That persistent milky haze trapped between your double-glazed panes is one of the more frustrating things a homeowner can encounter — especially on an otherwise fine NZ morning. The good news is that understanding what's caused it makes it much easier to decide on the right fix. The not-so-good news is that true DIY repair is very limited, and in most cases a professional glazier is your best option.
Why Does Double Glazing Go Foggy?
Double-glazed windows (also called insulated glass units or IGUs) consist of two panes of glass separated by a spacer bar and sealed around the edges to trap a layer of air or inert gas (usually argon) between them. This sealed gap is what gives the unit its insulating and acoustic properties.
When you see fogging or condensation between the panes, it means the perimeter seal has failed. Once the seal breaks down, moist outside air enters the cavity. As temperatures change throughout the day, that moisture condenses on the inner glass surfaces — and because it's trapped, it can't evaporate away. Over time, mineral deposits from the water can leave a permanent haze even when the glass is dry.
Common causes of seal failure include:
- Age — most IGU seals have a service life of roughly 10–20 years depending on quality and exposure
- UV degradation of the sealant, particularly on north-facing windows in NZ's high-UV environment
- Thermal stress from repeated heating and cooling cycles
- Physical damage — hard knocks, incorrect installation, or water pooling in the frame
- Poor original installation where the unit wasn't bedded or sealed correctly
Is It Definitely a Failed Seal?
Before assuming the worst, check where the condensation is forming:
| Location of condensation | Likely cause | Action needed |
|---|---|---|
| Between the panes | Failed IGU seal | Replace the sealed unit |
| On the room-side surface | High indoor humidity | Improve ventilation |
| On the outside surface | Normal in cool weather | No action needed |
Condensation on the outside of the glass on a cold morning is actually a sign your window is working well — it means very little heat is escaping through it. Condensation on the inside surface (room side) usually points to excess indoor humidity, which is worth addressing for your home's health under the Healthy Homes standards.
Can You Fix a Failed IGU Yourself?
Honestly, the options are limited. The sealed unit itself cannot be re-sealed at home — once the perimeter seal fails, the only proper fix is to replace the IGU.
You may come across online kits that claim to drill small holes in the glass, insert a desiccant, and re-seal the holes. While some homeowners try this as a temporary measure, results are mixed. The internal surfaces may already have mineral staining that won't clear, and drilling glass without the right equipment risks cracking the pane. BRANZ research consistently shows that replacing a failed unit is the most reliable long-term solution.
What you can do yourself:
- Check and clear any drainage holes or channels in the window frame so water doesn't pool against the unit
- Inspect the external glazing tape or sealant around the unit's edge and apply a compatible silicone sealant if you spot obvious gaps (this won't fix an already-failed seal, but may slow further deterioration)
- Improve indoor ventilation to reduce humidity and slow the rate of seal degradation on other windows
Replacing the Sealed Unit — What's Involved?
Replacing an IGU is the correct fix. A glazier will:
- Remove the window beading or glazing tape holding the unit in the frame
- Extract the failed unit carefully
- Measure and order a new IGU to match (glass thickness, gap width, any Low-E coating, and overall size)
- Install the new unit with correct setting blocks, glazing tape or sealant, and refit the beading
The frame itself (timber, aluminium, or uPVC) is usually reused, which keeps costs down. As a very rough guide, replacement IGUs for standard residential windows can range from a few hundred dollars upward depending on size, glass specification, and access — always get at least two quotes.
If your windows are safety glazing zones (beside doors, low to the floor, in bathrooms), the replacement glass must comply with NZS 4223 and the NZ Building Code — another reason to use a qualified glazier who knows the requirements.
Improving Your Home's Ventilation While You're At It
If multiple windows are fogging on the inside surface (room side), your home may have a humidity problem. NZ homes — particularly older ones — can trap moisture from cooking, showering, and breathing. Installing or using extractor fans, opening windows daily, and considering a heat recovery ventilator (HRV) or positive pressure system can all help. Energywise has practical guidance on managing moisture in NZ homes.
When to Call a Professional
Call a qualified glazier when:
- Condensation is confirmed to be between the panes — the IGU needs replacing
- The glass is cracked or broken (always a job for a professional)
- The window is in a safety glazing zone and must meet NZS 4223
- The frame is damaged and may need repair or replacement alongside the glass
- You're unsure of the glass specification needed (Low-E, laminated, toughened)
Glaziers are not licensed under a statutory registration scheme the way electricians or plumbers are, but choosing an experienced, reputable tradesperson matters — especially for safety-critical glazing. You can find qualified local glaziers on tradedirectory.co.nz.