Here is the mechanism in plain terms, what you can do about it this week, and what actually cures it.
The dew point, plainly
Warm air carries moisture invisibly; the warmer it is, the more it can hold. Cool a parcel of that air and at some temperature — the dew point — it can no longer hold what it is carrying, and the excess condenses as liquid. Air at a comfortable 20°C and typical winter indoor humidity has a dew point of roughly 12°C. So any surface in the room colder than about 12°C becomes a collection plate.
Solid brick walls — standard in Oxford housing built before the 1930s — conduct heat out roughly five times faster than an insulated wall, so on a cold night their inner faces sink well below that threshold. The wettest spots are the coldest spots: north-facing walls, upper corners where two cold surfaces meet, window reveals, and the wall behind the wardrobe, where furniture blocks warm room air from reaching the plaster at all. Mould follows within weeks, because mould needs nothing more than a persistently damp surface.
One distinction worth making before spending anything: condensation sits on the surface at cold spots and grows mould; rising damp leaves a tide mark low on ground-floor walls; penetrating damp maps to a defect outside — a gutter, a cracked sill. Cold-wall mould in bedroom corners is condensation, and no injected damp-proof course will touch it.
Immediate mitigations — this week, little cost
None of these cure the wall, but together they narrow the gap between surface temperature and dew point:
- Heat low and steady rather than in short blasts. Long cold gaps let wall surfaces sink far below the dew point; the plaster never catches up during a quick evening blast of heating.
- Produce less airborne moisture. Lids on pans, bathroom door shut with the window cracked or fan running after showers, and no clothes drying on radiators — an indoor washing load releases litres of water into the air with nowhere to go.
- Ventilate deliberately. Use extractor fans every time you cook or bathe, and give the house short purge airing rather than a permanently cracked window that chills the walls.
- Pull furniture away from external walls. A 50 mm gap lets room air warm the plaster behind wardrobes and beds — the classic hidden mould site.
- Clean mould off properly with a suitable fungicidal wash so you can see whether it returns, and where.
If the mould stops returning, mitigation may be enough for that room. If it comes back every cold snap, the wall itself is the problem.
The fabric answer
The cure is to make the wall’s inner face too warm to condense on — which is an insulation job, paired with ventilation:
Warm the wall. Internal wall insulation lifts the surface from cold-brick temperatures to within a degree or two of room temperature, removing the condensation mechanism entirely. On solid-wall Oxford houses we typically fit wood-fibre boards finished in lime plaster — a build-up chosen because it is vapour-open and hygroscopic, buffering day-to-day moisture swings rather than sealing them behind an impermeable layer. On an 1880s North Oxford Victorian our specification took the walls from a U-value of 2.2 W/m²K to a target of 0.39; the same specification carried the insulation around party-wall junctions and reveals, because any strip left cold becomes the room’s new condensation point.
Move the moisture out. A better-insulated house is a more airtight house, so extraction must be upgraded in the same breath. We install humidity-controlled continuous ventilation — quiet extract units in kitchens and bathrooms that run gently all the time and boost themselves automatically when moisture spikes — as standard on our whole-house retrofits, from Victorian semis to a 1920s conservation-area house in Iffley. The system, not the occupants’ discipline, manages the moisture.
That pairing — warm surfaces plus managed air change — is why properly retrofitted houses do not get winter mould, rather than merely getting less of it.
When to call a professional
Call us in when mould returns despite honest mitigation; when it is spreading across ceilings and multiple rooms rather than one corner; when a tide mark or an outside defect makes the diagnosis uncertain; or when you are planning insulation anyway and want the wall build-up and the ventilation designed as one system. A free survey with a thermal camera will show exactly which surfaces are falling below the dew point — the cold corners light up on screen — and our cold-room diagnosis guide explains what we look for. If you are a landlord, note that persistent damp and mould is also a housing-standards issue: the fabric cure and the EPC C deadline can be dealt with in one programme of works.