensions. As with all retrofit measures, correct assessment and installation are essential. Otherwise, mould — especially black mould — keeps coming back despite interventions.

Within Fix Cold Wall Condensation and Black Mould. ECOPROCORK® Trowel-able layered insulating cork system [TLICS™], DECO 00 Fino is used as a breathable cork-based thermal skim layer. Applied as part of a layered insulating system, it contributes to raising internal surface temperature while supporting vapour permeability. It is not a paint or a short-term treatment. It forms part of a system approach designed to improve wall surface performance in condensation-prone environments, especially where black mould keeps coming back after standard treatments.

If black mould keeps coming back, the first technical question to ask is simple: is the wall cold relative to the room air? If it is, surface temperature — not cleaning products — is the key variable. When the thermal behaviour of the wall changes, the recurrence risk changes with it.

Best Thin Internal Wall Insulation

Spray/Trowelable Cork DECO 00 FINO