How to keep a seven-day humidity diary before choosing ventilation

Set up a repeatable seven-day snapshot

Place reliable temperature and relative-humidity sensors in representative rooms, away from direct sun, open windows, heaters and wet surfaces. A bedroom, living area and the most affected room usually reveal more than moving one sensor throughout the house.

Record morning, late afternoon and bedtime readings for seven days. Also note a reading before and after major moisture events such as showering, cooking, floor washing or drying laundry indoors.

• Room and exact sensor position
• Temperature and relative humidity
• Windows, extract fans, air conditioning or dehumidifier status
• Occupancy, showering, cooking and indoor laundry

Add the pattern the sensor cannot see

Photograph condensation, mould marks and peeling from the same positions. Note recent rain, strong coastal humidity, a closed-house period and whether furniture is tight against an external wall. A low room reading does not exclude a cold surface, a hidden leak or a local damp patch.

EPA guidance recommends keeping indoor humidity below 60 percent and, where possible, in the 30–50 percent range. Use that as general context rather than a pass-or-fail test for one moment.

Turn the diary into a decision brief

Look for repeated peaks, rooms that stay elevated longest and whether extract or open windows actually shorten recovery. If outdoor air is warm and humid, opening more windows may add moisture rather than remove it; a professional should consider dew point and the building, not only relative humidity.

Send the readings with a simple room plan and photographs. This helps decide whether to investigate a leak, improve local extract, plan whole-house air movement, add dehumidification or examine cold surfaces and insulation.