That persistent haze or condensation trapped between your double-glazed panes is one of the most frustrating window problems a homeowner can face — and it only gets worse over time. The good news is that the cause is almost always the same, and there are clear options for fixing it. Here's what you need to know.
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 double glazing its insulating and noise-reducing properties.
When you see fog, mist, or streaks between the panes — not on the inside or outside surface — it means the perimeter seal has failed. Moisture-laden air has entered the gap, and when temperatures change, that moisture condenses on the inner glass surfaces. The desiccant (a moisture-absorbing material) inside the spacer bar becomes saturated and can no longer do its job.
Seal failure is a normal end-of-life issue, but it can be accelerated by:
- UV exposure breaking down the sealant over time
- Thermal cycling — repeated expansion and contraction of the frame and glass
- Poor original installation or low-quality sealant
- Physical damage to the frame or edge of the unit
- Water pooling in the frame rebate, softening the seal from the outside
In New Zealand's variable climate — particularly in regions with high UV like Canterbury or with high humidity like Northland and the West Coast — seals can fail faster than the manufacturer's warranty period.
Common Symptoms and What They Mean
| Symptom | Likely Cause |
|---|---|
| Persistent fog between panes | Seal failure, moisture ingress |
| Fog appears in morning, clears by afternoon | Early-stage seal failure |
| White mineral deposits or streaks | Long-term moisture cycling, mineral build-up |
| Condensation on inside surface of inner pane | Poor room ventilation, not a glazing fault |
| Condensation on outside surface of outer pane | Normal in cold mornings — sign the unit is working well |
It's worth noting: condensation on the outside of your outer pane on a cold morning is actually a sign your double glazing is performing well — the outer pane is staying cold because heat isn't escaping through it.
How to Diagnose the Problem Step by Step
Step 1: Identify where the condensation is
On a cool morning, look closely at the window. Use a torch if needed. Touch the inner pane — if the fog is between the panes and your finger doesn't wipe it away, the seal has failed.
Step 2: Check the frame and rebate
Look around the outside of the window frame at ground level and sill level. Is water pooling? Is the sealant cracked or missing? Frame issues can accelerate seal failure and may need attention regardless.
Step 3: Check how many panes are affected
If multiple units in the same window or throughout the house are fogging, it may point to a systemic issue — age of the installation, a particular batch of units, or ongoing frame movement.
Step 4: Note whether it clears
If the fog clears completely on warm days, the seal has only recently begun to fail. If it never clears, or you can see mineral deposits, the unit has been compromised for some time.
What Can a Homeowner Fix Themselves?
Unfortunately, there is no reliable DIY fix for a failed sealed unit. Some products on the market claim to drill small holes in the unit and inject a desiccant or drying agent — while this may temporarily reduce fogging, it does not restore the insulating gas layer or the unit's thermal performance, and it can void any remaining warranty.
What you can safely do yourself:
- Clean the outer surfaces of the glass thoroughly with a good glass cleaner
- Improve ventilation inside your home to reduce interior condensation (check Energywise for guidance on healthy home ventilation)
- Clear drainage channels in the window frame — many frames have small weep holes that can block with dirt
- Apply fresh exterior sealant around the outside of the frame where it meets the wall, if you can see cracking — this won't fix the unit but prevents further water ingress into the frame
Your Real Fix Options
Option 1: Replace the sealed unit (IGU)
This is the correct and most common solution. A glazier removes the failed unit from the existing frame and installs a new IGU. The frame itself is reused. This is cost-effective when the frame is in good condition and is the standard approach for aluminium-framed windows common in NZ homes.
Option 2: Replace the entire window
If the frame is damaged, corroded, or the window is very old, full replacement may make more sense. This is also an opportunity to upgrade to a higher-performance unit — for example, low-E glass or a wider air gap for better thermal performance in line with MBIE Building Performance requirements under the NZ Building Code.
Option 3: Secondary glazing
If budget is tight and the window is otherwise sound, adding a secondary glazing panel to the interior can restore some thermal performance, though it won't fix the appearance of the fogged unit.
As a rough guide, IGU replacement by a glazier typically costs less than full window replacement, but pricing varies significantly depending on glass size, type, and your location. Get at least two quotes.
How to Prevent It Recurring
- Keep frames clean and clear — don't let water pool in the sill rebate
- Repaint or reseal timber frames regularly to prevent moisture damage
- Ensure good ventilation inside your home to reduce humidity levels — this matters for Healthy Homes compliance in rental properties
- Choose quality units — look for IGUs with warm-edge spacer bars and quality sealant when replacing
- Ask about warranty — reputable glaziers and manufacturers typically offer a warranty on sealed units; BRANZ research suggests quality units should last many years under normal NZ conditions
When to Call a Professional
You should call a qualified glazier when:
- The fog between panes is confirmed (seal failure is not a DIY fix)
- The glass is cracked or broken — broken glass is a safety hazard and should be dealt with promptly
- The frame is damaged, rotten, or corroded
- You're unsure whether the condensation is between the panes or on the surface
Glazing work itself is not legally restricted work in New Zealand in the same way as electrical or plumbing, but working at height carries real risks under WorkSafe NZ guidelines — don't attempt to remove or handle large panes of glass yourself. A professional glazier has the tools, training, and insurance to do this safely.
If your replacement window work involves structural changes or new openings, that may require a building consent — check with your local council or visit MBIE Building Performance for guidance.
You can find qualified local glaziers on tradedirectory.co.nz to get the job done safely and correctly.
Window & Glass Association
