Every home draws in outside air — through a dedicated fresh-air unit, through the AC's fresh-air connection, or by infiltration around doors and windows. In Dubai that air is often 35–45 °C and 50–90% humidity. If it is delivered without being properly cooled and dehumidified first, it dumps a large moisture load into the space that the room AC then has to fight — and often loses to, leaving humidity high.
Key takeaways
- Dubai fresh air is a major moisture source before it is treated.
- An oversized or untreated fresh-air supply keeps indoor humidity high.
- Negative pressure pulls extra humid air in through every gap.
- Balancing and pre-treating fresh air is an FM/HVAC task.
The fresh-air moisture load
Ventilation standards require a continuous supply of outdoor air for health and odour control. The energy and moisture in that air have to be removed somewhere. In a well-designed system a dedicated unit cools and dehumidifies the fresh air close to room conditions before it enters. Where that treatment is missing, undersized or switched off, the raw humid air enters the rooms and the split units or FCUs cannot keep up.
Pressure and infiltration
If a home extracts more air (kitchen hood, bathroom fans, a strong exhaust) than it supplies, it goes slightly negative and pulls make-up air in through every gap around doors, windows and service penetrations — unfiltered, untreated and humid. Running powerful extract fans continuously in a tightly sealed apartment can quietly raise humidity for this reason.
Signs fresh air is the issue
- Humidity stays high across the whole apartment, not just one room, despite the AC working.
- It is worst near a fresh-air grille or an entrance door.
- You feel a warm, humid draught from a supply diffuser that should be delivering cool air.
- Condensation appears on the fresh-air ductwork or grille itself.
What can be done
This is largely a building-services matter. An FM or HVAC engineer can check that the fresh-air unit is operating and its cooling coil is working, that the supply volume matches the design, and that the building is not running negative. Residents can help by not running all extract fans continuously, keeping external doors closed, and reporting a humid supply draught. See humid even with the AC on.
If your whole apartment reads above 60% humidity and the room AC is clearly working, look at how fresh air is being brought in — that is often where the moisture is coming from.