The short answer
Hong Kong's mould season generally peaks around February to April, during 回南天 ("returning-south weather"), but the preparation that actually works happens earlier, in the drier November-to-January window — by the time walls are already sweating, you're usually already behind. Per our Hong Kong humidity tracker page, over the last 12 months average monthly humidity was 60.4% in November and 62.7% in January, then climbed to 76.1% in February, 78.1% in March, and 80.5% in April (May through July run even higher, above 81%). Preparation means: leaving an air gap behind wardrobes and mattresses, checking and resealing bathroom silicone early, cleaning air-conditioner filters, and airing out leather goods on dry days. This is practical, operational guidance, not an academic paper — you can always check the humidity tracker page for the current figures.
Why start in November instead of waiting for visible mould?
Mould needs both moisture and time to actually grow — a wall being damp for a day usually isn't a problem, but a wall staying damp for a week is when you start seeing mycelium. So effective mould prevention isn't reacting to beads of moisture on a wall in February — it's checking and treating the high-risk spots (behind wardrobes, north-facing walls, bathroom silicone) earlier, while humidity is still relatively low, from November through January. This isn't superstition, it's a timing game against how long mould actually takes to establish — get there first and it has one less foothold.
Prep timeline: what to do November through January
| Month | Humidity reference (trailing 12 months) | What to prep |
|---|---|---|
| November | ~60.4% monthly average | Leave an air gap behind wardrobes, mattresses and furniture; check window seals for wear |
| December | ~70.1% average (can rebound) | Don't assume the whole winter stays dry — watch for sudden humid spells, test-run the dehumidifier |
| January | ~62.7% monthly average | Check and reseal bathroom silicone; air out leather goods and books on dry days |
This is practical, operational guidance, not a weather forecast — that December figure of 70.1% is a reminder that even inside the "dry window," you'll still get humid spells. Don't run a fixed schedule by calendar month alone; watch the actual weather too.
Peak season, February to April: how to respond
The humidity tracker records monthly averages of 76.1% for February, 78.1% for March and 80.5% for April. During damp periods, watch for condensation, musty clothing and persistently wet areas, and check the moisture source promptly. Water-damaged areas and items should be dried within 24–48 hours; that is prevention guidance, not a way to diagnose mould severity. Adjust dehumidification and window opening to indoor and outdoor humidity and the weather. Recurring dampness, mould or suspected leakage warrants further assessment.
Which spots fail first?
- Behind wardrobes and under mattresses — pressed against the wall with no airflow, this is one of the earliest spots to show mycelium, especially against a north-facing wall.
- North-facing and corner walls — less sun exposure, lower surface temperature, so moisture condenses here first; visible beads of moisture inside the home usually start on these walls.
- Bathroom silicone and grout — persistently damp, poor ventilation, and a direct breeding spot; once silicone goes black, surface cleaning rarely restores it fully.
- Air-conditioner filters and drip pans — condensation plus dust is a near-perfect mould bed, and a dirty filter blows the musty smell and spores straight into the room.
- Leather goods, books and cardboard boxes — these organic materials are literally mould's food source, and long-term storage in a sealed wardrobe or under a bed is especially high-risk.
Dehumidifying and ventilating: what actually works
A dehumidifier isn't just "switch it on and forget it" — placement and habits matter more. Put it in the middle of the room or near the high-risk spot (wardrobe, bathroom), not wedged in a corner where furniture blocks the airflow, and empty the auto-stop water tank daily rather than letting it sit full. On ventilation, close the windows during 回南天 or rainy stretches — outdoor humidity is usually higher than indoor at that point, so opening up only lets more moisture in; save cross-ventilation for the rare sunny, low-humidity day. An air conditioner's dry mode also helps during peak season, but only if the filter is cleaned regularly — otherwise you're just blowing a musty smell around the room.
Early warning signs and the DIY-vs-professional line
Early warning signs are usually small — a light bead of moisture on a wall, a musty smell in clothing without visible mould yet, silicone starting to discolour at the edge. These are usually manageable with surface cleaning (diluted alcohol or bleach, plus stepped-up dehumidifying). But if the same spot comes back within a month, the area keeps expanding, or you suspect a leak rather than plain humidity, spraying a cleaning product yourself usually isn't going to solve it — that's when it's worth having someone check the source. Worth noting: mould-friendly conditions overlap almost exactly with peak season for damp-loving pests like booklice — if you're also seeing tiny pale, fast-moving bugs around the house, see our complete booklice guide.