Why did mould appear in cold corners after insulation?

Mould in a corner after insulation can mean moisture is condensing on a surface that remains colder than the rest of the room. The work may also have reduced the air that used to pass through gaps and cracks. Those are possibilities to investigate, not proof that the insulation is defective. Check the cold spot and the home's ventilation before arranging more work.

Why can a corner stay cold?

Condensation forms when moisture in indoor air cools on colder parts of a building. Corners, window frames and low sections of wall behind furniture are examples. Insulating a broad wall surface does not by itself tell you how warm every nearby detail has become. A colder junction can remain a place where moisture gathers, even while the main wall feels warmer.

People often call a colder junction a thermal bridge. A gap in insulation is one possible reason for a particular cold spot, but appearance alone cannot establish whether a gap exists or where it is. Record which surface is affected and ask for the completed insulation and nearby junctions to be assessed. That is more useful than assuming the visible mould marks the precise position of a hidden defect.

Could ventilation have changed?

Energy improvements often seal gaps and cracks that previously let air through. If the home's ventilation is not considered alongside that change, moisture from normal activities can remain indoors and condense on cold surfaces. The government's ventilation guide says energy efficiency measures must not make ventilation worse. This is a reason to review the whole room, not to blame the people living in it for cooking, washing or bathing.

Check whether kitchen and bathroom extractors work, whether vents are blocked and whether the ventilation arrangement has changed since the retrofit. Make sure the people using the room know how its vents and fans operate. If ventilation is inadequate, it needs attention alongside any cold area. Adding another layer of insulation without understanding the air movement may leave the moisture problem unresolved.

What should you check before further work?

• Where exactly does the mould appear: corner, window reveal, low wall or behind furniture?
• Which insulation areas and junctions were changed, and are there visible cold spots?
• Are extractors and vents working and unobstructed?
• Is there another source of damp that needs to be ruled out?

These observations give an assessor a clearer starting point. Damp and mould can have more than one underlying cause, and professional advice can help avoid misdiagnosis. If the affected area has reappeared after earlier work, that is also useful to report. Surface cleaning alone does not identify the building or ventilation issue that allowed mould to return.

Cold reveals in a Bradford, Yorkshire terrace

In a retrofitted Bradford terrace, the insulated wall may feel comfortable while a window reveal or joist junction still feels noticeably colder. Treat that difference as a prompt to look at the junction and the room's ventilation. It does not tell you that insulation is missing behind the finish. Note the exact location of the mould and what changed during the work so a surveyor can assess the pattern in context.

Choose the next repair from the cause

Ask an independent, suitably qualified surveyor to investigate if the cold spot persists, mould returns or the cause remains uncertain. They can consider the insulation, ventilation and other damp routes together. Any further retrofit proposal should explain how it deals with the colder detail and preserves adequate ventilation. The useful answer is a diagnosis of this room, not a promise that one more material will fix every mould patch.

For a rented home, a clear report of where and when the mould appeared helps the person responsible for the property assess it. The same record helps an owner discuss the work with a surveyor or installer. Keep the emphasis on the condition of the building and the functioning of its ventilation. Normal household moisture has to be managed by a home that works as a whole.

This page gives general information, not legal or medical advice. For your own situation, use the organisations listed under "Where this comes from" at the foot of the page.