When the same patch of wall grows mould every winter, the cause is often the wall itself. Here is how to find a cold spot and what to do about it, in order.
A cold spot, or cold bridge, is a part of a wall where heat escapes faster than through the surface around it, such as a lintel, a window reveal or a missing patch of insulation. Moisture in room air condenses there first, so the mould returns to the same place. Warm the room steadily, cut the moisture, let air reach the wall and ask a surveyor if it keeps returning.
Mould that returns to the same few places is a clue. BRE describes cold spots, also called thermal bridges, as places where heat escapes faster through the wall, commonly at lintels, window reveals, gaps in insulation or mortar bridging a cavity. Look in these places on a cold morning.
Press the back of your hand against the spot and then the wall beside it. A surface that feels clearly colder, with beads of moisture or a dark bloom, is a cold spot. A patch that is wet to the touch even on a mild, dry day is a different problem, so see rising, penetrating or condensation damp.
GOV.UK guidance on damp and mould lists window frames, corners and low points behind sofas and wardrobes as the places where moisture condenses on colder surfaces, which matches what you find in practice.
A cold spot only grows mould if the room air is humid enough to condense on it. Run extractors during and after showers and cooking, keep lids on pans and dry washing somewhere ventilated. A cheap hygrometer will show whether the room stays humid all day. BRE notes that mould is a risk at a cold spot when ventilation is poor, such as behind furniture or pictures. See our guides to extraction and drying washing indoors.
If the same spot keeps returning after rungs 1 to 3, ask a surveyor to look at the construction detail. A cold bridge at a lintel or reveal is a building fix, and insulation added in the wrong place can push the cold surface and the moisture elsewhere. If indoor humidity then still runs high in an owner-occupied home with a loft, a loft-mounted dehumidifier such as Dryzen is one option. Each unit covers 150 square metres and extracts 12 litres a day, with an activated carbon filter. It is not suitable for rentals unless the landlord authorises the changes, it does nothing for a leak, and it does not remove a cold bridge.
Medway Council's conservation area appraisal for New Road in Chatham describes the area as densely settled and made up entirely of terraced houses, with all buildings of architectural or historic interest and many listed. The appraisal mentions gauged brick lintels over the windows of these terraces, and lintels and window reveals are exactly the places cold spots form, so look there first. If your home is listed or in a conservation area, check with the council before altering walls or windows. See our pages for Rochester and Brompton for how this plays out nearby.
It is a spot where heat escapes through the wall faster than through the wall around it, for example at a lintel above a window, in a window reveal or where insulation is missing. The inside surface there stays colder, so moisture in the room air condenses on it and mould can follow.
Look at timing. A cold-spot patch tends to appear in cold weather, often at a corner, reveal or strip above a window, and eases when the room warms. A leak keeps growing after rain, or stays damp in mild weather. BRE describes a case where a wall patch looked like a leak but was condensation on a thermal bridge.
Steady background heat warms the surface and helps, but it does not remove the cold bridge. Cutting indoor moisture and letting air reach the wall matter just as much. If the same spot keeps coming back, ask a specialist to look at the construction detail.
Be careful. Insulation added in the wrong place can move the cold surface and the moisture to another point. For anything beyond a small fix, ask a competent surveyor or installer to check the whole wall first.
Sources: BRE: Condensation can be mistaken for damp; Property Care Association: Condensation in homes; GOV.UK: Understanding and addressing the health risks of damp and mould in the home; Medway Council: New Road Chatham conservation area appraisal
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