How to Fix Window Condensation in Hong Kong Apartments
If you live in Hong Kong, you have almost certainly woken up to find water droplets streaming down your windows. Window condensation is one of the most common household nuisances in the city, particularly between February and April when the humid spring air meets cool glass surfaces. Left untreated, condensation leads to mould growth, peeling paint, rotting window frames, and even health problems for residents with allergies or asthma. The good news is that condensation can be managed — and in many cases eliminated — with the right combination of strategies.
Why Windows Condensate in Hong Kong
Condensation forms when warm, moisture-laden air comes into contact with a surface that is below the dew point temperature. In Hong Kong, relative humidity regularly exceeds 90% during spring and summer. When you run your air conditioner, the interior glass surface cools well below the dew point of the outdoor air, and moisture condenses on the outer surface. Conversely, in winter and early spring, unheated indoor air can be cool enough for interior condensation to form, especially on north-facing windows that receive no direct sunlight.
Several factors make Hong Kong apartments particularly prone to condensation:
- Small, sealed spaces — Hong Kong flats are compact and often have limited natural ventilation. Cooking, showering, and even breathing add significant moisture to a small volume of air.
- Single-glazed aluminium windows — The vast majority of Hong Kong residential buildings use single-glazed aluminium-framed windows. Aluminium is an excellent conductor of heat, meaning the frame and glass quickly reach the temperature of the outside air, creating an ideal condensation surface.
- Tile and concrete construction — These materials absorb and release moisture slowly, contributing to elevated indoor humidity levels, especially in newly renovated flats where concrete may still be curing.
- Proximity to the sea — Coastal districts such as Sai Kung, Tseung Kwan O, Discovery Bay, and Aberdeen experience higher ambient humidity than inland areas.
Single Glazing vs Double Glazing
The single most effective long-term solution for interior condensation is upgrading from single-glazed to double-glazed (or even triple-glazed) windows. Double glazing works by creating an insulating layer of air or argon gas between two panes of glass. This keeps the interior glass surface closer to room temperature, well above the dew point, so condensation cannot form.
In Hong Kong, double glazing is still relatively uncommon in older residential buildings, but it is increasingly specified in new developments. Retrofitting double-glazed windows in an existing flat typically costs between HK$3,000 and HK$8,000 per window, depending on size, frame material, and whether the existing frame can be reused. Aluminium frames with a thermal break are recommended to prevent condensation on the frame itself. If you live in a building managed by an Owners' Corporation, check your DMC before replacing windows, as external alterations often require approval.
For tenants or owners who cannot replace their windows, secondary glazing — a separate pane fitted inside the existing window — offers a more affordable alternative at around HK$1,500 to HK$3,500 per window. Magnetic secondary glazing kits are available from local suppliers and can be removed without damaging the original frame.
Dehumidifiers: Your First Line of Defence
In Hong Kong's climate, a dehumidifier is virtually a household essential. By reducing indoor relative humidity to between 50% and 60%, you raise the dew point threshold and make condensation far less likely. When choosing a dehumidifier for a Hong Kong flat, consider:
- Capacity — For a typical 400–600 sq ft flat, a unit rated at 12 to 20 litres per day is sufficient. Larger village houses or ground-floor units may need 25 litres or more.
- Energy efficiency — Look for units with Grade 1 energy labels under the EMSD's Mandatory Energy Efficiency Labelling Scheme. Compressor-based dehumidifiers are far more efficient than desiccant types in Hong Kong's warm climate.
- Continuous drainage — Models with a gravity drain hose can be left running continuously without emptying the tank, which is ideal during the wettest months.
- Noise level — If you plan to run the unit overnight in a bedroom, check decibel ratings. Units below 40 dB are considered quiet.
Ventilation Strategies
Proper ventilation removes moisture-laden air before it can condensate. In Hong Kong, this requires a balanced approach because opening windows during humid weather can actually make things worse. Effective strategies include:
- Use exhaust fans — Always run the exhaust fan during and for at least 20 minutes after cooking or showering. If your bathroom lacks a window, ensure the exhaust fan vents to the outside, not just into a ceiling void.
- Cross-ventilation on dry days — When relative humidity drops below 70% (check the Hong Kong Observatory website or app), open windows on opposite sides of the flat to create cross-ventilation.
- Trickle vents — Some modern windows include trickle vents that allow a small, controlled flow of air even when the window is closed. These can be retrofitted to existing aluminium frames.
- Mechanical ventilation — For internal rooms with no windows, a positive input ventilation (PIV) unit or a heat recovery ventilation (HRV) system can introduce filtered, drier air. These are increasingly popular in Hong Kong renovations.
Anti-Condensation Film
Anti-condensation window film is a low-cost, easy-to-install option that reduces surface condensation. The film works by adding a thin insulating layer to the glass, raising its surface temperature slightly. It also absorbs a small amount of moisture, preventing visible droplets from forming. Application is straightforward: clean the glass thoroughly, cut the film to size, and apply with a spray of soapy water, smoothing out air bubbles with a squeegee.
Films typically cost between HK$50 and HK$200 per square metre and last one to three years before needing replacement. While they are not as effective as double glazing, they can make a noticeable difference when combined with dehumidification and ventilation. Some films also offer UV protection, which helps prevent furniture and flooring from fading — a useful bonus in sun-drenched Hong Kong flats.
When to Call a Professional
If condensation persists despite your best efforts, or if you notice water pooling on window sills, black mould spreading into walls, or visible damage to window frames, it is time to call a professional. A qualified window contractor can assess whether your frames are failing, whether there are hidden gaps allowing moisture ingress, and whether a full window replacement is warranted. In Hong Kong, the Buildings Department requires that window replacement in buildings over three storeys be carried out by a Registered Minor Works Contractor under the Minor Works Control System. Always check your contractor's registration before engaging their services.