Black mould on a cold wall when the heating stays low.

If someone in the home is unwell

Government guidance for England, updated 1 April 2026, says damp and mould mainly affect the airways and lungs, and can also affect the eyes, the skin and mental health. Children, older adults, and people with certain health conditions are at greater risk of more severe effects. Anyone worried about symptoms should speak to a healthcare professional. In England, NHS 111 is the route when you need medical help now and it is not a 999 emergency. The same NHS page lists NHS 24 on 111 for Scotland, NHS 111 Wales, and nidirect for illness advice in Northern Ireland.

The same government guidance tells landlords not to wait for a medical letter before dealing with the mould. An owner should not wait for one either before deciding who to phone. This page does not say whether a cough is caused by the wall.

Why a cold wall grows mould this month

From 1 October to 31 December 2026, Ofgem says the price cap is 4% higher for a typical household on a default tariff who pay for electricity and gas by Direct Debit. On that tariff the average gas unit rate across England, Scotland and Wales is 7.97 pence per kWh, including VAT at 5%. Ofgem published the figures on 26 August 2026. A fixed tariff sits outside this cap. Northern Ireland is outside it. These are published averages, not a repair price and not a rate for one city.

A home that is heated less has colder outside walls. The Met Office said on 15 October 2026 that the weekend overnight would be particularly chilly, with a wide risk of frost in parts of the north, and that rain was more likely for parts of England and Wales from early the following week. Cold surfaces, plus moisture from cooking, showers and clothes dried indoors, are how condensation starts.

The England guidance calls condensation the most common form of damp: moisture from the home settling on colder parts of the building, such as window frames, corners, and low points on walls behind sofas or wardrobes. It also says homes that are insufficiently heated or insulated are more prone to that film on the inside of external walls and windows. Wiping the marks off leaves the cause in place. The guidance says simply removing surface mould does not stop it returning.

Read the wall before you book an injection

Black mould in a cold corner, on a window reveal, behind furniture on an outside wall, or on a bathroom ceiling, with no stain that tracks the rain, is the condensation pattern. A very small amount is the case the England guidance says a householder can usually clean with a mould and mildew product. It prefers that to bleach, and it says larger areas should be dealt with by a qualified professional. This page does not set out a cleaning method. If someone in the home is unwell, treat the patch as a reason to phone, including when it still looks small.

A smell of mould with nothing you can see can mean growth behind wallpaper, panelling or furniture. The same guidance says disturbing those surfaces can release a lot of spores, and to ask a trained professional if you are concerned. That is a reason to book a visit before you strip the wall.

Other patterns belong on other notes:

• A stain that darkens in the rain, high on a wall or across a ceiling, is often water getting in. Read a damp patch that appears after rain. A chemical damp course does not repair a gutter, a flashing or a roof.
• A tide mark near the floor that is still there after a dry week is the pattern people associate with moisture from the ground. A survey comes before an injection price. What a published treatment band leaves out is in what sits inside a price for damp treatment.
• Water you can see from a pipe, a bath or a shower is a plumbing job, on the plumbing and heating list.

If the boiler will not heat the house, the cold wall is downstream of that fault. Use a radiator that stays cold when the heating comes on, or a boiler that keeps losing pressure, before you treat the mould as the only job. An extractor that does not run is a ventilation fault. The England guidance classes installing ventilation as controlled building work. Phone an electrician on the electrical list. New plaster, on the plastering list, comes after the wall is dry.

Who you phone

You own the home

Phone a damp specialist when any of these are true:

• The mould is more than a very small patch, or it returns after the room has been heated and ventilated.
• You can smell mould and you cannot see where it is.
• You cannot tell this pattern from a leak or from a tide mark at floor level.

Ask whether the visit is a survey or a treatment. The published price guide prices them separately, and the per-metre figure leaves out plaster, decoration, drying and the survey. A listing on this site shows TrustMark only when the record already has it. You still ask what the visit includes. Open the damp proofing list, including London. You phone, email, or use the website on the record.

If the home is hard to keep warm, whether a grant is still open is a different page: the Warm Homes Local Grant for eligible private homes in England, and whether ECO4 is still a route for a new application. Those pages say what is closed. This one does not take an application.

You rent from a council or housing association in England

Report the mould to the landlord. Awaab’s Law has applied to social housing in England since 27 October 2025. The government collection says social landlords must investigate a potential significant hazard within 10 working days of becoming aware of it, give the tenant a written summary within 3 working days of the investigation ending, and do the relevant safety work within 5 working days of the investigation concluding if a significant hazard is found. An emergency hazard, which can include mould that is already having a material effect on health, has a shorter clock: the investigation and the safety work both within 24 hours of the landlord becoming aware. The landlord classifies the report. This site does not send it.

The same collection says phase 2 comes into force on 30 November 2026 and will add excess cold, among other hazards. On 16 October 2026 that extension is not in force. Damp and mould are already in the duty that started last October. Tenant guidance, first published the day the regulations took effect, is the place for routes of redress. A broken boiler can already be reported. Do not hold a mould report until the end of November.

You rent from a private landlord in England

Report the mould to the landlord, in writing, and keep a copy. The England guidance says private landlords also have duties that cover damp and mould, including the Homes (Fitness for Human Habitation) Act 2018. The day-counts in the section above are the social-housing rules. This page does not give a private tenancy its own timetable. If the heating has failed, report that as well as the mould.