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. ⌄
- Water on inside of glass
- Puddles on sill
- Mould around frames
- Condensation worse overnight
- Moisture trapped between panes
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 — Confirm where the moisture is forming Reduce moisture, improve airflow and rule out leaks before treating finishes. ⌄
Condensation on the room side differs from moisture between double-glazed panes.
Step 2 — Reduce moisture at source Reduce moisture, improve airflow and rule out leaks before treating finishes. ⌄
Use extraction, lids on pans and avoid drying clothes indoors without ventilation.
Step 3 — Improve airflow and surface warmth Reduce moisture, improve airflow and rule out leaks before treating finishes. ⌄
Open trickle vents, keep heating steady and allow air to circulate around the frame.
Why condensation forms How this moisture pattern develops. ⌄
Condensation forms when warm, moisture-laden air meets a cold window surface. Persistent moisture usually points to high indoor humidity, weak ventilation, low surface temperature or a combination of all three.
Advanced checks Further diagnosis for recurring or severe condensation. ⌄
- Measure indoor relative humidity.
- Check extractor performance.
- Inspect seals and drainage around the frame.
- Assess glazing performance and failed sealed units.
- Check whether heavy curtains or blinds are trapping moisture.
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
- Replacement extractor fan
- Trickle vent
- Window seal or gasket
- Replacement sealed glazing unit
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.