Why is black mould growing on my walls, and how do I stop it?

Why does black mould grow on walls?

Because a patch of wall stays damp for long enough, and in most homes that dampness is condensation on a cold surface. Mould does not need running water: the Property Care Association Code of Practice says it can grow at a relative humidity of around 80% at an external wall. Scottish building standards guidance says the risk rises when a room's average humidity stays at or above 70% for long periods, because the air at the wall is wetter than the room average. The two figures measure different things, but they point the same way.

The moisture comes from ordinary living. NHBC lists breathing, showers and baths, washing and drying clothes, cooking and boiling kettles. The cold comes from outside: at Penkridge, the nearest Met Office station to Stafford, January averages 7.3°C by day and 1.8°C at night (1991 to 2020 averages), so outside walls spend much of the winter far colder than the rooms they enclose.

UKHSA and DHSC guidance says the root causes are usually building defects, inadequate ventilation, heating or energy efficiency, rather than the people living in the home.

Where does mould show first?

In the coldest, stillest parts of a room. NHBC names windows, colder parts of walls, the area around door and window openings, the line where ceilings and floors meet outside walls, and cupboards or furniture standing against outside walls. The Scottish guidance adds thermal bridges such as lintels, sills and the junction where an inside wall meets an outside one.

Bedrooms are a common starting point, which is why "condensation in bedroom" is such a frequent search. People breathe out moisture all night behind a closed door, and a wardrobe pushed against an outside wall keeps that stretch of wall cold and out of the air flow. Mould behind furniture is often the first sign anyone notices.

Is black mould harmful to health?

It can be, and some people are far more at risk than others. UKHSA and DHSC guidance says the main effects are on breathing: coughing, wheezing, shortness of breath, a higher risk of airway infections, and new or worse allergic conditions such as rhinitis, asthma and COPD. In severe cases these can be fatal. It also lists eye irritation, skin problems such as eczema and rashes, and effects on mental health including anxiety and distress.

The people it names as most at risk include those with asthma, COPD, cystic fibrosis or other lung or heart conditions; people with weakened immune systems; people with a mental health condition; pregnant women and women who have recently given birth; children and young people; older people; and anyone bedbound, housebound or with limited mobility. If someone in the household is in one of those groups, mould needs dealing with promptly, not at the next redecoration.

What actually stops mould coming back?

Less moisture in the air and warmer wall surfaces. In practice that means extraction that works, steady heating, and somewhere for washing to dry other than a bedroom radiator.

• Extractor fans. The UKHSA guidance asks that kitchen and bathroom fans work and are powerful enough, and suggests humidity-controlled fans or fans with an overrun. Building Regulations guidance summarised by BEAMA sets minimum intermittent extract rates of 60 litres a second for a kitchen (30 through a cooker hood), 30 for a utility room, 15 for a bathroom and 6 for a toilet, and warns that fans may fall short of their rated figure once ducts and grilles are fitted. NHBC suggests running the fan or opening a window during cooking and bathing and for about 20 minutes after.
• Heating. RICS suggests keeping indoor temperatures around 18 to 21°C, and names drying clothes over radiators, poor ventilation and insufficient heating as key contributors to condensation.
• Whole-house ventilation. Positive input ventilation uses a fan, usually in the loft, to supply filtered air through a ceiling diffuser and push stale air out. It is not an extract system, so kitchens and bathrooms still need their fans, and it may not suit homes without a loft or where loft air is cold in winter.

Our condensation and mould service looks at which of these a house is short of before suggesting anything.

Why does mould wash on its own not work?

Because it removes the mould, not the moisture that grew it. UKHSA puts it plainly: "Simply removing surface mould will not prevent the damp and mould from reappearing." RICS says the same of mould treatment generally. The Awaab's Law guidance for social landlords allows mould washes and dehumidifiers as part of making a home safe, but says the underlying cause must also be dealt with.

Covering the problem can make it worse. Historic England warns that impermeable paints over traditional finishes reduce a wall's ability to regulate humidity and can increase surface condensation and mould. The UKHSA guidance also says that tenant steps such as opening windows and drying laundry outside "must never be a substitute" for dealing with underlying causes, and that blaming damp and mould on lifestyle choices is "totally unreasonable".

What if the mould is not condensation at all?

Then better ventilation will not cure it, and the source has to be found. Mould only needs damp, and the damp can come from rain, a leaking pipe or the ground as well as from the air. UKHSA describes penetrating damp as water getting in through defects in walls, roofs, windows or floors, and RICS gives a broken roof tile or failed window sealant as examples.

Signs that point away from condensation include a patch that grows after wet weather rather than in cold spells, mould in one spot on a wall instead of in every cold corner, staining below a gutter or chimney, and damp that stays even when the room is warm and aired. The penetrating damp page covers leaks from outside, and our guide to telling rising damp from condensation covers the ground. Stafford Damp Proofing's damp surveys check outside as well as in, wherever you are in our area of damp and mould work around Stafford.

What should tenants do about mould?

Report it to the landlord in writing, with dated photos, and keep a copy. The UKHSA guidance asks landlords to respond sensitively and urgently, not to wait for medical evidence before acting, to photograph and record mould before removing it, and to inspect again at least six weeks after remedial work.

In social housing in England, the Awaab's Law guidance has set deadlines for investigating and fixing serious damp and mould since 27 October 2025; our guide to Awaab's Law sets out the timescales and where private renting stands. Private tenants in Stafford borough whose landlord does not act can contact Stafford Borough Council's Housing Standards Team, which can give advice or serve a legal notice requiring repairs.

Questions we get asked

Should I wipe black mould off before anyone sees it?

Photograph it first. UKHSA guidance asks landlords to record mould before removing it, and a dated photo helps whoever investigates the cause.

Will a dehumidifier fix mould?

It can help in the short term, and the Awaab's Law guidance allows dehumidifiers as part of making a home safe. The cause, such as weak extraction or a cold wall, still needs fixing.

Is mould in a bedroom always condensation?

Usually, but not always. A leak, a blocked gutter or rain getting through a wall can cause it too, so a patch that grows after wet weather needs checking from outside.

Will a damp-proof course stop black mould?

Not if the mould comes from condensation, which is the most common cause. A damp-proof course only deals with ground moisture rising in a wall.

Where these facts come from

• Property Care Association: Code of Practice for the investigation and control of dampness in buildings (March 2017)
• Scottish Government: building standards technical handbook 2022, condensation
• NHBC: Condensation in your home (homeowner leaflet)
• Met Office: Penkridge climate averages, 1991 to 2020
• GOV.UK: Understanding and addressing the health risks of damp and mould in the home (first published 7 Sep 2023, updated 1 Apr 2026)
• BEAMA: guidance on Part F of the Building Regulations for dwellings in England (2022)
• RICS: damp and mould, a consumer guide (16 June 2025)
• Homebuilding & Renovating: Positive Input Ventilation explained (9 Sep 2021, updated 12 Jul 2024)
• GOV.UK: Awaab's Law: guidance for social landlords: timeframes for repairs in the social rented sector (17 Oct 2025, updated 16 Jul 2026)
• Historic England: Risks of energy efficiency interventions
• Stafford Borough Council: damp, condensation and mould