Mould on the inside of an external wall: what it means and who fixes it
The inside face of an outside wall is one of the coldest surfaces in most rooms, so it is where mould often appears first. Sometimes the wall is simply cold and damp air is condensing on it; sometimes water is getting in from a defect outside. The two look different, are fixed differently, and the second is almost always the landlord's job. This guide shows you how to tell them apart and what to record.
In short
Mould on an external wall is usually one of two things: condensation forming on a cold wall surface, or moisture coming through the wall from an outside defect such as failed pointing, cracked render or a leaking gutter. Condensation mould tends to sit in lower corners, behind furniture and around window reveals as speckled black growth; penetrating moisture tends to appear as an irregular patch that gets worse after rain, often with a stain or tide mark. Photograph the inside wall and, from ground level, the outside of the same wall, then report it in writing. In England, Wales and Scotland the structure and exterior of a rented home are the landlord's by statute; in Northern Ireland the tenancy agreement sets out who does which repairs, and where it is not clear the landlord is responsible for the structure and exterior. Either way, a wall that is too cold or too wet to stay mould-free is a repair issue, not a cleaning issue.
- Private and social tenants in England, Wales, Scotland and Northern Ireland
- Anyone with mould on a wall that faces outside, in a house or a flat
- Leaseholders whose flat has an external wall (check the lease for who is responsible for the outside)
- Look at where the mould sits on the wall and whether it changes after rain
- Photograph the inside wall in wide, medium and close-up views
- Photograph the outside of the same wall from ground level, without climbing
- Move furniture off the wall and keep the room ventilated while you wait
- Report it in writing with the photographs and ask for an inspection
- UK-wide for the diagnosis and evidence steps. The repairing duty sits with the landlord in England (Landlord and Tenant Act 1985 s.11 and the Homes (Fitness for Human Habitation) Act 2018), Wales (Renting Homes (Wales) Act 2016 ss.91 and 92) and Scotland (the repairing standard in the Housing (Scotland) Act 2006 for private tenancies; Schedule 4 of the Housing (Scotland) Act 2001 for council and housing association tenancies). In Northern Ireland repair responsibilities are set out in the tenancy agreement and statement of tenancy terms, and the landlord is responsible for the structure and exterior where the terms are not clear. Awaab's Law timescales apply to registered providers of social housing in England now and to all private and social tenancies in Scotland from 6 October 2026.
- Photographs show pattern and change, not the cause; only an inspection can confirm whether moisture is coming through the wall
- Condensation and penetrating moisture can happen on the same wall at the same time
- This guide does not cover rising damp or leaks from a flat above, which follow different patterns
- This guide is not legal advice; Shelter, Shelter Scotland, Shelter Cymru, Housing Rights and Citizens Advice give free advice
- Private renting: repairs (what your landlord must do)
- Landlord and Tenant Act 1985, section 11
- Understanding and addressing the health risks of damp and mould in the home
- Awaab's Law: guidance for tenants in social housing
- Renting Homes (Wales) Act 2016, section 91 (fitness for human habitation)
- Renting Homes (Wales) Act 2016, section 92 (landlord's obligation to keep dwelling in repair)
- Housing (Scotland) Act 2006, section 13: the repairing standard
- Housing (Scotland) Act 2006, section 12: tenancies the repairing standard applies to
- Housing (Scotland) Act 2001, Schedule 4 (Scottish secure tenancies: landlord's repairing obligations)
- First-tier Tribunal for Scotland (Housing and Property Chamber)
- Repairing your home: private tenants
- Housing standards in rented accommodation
- Leasehold property
- Repairs and housing conditions
- How long should a private landlord take to do repairs?
Put your photographs and dates in one case file. The first check is free and needs no account.
Start a free caseWhy the inside of an outside wall grows mould
Warm indoor air carries moisture from cooking, washing, drying clothes and breathing. When that air meets a surface colder than itself, the moisture condenses onto it. An external wall is colder than the internal walls because one side of it is outdoors, so it is the first surface in the room to reach that point. Mould follows the moisture.
Several things make an external wall colder than it should be. Insulation that was never fitted, or cavity insulation that has slumped or got wet, leaves the wall close to outside temperature. Corners where two outside walls meet lose heat in two directions, which is why mould so often starts in the top or bottom corner of a room. Furniture pushed against the wall stops warm air reaching the surface, so the patch behind a wardrobe or bed is often the worst in the room. Window reveals are thin and cool quickly.
The other route is water coming through the wall itself. Pointing between bricks erodes; render cracks; a gutter overflows or a downpipe leaks and soaks one strip of wall every time it rains. Water that gets into masonry keeps the inside face damp long after the rain stops, and mould grows on the damp plaster.
Condensation or penetrating moisture: reading the pattern
You cannot prove the cause from inside, but the pattern usually points one way. Use this table to describe what you see when you report it.
| What you see | Points towards | Why |
|---|---|---|
| Speckled black growth in the lower corners, behind furniture, on the ceiling edge or in window reveals | Condensation on a cold surface | These are the coldest spots and the ones warm air cannot reach |
| Worse in cold weather, better in summer; windows stream in the morning | Condensation | Cold outside means a cold wall and more moisture in the air indoors |
| An irregular patch, often with a brown or yellow stain or a tide mark, in one place only | Moisture coming through the wall | Water enters at one defect and spreads from it |
| Gets noticeably worse in the days after heavy rain | Moisture coming through the wall | Rain is the source; condensation tracks cold spells, not individual downpours |
| Plaster feels soft, blown or crumbly; paint bubbles or wallpaper lifts | Moisture coming through the wall | Sustained wetting from within the masonry damages the finish |
Both at once
A wall with a leaking gutter above it is also a cold wall. It is common to have a penetrating patch in one place and condensation mould along the same wall. Describe both rather than choosing one.
What to photograph inside
Take three views of each affected area: a wide shot that shows which wall it is, a medium shot of the patch with something for scale, and a close-up of the growth or stain. Add one of anything that supports the pattern: the corner it starts from, the gap behind the wardrobe, the window reveal, a bubbled patch of paint.
Repeat from the same spot weekly and after every heavy rain. A patch that is bigger after rain and unchanged after a dry cold week says more about the cause than any single picture can. Keep the originals uncropped so the file dates stand.
What to photograph outside, safely
Go outside and find the same wall. Work out roughly where the inside patch sits: which floor, how far from the corner, whether it is under a window. Then photograph from ground level only. Do not use a ladder, stand on a wall or lean out of a window; a zoomed-in photograph from the ground is enough for the record.
- The whole elevation, so the wall can be identified later.
- The gutter and any downpipe above or near the patch, ideally while it is raining, if you can see overflow, a drip or a green streak on the wall beneath.
- Pointing between bricks: gaps, crumbling mortar or plants growing in the joints.
- Render: cracks, hollow-sounding or missing sections, staining below cracks.
- Window sills and the joint where the frame meets the wall: failed sealant, a sill with no drip edge, water marks below.
- Ground level: soil, paving or a raised path against the wall above the damp-proof course, blocked air bricks, a leaking outside tap or drain.
What you can do while you wait
- Pull furniture a hand's width off the external wall so air can move behind it.
- Keep the room heated to a steady low level rather than blasts of heat; a wall that never warms up stays below the point where moisture condenses.
- Use extractor fans when cooking and washing, keep lids on pans, and close the kitchen and bathroom doors so moisture does not drift into the coldest room.
- Dry clothes outside or in a room with a window open and the door shut; if you must use the room with the mould, do not dry clothes against that wall.
- Wipe the growth only after it is photographed and reported, and only if the patch is small; government health guidance (UKHSA) says people with pre-existing respiratory conditions such as asthma and COPD, people with weakened immune systems, children, older people and pregnant women are at greater risk from damp and mould, so do not do this if that describes anyone in the home.
- Do not paint over it. Government guidance says removing surface mould on its own will not stop it coming back, and painting over it loses the record of the condition.
What needs the landlord
In England, Wales and Scotland, anything on the outside of the building is the landlord's to inspect and repair: gutters, downpipes, pointing, render, sills, sealant, air bricks and the damp-proof course. In England, section 11 of the Landlord and Tenant Act 1985 puts the structure and exterior of the home, expressly including drains, gutters and external pipes, on the landlord. Wales and Scotland place the same kind of duty on the landlord in their own legislation. In Northern Ireland the tenancy agreement and statement of tenancy terms set out who does which repairs, and where they are not clear the landlord is responsible for the structure and exterior, including drains, gutters and external pipes.
Missing or failed insulation is also a landlord matter. A tenant cannot fix a cold wall by opening windows, and a home that cannot be kept free of mould by ordinary living is a condition problem, not a lifestyle one. Government guidance to rented housing providers is clear that it is unreasonable to blame damp and mould on lifestyle choices and that landlords should find and fix the underlying cause.
When you report it, say which wall, describe the pattern using the table above, attach the inside and outside photographs, and ask for an inspection that looks at both the outside of the wall and the insulation. Ask for the findings in writing.
Wording that works
"There is mould on the inside of the front bedroom wall, which is an external wall. The patch is below the window, gets worse after rain, and the gutter above it overflows. Photographs of the inside and outside are attached. Please inspect the outside of the wall and the insulation and tell me in writing what you find."
Who is responsible, by nation and tenure
Where there is no fixed timescale, Shelter's guidance for private tenants in England is that the landlord should do repairs in a reasonable time, and that what is reasonable depends on how serious or urgent the problem is and how vulnerable the people living there are. Government health guidance also tells landlords to act with urgency when concerns have been raised about a tenant's health. So say in your report if anyone in the home has a breathing condition, is very young, older or pregnant.
| Where and who | The landlord's repairing duty | Fixed timescales? |
|---|---|---|
| England, private tenant | Landlord and Tenant Act 1985 s.11 (structure and exterior, including drains, gutters and external pipes); Homes (Fitness for Human Habitation) Act 2018; council enforcement under the Housing Health and Safety Rating System, which assesses damp and mould growth as a hazard | No statutory days. Shelter's guidance is that repairs should be done in a reasonable time, and that what is reasonable depends on how serious or urgent the problem is and how vulnerable the people living there are |
| England, council or housing association tenant | The same repairing duties, plus Awaab's Law for registered providers of social housing | Yes: damp and mould investigated within 10 working days; written summary within 3 working days of the investigation concluding; safety work begun within 5 working days of the investigation concluding; emergency hazards within 24 hours (Phase 1, in force since 27 October 2025; Phase 2 from 30 November 2026) |
| Wales, most contract-holders (secure contracts and standard contracts of under seven years) | Renting Homes (Wales) Act 2016: the landlord must keep the structure and exterior of the dwelling (including drains, gutters and external pipes) in repair (s.92) and ensure the dwelling is fit for human habitation (s.91) | No statutory days |
| Scotland, private tenant | The repairing standard (Housing (Scotland) Act 2006 s.13): the house must be wind and water tight and in all other respects reasonably fit for human habitation, and the structure and exterior (including drains, gutters and external pipes) must be in a reasonable state of repair; enforced by the First-tier Tribunal for Scotland (Housing and Property Chamber); Awaab's Law from 6 October 2026 | Yes from 6 October 2026 |
| Scotland, council or housing association tenant | The repairing standard and the tribunal route do not apply to Scottish secure tenancies (2006 Act s.12). Instead, Schedule 4 of the Housing (Scotland) Act 2001 requires the landlord to ensure the house is wind and watertight and in all other respects reasonably fit for human habitation and to keep it in that condition throughout the tenancy; complain to the landlord first, and Shelter Scotland can advise on what comes next; Awaab's Law from 6 October 2026 | Yes from 6 October 2026 |
| Northern Ireland, private tenant | Repair responsibilities are set out in the tenancy agreement and statement of tenancy terms; where they are not clear, the landlord is responsible for the structure and exterior of the property, including drains, gutters and external pipes (nidirect). If the landlord does nothing about a hazard, you can contact an environmental health officer at the council, who can take action against the landlord (nidirect) | None set out in the sources cited here; check the tenancy agreement |
| Northern Ireland, Housing Executive or housing association tenant | Check your tenancy agreement for the landlord's repair responsibilities; Housing Rights gives free advice on repairs in Northern Ireland | None set out in the sources cited here |
| Leaseholder in a flat | The lease sets out whether you or the landlord (freeholder) is responsible for repairs (GOV.UK). Where the lease puts the structure and exterior on the freeholder or management company, report the defect to them or the managing agent | Depends on the lease |
If the landlord says it is condensation and does nothing
Ask, in writing, what inspection was done of the outside of the wall and the insulation, and for the finding in writing. If nobody looked outside, say so in a formal complaint: a landlord who has not inspected the exterior cannot reasonably rule out water coming through it. Keep photographing after rain. The escalation route for each nation is in the guide on a landlord ignoring mould.
Frequently asked questions
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