What is a cold bridge and why does it cause damp?

This is the most misdiagnosed damp problem in British homes. It gets treated as a leak, a rising damp problem or a cleaning failure, when it is a temperature problem with a ventilation component.

Step by step

1. Recognise the pattern.Same place every winter, worst in cold weather, dry through summer, often behind furniture or in a corner. That seasonality is the signature — a leak does not care what month it is.
2. Know where they occur.External corners, window and door reveals, lintels, the junction where a floor meets an outside wall, steel beams, balcony slabs, and anywhere insulation stops or was never continuous.
3. Understand the mechanism.Air holds moisture in proportion to its temperature. Against a cold surface it cools below its dew point and the moisture comes out as liquid. Mould needs about 80% humidity at the surface, which happens long before you see water.
4. Confirm it with a thermometer.An infrared thermometer costs very little. Compare the suspect spot with the middle of the same wall on a cold day. A few degrees colder confirms it and tells you exactly where the bridge is.
5. Rule out the alternatives.A leak stains and spreads. Rising damp shows a tide mark low down with salts. Penetrating damp tracks to a defect outside. Cold-bridge mould is surface-only and wipes off to reveal sound plaster.
6. Reduce the moisture in the air first.It is the cheapest half of the fix. Extractor fans that run long enough, lids on pans, drying washing outdoors or in a ventilated room, and trickle vents left open.
7. Get air moving over the cold surface.Furniture hard against an outside wall creates a still, cold pocket — that is why mould appears behind wardrobes. A 50mm gap makes a real difference for no cost.
8. Warm the surface rather than the room.Intermittent bursts of high heat leave surfaces cold. A lower, steadier background temperature keeps the surface above the dew point, which is what stops condensation.
9. Insulate the bridge where you can.Insulated plasterboard on a reveal, insulating the lintel, or continuing loft insulation right over the wall plate. Small targeted work often solves it.
10. Avoid the trap of insulating around it.Insulating a wall but not the reveals concentrates the problem at the reveals and makes the mould worse. Continuity matters more than thickness.
11. Treat the mould, but expect it back.Clean it off with an appropriate product. If the surface still runs cold and the air stays damp, it will return — cleaning is not the fix.
12. Get advice for whole-house cases.Widespread cold bridging, or mould after external wall insulation was fitted, needs a proper assessment. Badly detailed insulation work causes exactly this.

Tips

• Same corner, every winter, dry in summer. That trio is a cold bridge and nothing else.
• An infrared thermometer turns a guess into a measurement, and costs less than a tin of anti-mould paint.
• Pull furniture 50mm off outside walls. Free, and it clears a surprising proportion of bedroom mould.

Common mistakes

• Treating it as a leak and chasing the outside wall — The seasonality gives it away. Cold-bridge condensation is surface moisture from inside air, so there is no external defect to find.
• Sealing the room up to keep the cold out — It raises indoor humidity, which is the other half of the equation. Ventilation is part of the fix, not the problem.

If it doesn't work

Questions people ask