Do not block required ventilation without understanding its purpose
Background vents, extract systems and combustion-air openings may be essential. Improve comfort by balancing or redirecting airflow rather than sealing vents completely.
MOST LIKELY CAUSES
Start with moisture sources, airflow and temperature
Symptom variations Match the humidity, airflow and timing pattern. ⌄
- Room becomes stuffy
- Windows wet in morning
- Mould worsens in winter
- Heat lost with windows open
- Humidity spikes after bathing or cooking
Quick checks Simple checks before replacing equipment. ⌄
- Measure humidity with a digital hygrometer.
- Check extract fans are moving air outdoors.
- Inspect visible grilles, filters and duct routes.
- Note whether doors and windows affect airflow.
- Check for hidden leaks or persistently damp materials.
Step 1 — Use short periods of wide window opening Control moisture at source, confirm real airflow and keep surface temperatures stable. ⌄
Ventilate effectively for several minutes rather than leaving a small gap all day.
Step 2 — Use extractors at source Control moisture at source, confirm real airflow and keep surface temperatures stable. ⌄
Run kitchen and bathroom fans during and after moisture-producing activities.
Step 3 — Keep background vents open Control moisture at source, confirm real airflow and keep surface temperatures stable. ⌄
Use trickle vents and maintain a steady indoor temperature.
Why the problem happens How moisture, airflow and temperature interact. ⌄
Winter ventilation works best through short, effective air changes, functioning extractors and background vents rather than leaving windows slightly open all day.
Advanced checks Further diagnosis for persistent humidity or poor airflow. ⌄
- Monitor humidity before and after ventilation.
- Check extractor run-on time.
- Assess cross-ventilation routes.
- Review heat-recovery ventilation where appropriate.
- Balance comfort, air quality and moisture control.
Tools and information needed Useful items for accurate checks. ⌄
- Digital hygrometer
- Airflow or tissue test
- Fan and duct specifications
- Room temperature readings
- Inspection for leaks, damp and blocked vents
Parts or improvements that may help Common items linked to this problem. ⌄
- Digital hygrometer
- Trickle vents
- Extractor fan
- Timer or humidistat control
- Mechanical ventilation with heat recovery where suitable
When to call a professional Conditions that should end basic DIY diagnosis. ⌄
- Required ventilation or combustion air may be affected.
- Fans need electrical or duct alterations.
- Humidity stays high despite normal ventilation.
- Hidden water ingress or damp is suspected.
- Whole-house ventilation needs balancing or redesign.
- Mould or condensation is widespread and recurring.
Frequently asked questions Common ventilation and humidity questions. ⌄
Should vents be closed in winter?
Usually not. Background ventilation helps control moisture. Draughts should be managed with suitable vent design or balancing.
Is 70% humidity too high?
Persistent levels around 70% are high enough to increase condensation and mould risk, especially on cold surfaces.
Will a dehumidifier replace ventilation?
No. It can help control moisture, but it does not provide fresh air or remove pollutants in the same way ventilation does.