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Black Mould on Cold Walls in a Flat in Edgware HA8
A flat at Middlesex House in Edgware had black mould on its walls. The surveyor described the cause as a perfect storm of cold walls, high humidity and switched off ventilation.
Project at a Glance
The Brief
The occupants wanted the mould dealt with and to stop it coming back.
The Survey
The mould was caused by cold wall surfaces, because outside ground levels sit about 1.5 metres high against the walls, combined with 80 percent humidity, inconsistent heating and ventilation that had been turned off.
Because the ground outside could not be dug away, the property remains at high risk, so internal works and changes to how the flat is heated and ventilated were the only practical route.
The Solution
The report recommended:
- A professional mould removal clean with a biocidal solution.
- A penetrating damp treatment: hack off, salt additive, waterproof render and skim.
- Keeping ventilation on and heating more consistent.
The Work
The damp proofing and electrical work was carried out on 24 and 25 October 2026.
The Result
Cleaning, treating the affected walls and restoring ventilation tackle the conditions that let mould grow. See our page on damp proofing in Edgware.
Frequently Asked Questions
Why do walls below ground level get mould?
Earth outside keeps the wall cold, so moisture in the air condenses on it and mould follows.
Is cleaning mould enough?
Cleaning removes it, but humidity, heating and ventilation must change or it returns.
Should I turn off ventilation to save heat?
No. Switching ventilation off traps moisture and is a common cause of mould.
Black mould in a flat?
Our CSSW and CSRT trained surveyors find the real cause before recommending any work. Non-invasive survey with a clear written report. No obligation.
Survey carried out by Michael Young at Advanced Damp. Project completed in Edgware, HA8.
Sam Fitzgerald is the founder of Advanced Damp and a working damp and structural waterproofing surveyor diagnosing and treating moisture problems in residential and commercial properties across London and the South East. He is FODCS certified (a Federation of Damp Certified Surveyor) and a member of the Federation of Damp. His hands-on work covers rising damp, penetrating damp, condensation, basement waterproofing to BS 8102, and thermal imaging surveys, and he has personally completed over 400 damp surveys across properties of every type and age. Everything he writes draws directly from jobs he has carried out. He does not write about treatments, products or causes he has not dealt with first-hand in the field.
