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VENTILATION & HUMIDITY

Room Humidity Too High

High indoor humidity usually comes from moisture-producing activities combined with weak ventilation, cold surfaces or hidden water ingress.

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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

Cooking, bathing and drying clothes Normal household activities can release large amounts of moisture.
Poor ventilation Moist air remains trapped instead of being removed.
Low or uneven heating Cool surfaces reach dew point more easily.
Hidden damp or leak Water ingress can keep humidity elevated even with ventilation.
◫ Symptom variations Match the humidity, airflow and timing pattern. ⌄
  • Windows wet in morning
  • Musty smell
  • Mould in corners
  • Humidity stays above 65%
  • Rooms feel clammy
✓ Quick checks Simple checks before replacing equipment. ⌄
  1. Measure humidity with a digital hygrometer.
  2. Check extract fans are moving air outdoors.
  3. Inspect visible grilles, filters and duct routes.
  4. Note whether doors and windows affect airflow.
  5. Check for hidden leaks or persistently damp materials.
Important: A noisy fan is not proof of useful airflow. Check actual extraction at the grille.
1 Step 1 — Measure humidity over time Control moisture at source, confirm real airflow and keep surface temperatures stable. ⌄

Use a digital hygrometer in different rooms and at different times.

2 Step 2 — Control major moisture sources Control moisture at source, confirm real airflow and keep surface temperatures stable. ⌄

Use lids on pans, extract during bathing and avoid unvented indoor drying.

3 Step 3 — Improve ventilation and steady heating Control moisture at source, confirm real airflow and keep surface temperatures stable. ⌄

Use working extractors, trickle vents and avoid large temperature swings.

⌁ Why the problem happens How moisture, airflow and temperature interact. ⌄

High indoor humidity usually comes from moisture-producing activities combined with weak ventilation, cold surfaces or hidden water ingress.

⌘ Advanced checks Further diagnosis for persistent humidity or poor airflow. ⌄
  1. Check extractor airflow.
  2. Compare humidity before and after activities.
  3. Inspect for leaks or damp materials.
  4. Review whole-house ventilation routes.
  5. Assess whether dehumidification is only masking a building defect.
◇ 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
  • Extractor fan
  • Trickle vents
  • Dehumidifier where appropriate
  • Leak or damp repair materials
! 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.