Rule out leaks and building defects before treating the surface
Condensation is caused by moisture meeting a cold surface, but similar-looking damp patches can come from plumbing leaks, roof defects or penetrating damp. Fix the source before repainting or using mould-resistant products.
MOST LIKELY CAUSES
Start with moisture, airflow and surface temperature
Symptom variations Match the exact location, timing and moisture pattern. ⌄
- Damp patches on external walls
- Black mould in corners
- Moisture behind furniture
- Problem worse in winter
- Wall feels cold but not actively leaking
Quick checks Simple checks before making repairs. ⌄
- Check whether the moisture appears after cooking, showering or overnight occupancy.
- Measure indoor humidity if possible.
- Check extractor fans, trickle vents and airflow paths.
- Look for cold spots, insulation gaps and blocked surfaces.
- Rule out plumbing leaks, roof leaks and penetrating damp.
Step 1 — Check whether the patch follows a cold area Reduce moisture, improve airflow and rule out leaks before treating finishes. ⌄
Look for corners, lintels, external walls and areas behind furniture.
Step 2 — Improve airflow Reduce moisture, improve airflow and rule out leaks before treating finishes. ⌄
Move furniture slightly away from walls and avoid blocking vents.
Step 3 — Reduce humidity and keep heating steady Reduce moisture, improve airflow and rule out leaks before treating finishes. ⌄
Use extraction and avoid large temperature swings.
Why condensation forms How this moisture pattern develops. ⌄
Condensation on walls usually appears where the surface is colder than the surrounding room, especially at corners, external walls and behind furniture.
Advanced checks Further diagnosis for recurring or severe condensation. ⌄
- Compare surface and room temperatures.
- Measure humidity over several days.
- Inspect insulation continuity.
- Check for penetrating damp or plumbing leaks.
- Assess thermal bridging around openings and structural elements.
Tools and information needed Useful items for accurate diagnosis. ⌄
- Digital humidity meter
- Surface-temperature thermometer
- Extractor and ventilation checks
- Moisture meter used with care
- Inspection of insulation, seals and airflow paths
Parts or improvements that may help Common items linked to this problem. ⌄
- Humidity meter
- Thermal imaging assessment
- Insulation materials
- Extractor fan
- Mould-resistant finish after the cause is corrected
When to call a professional Conditions that should end basic DIY diagnosis. ⌄
- The damp patch grows regardless of occupancy or weather.
- There is visible roof, plumbing or external water ingress.
- Timber, plaster or insulation is saturated.
- Large areas of mould keep returning.
- Loft timbers show decay or persistent wetting.
- Insulation or ventilation changes need proper design.
Frequently asked questions Common condensation questions. ⌄
Will a dehumidifier solve condensation?
It can reduce indoor humidity, but the underlying ventilation, heating or insulation problem may still need attention.
Should windows be opened all day?
Usually not. Short, effective ventilation and working extraction are often more practical than leaving windows open continuously.
Is black mould proof of a leak?
No. Black mould often develops on cold surfaces with persistent condensation, though leaks should still be ruled out.