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VENTILATION & HUMIDITY

Best Humidity Level for a Home

For many homes, indoor relative humidity around 40% to 60% is a practical target. Persistent levels above this increase condensation and mould risk, while very low levels may feel dry.

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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

Around 40–60% RH This range is generally comfortable and reduces condensation risk.
Above 60–65% RH Mould and dust-mite risk rises if levels remain high.
Below roughly 35–40% RH Air may feel dry, especially in heated winter rooms.
Room temperature matters The same humidity can condense on colder surfaces.
◫ Symptom variations Match the humidity, airflow and timing pattern. ⌄
  • Windows misting
  • Static or dry throat
  • Mould in corners
  • Humidity changes by room
  • Levels spike after showers
✓ 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 — Use a reliable hygrometer Control moisture at source, confirm real airflow and keep surface temperatures stable. ⌄

Measure at breathing height away from radiators and windows.

2 Step 2 — Look at trends, not one reading Control moisture at source, confirm real airflow and keep surface temperatures stable. ⌄

Check morning, evening and after moisture-producing activities.

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

Aim for steady conditions rather than chasing a single exact number.

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

For many homes, indoor relative humidity around 40% to 60% is a practical target. Persistent levels above this increase condensation and mould risk, while very low levels may feel dry.

⌘ Advanced checks Further diagnosis for persistent humidity or poor airflow. ⌄
  1. Compare humidity between rooms.
  2. Check surface temperatures at cold bridges.
  3. Review overnight bedroom levels.
  4. Assess extractor performance.
  5. Investigate persistent high readings despite normal ventilation.
◇ 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
  • Heating and insulation improvements
! 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.