The AC runs all day, the room is cold, and the hygrometer still says 65%. Eight causes explain it; most homes have two or three at once.
The eight causes
- An oversized AC that short-cycles. It cools the room in minutes and stops before it has condensed much water.
- Supply air that carries moisture. In a Dubai inspection reported by The National, the rooms read 64 to 70% and the air leaving the AC supply vent read 88.5%.
- A wet coil or blocked drain. Water sits in the unit and evaporates back into the airflow.
- Sealed glazing. Double-glazed high-rise units hold shower, cooking and laundry moisture indoors.
- Night-time humidity. Forecasts of 45% by day hide 60% overnight, when the AC also cycles less.
- Doors and windows left open to the balcony or garden in summer.
- Leaky ducts. Older villas may have cracks in ductwork or walls.
- The AC switched off for the day or a holiday: residents came back to mouldy homes.
How to measure it
Put a hygrometer at waist height in the bedroom, away from the vent. Read it at dawn and before bed for a week. See how to measure humidity at home.
What does not fix it
A colder set-point. The AC still stops at the temperature you choose, and colder surfaces sweat more.
The fix that holds 50%
Fix the drain and coil, close the envelope, stop drying laundry indoors. Then let a dehumidifier hold the set-point the AC cannot. See high indoor humidity.
Key points
- An oversized AC cools fast and short-cycles, so it never runs long enough to dry the air.
- Supply air can carry moisture: one Dubai tenant's inspection measured 88.5% RH at the vent.
- Sealed glazing keeps shower, cooking and laundry moisture indoors; humidity peaks overnight at about 60%.
- The fix is a dedicated dehumidifier controlled to 50% RH, not a colder AC setting.
Where this applies
