Condensation on windows is one of the most common complaints from New Zealand homeowners, especially during the cooler months. While it might seem like a minor nuisance, persistent condensation can lead to mould, rot, and health issues if left unchecked. Understanding why it happens is the first step to fixing it.
What Actually Causes Window Condensation?
Condensation forms when warm, moist air inside your home meets a cold surface — like a window pane. The air cools rapidly at the glass and can no longer hold as much moisture, so water droplets form. This is the same principle as a cold drink "sweating" on a warm day.
In New Zealand, our climate plays a big role. Many parts of the country experience cool, damp winters, and older NZ homes — particularly those built before the 1990s — were often constructed with single-glazed aluminium-framed windows that get very cold. These are prime candidates for heavy condensation.
Key factors that increase condensation:
- Cooking, showering, and drying clothes indoors
- Poor ventilation or extractor fans that don't vent outside
- High occupancy (more people = more moisture in the air)
- Single glazing or cold window frames
- Inadequate or no ceiling and underfloor insulation
The Difference Between Surface and Internal Condensation
It's worth knowing there are two types of condensation you might encounter:
Surface condensation
This appears on the inside face of your window glass or frame. It's the most common type and is caused by the indoor humidity and temperature conditions described above.
Inter-pane condensation
If you have double-glazed windows and you notice fogging or moisture between the two panes, this means the sealed unit has failed. The inert gas or dry air that was sealed inside has been replaced by moist air. This cannot be fixed by ventilation — the sealed unit needs to be replaced by a glazier.
Health and Home Risks of Persistent Condensation
Condensation that isn't managed can cause real problems. Moisture on windows runs down onto sills and frames, encouraging mould growth. Mould spores are a known health hazard, particularly for people with asthma or respiratory conditions. BRANZ research consistently highlights moisture as one of the leading causes of poor indoor air quality in NZ homes.
Under the Healthy Homes Standards, rental properties must meet specific requirements around ventilation, heating, and moisture control. Landlords should be aware of their obligations — more detail is available from Tenancy Services.
Practical Ways to Reduce Window Condensation
1. Improve ventilation
Open windows briefly each morning to flush out moist air. Make sure extractor fans in kitchens and bathrooms are working properly and venting outside (not into the ceiling cavity). A heat recovery ventilation (HRV) or positive pressure ventilation system can also help maintain balanced indoor humidity.
2. Use a dehumidifier
A portable dehumidifier is one of the most effective short-term solutions, particularly in bedrooms and living areas. Aim to keep indoor relative humidity between roughly 40–60%.
3. Heat your home consistently
Cold rooms are more prone to condensation. Keeping your home at a reasonable temperature — even overnight — helps keep window surfaces warmer. Energywise has good guidance on efficient home heating options.
4. Dry clothes outside or use a vented dryer
Drying laundry indoors releases a significant amount of moisture into the air. If you must dry inside, do so in a well-ventilated room.
5. Upgrade your glazing
This is the most effective long-term fix. Replacing single-glazed windows with double glazing (or even secondary glazing) dramatically reduces the temperature difference between the indoor air and the glass surface, greatly reducing condensation. Modern double-glazed units with warm-edge spacers and thermally broken frames perform significantly better than older aluminium single-glazed windows.
6. Add window insulation film or thermal curtains
As a lower-cost interim measure, insulating window film or heavy thermal curtains can reduce heat loss through glass. These won't eliminate condensation but can reduce its severity.
Comparing Your Options
| Solution | Cost (as a guide) | Effectiveness | DIY Friendly? |
|---|---|---|---|
| Ventilation habits | Free | Moderate | Yes |
| Dehumidifier | Low–moderate | Good short-term | Yes |
| Thermal curtains | Low–moderate | Moderate | Yes |
| Secondary glazing | Moderate | Good | Partly |
| Full double glazing replacement | Higher | Excellent | No — trade required |
When to Call a Professional
If you have inter-pane condensation in a double-glazed unit, the sealed unit has failed and needs professional replacement — this is not a DIY job. A qualified glazier will remove the failed unit and install a new sealed double-glazed panel.
If you're considering a full window replacement, this work may involve structural or weathertightness considerations under the NZ Building Code (particularly Clause E2 — External Moisture). Depending on the scope, a building consent may be required. Check with your local council or visit MBIE Building Performance for guidance.
For significant glazing work, it's always best to use a qualified tradesperson. You can find experienced local glaziers on tradedirectory.co.nz.