When fog or water droplets form between the two panes of your double glazing, the airtight seal around the glass unit has failed. It won’t clear up on its own.

The good news? You probably don’t need to replace the whole window. In most cases, we can simply swap out the sealed glass unit while leaving your existing frame untouched.

Below, we explain how to diagnose your window condensation, why seals break down over time, and your best options for fixing it.

Inside, outside or between the panes? How to tell

Not all window condensation signals a problem. In fact, failed seals are rare in modern double glazing.

The two most common condensation issues we see during service calls are moisture on the outside of the glass and a narrow strip of condensation along the bottom of the inside pane.

Neither means your double glazing has failed. The moisture’s location tells you what’s going on and whether you need to take action.

The Quick Cloth Test: Wipe both sides of the glass with a dry cloth. If the fog remains, it’s likely trapped between the panes.

If your condensation sits on the outside or room side of the glass, our article on morning window condensation explains what causes it and how to manage it.

How a sealed double glazing unit works

A double glazing unit – also known as an insulated glass unit, consists of two panes of glass held apart by a spacer bar around the perimeter. The gap between the panes is sealed airtight and filled with dry air or an insulating gas like argon.

Tucked inside that spacer bar is a desiccant, a specialised drying agent that absorbs any lingering moisture trapped during manufacturing.

That dry, sealed gap gives double glazing its insulating capability. It slows heat transfer through the window, keeping your home warm and the inner pane close to room temperature. Our glazing guide covers the different glass options in more detail.

When the edge seal fails, moist air gradually creeps into the gap. The desiccant absorbs this moisture at first, but once saturated, the excess water condenses on the cold inner surfaces as visible fog or droplets.

Why does double glazing fog up between the panes?

1. Age and natural wear

Seals don’t last forever. Over time, the sealant around the unit’s edge gradually degrades, and older units are naturally most vulnerable.

2. Sun exposure and thermal expansion

During the day, solar heat warms the trapped air or gas inside the unit, causing it to expand. At night, it cools and contracts. This daily expansion cycle puts continuous stress on the edge seal, so windows in full sun work hardest.

3. Water pooling around the glass

The edge seal needs to stay dry to preserve its structural integrity. If frame drainage slots become clogged with dirt or debris, standing water around the base of the glass unit slowly degrades the sealant.

4. Incorrect installation or glazing

Glass units rest on small plastic spacers (packers) inside the frame that keep them square and support their weight. If these packers are the wrong size, slip out of position, or the unit is forced, constant stress points can cause the seal to fail early.

5. Physical damage

A sharp impact, a high-pressure washer aimed at the frame edges, or harsh chemical cleaners can damage the seals and allow moisture inside.

Is a failed seal a problem?

A failed seal won’t cause structural damage, but leaving it unaddressed will affect your home’s comfort and appearance. Here is why it’s best addressed early:

• Reduces insulation performance: A failed seal can let insulating gas escape and moisture enter. This reduces the unit’s thermal performance and can affect your energy savings, although the comfort difference may not be immediately noticeable.
• Spoils your view: Trapped condensation creates a permanent haze, making rooms look tired and blocking clear sightlines.
• Causes permanent glass staining: Trapped moisture leaves mineral deposits on the inside of the glass over time, creating permanent stains that won’t wipe away.
• Worsens over time: A compromised seal lets more moisture accumulate with every seasonal shift.

How to fix condensation between double glazing

Replace the glass unit

This is the most reliable, long-term solution. A new sealed unit is manufactured to match your exact dimensions and fitted directly into your existing frame. It restores full glass clarity and the thermal insulation you paid for.

It also offers a convenient opportunity to upgrade. If your original windows use standard double glazing, upgrading to Low-E glass with argon gas will make your room noticeably warmer. 

Demisting services

Some providers demist units by drilling small holes in the glass, drying out the gap, and resealing the holes. While this temporarily clears visible fog, it does not restore the original airtight seal or replace lost argon gas. Think of it as a temporary cosmetic fix, not a lasting repair.

When a full window replacement makes sense

If multiple units fail at once or your frames are deteriorating, swapping individual glass units may only patch a broader issue.

For instance, old aluminium frames conduct heat readily, so even with new glass, cold frames will still generate room-side condensation.

In those situations, it pays to weigh up whether to replace or retrofit and review how frame materials affect overall durability and warmth.

Is it covered by warranty?

NK Windows products cover glass for 10 years, subject to our glass suppliers’ standard conditions. Review the specifics on our guarantees page. 

If you notice fogging in your NK windows, contact us promptly. Submit warranty claims in writing within 7 days of noticing the defect. You can book a service call, and our technicians will assess the unit.

For windows supplied by another company, check their original warranty terms first, as glass units frequently carry independent coverage.

How to make your double glazing last longer

A well-maintained sealed unit can last for years. A few simple practices help extend its life:

• Clear frame drainage slots: Inspect the small drainage slots at the bottom of your frames every few months and remove any accumulated dirt.
• Clean gently: Use warm, soapy water and a soft cloth. Avoid abrasive cleaning agents or strong solvents near seals and gaskets. Follow our step-by-step guide to cleaning uPVC window frames.
• Avoid high-pressure washing: Don’t use high-pressure water on glass edges, as it can bypass perimeter seals.
• Address early damage quickly: Have glass edge chips or loose frame beading inspected before moisture penetrates the seal.

For comprehensive maintenance guidance, refer to our aftercare advice.

How long should double glazing last?

Modern double glazing is built to last. Under the New Zealand Building Code, insulated glass units have a minimum durability requirement of 15 years.

In practice, you should expect a quality glass unit to last much longer. A well-made and correctly installed unit can last 30 years or more.

Over time, some of the argon gas between the panes may escape. This can reduce the glass unit’s thermal performance by around 15%, although you’re unlikely to notice much difference in your home’s comfort.

Losing some argon gas doesn’t automatically mean you need to replace your window. The main warning sign is persistent condensation trapped between the panes.

Frequently asked questions

Will condensation between the panes go away on its own?

No. While moisture levels may fluctuate with changing weather—appearing on chilly mornings and clearing in afternoon heat—the underlying seal remains broken. Replacing the glass unit is the only permanent solution.

Can you fix foggy double glazing without replacing it?

Demisting services clear visible moisture by drilling and drying the gap, but they cannot restore the airtight seal or argon gas. Replacing the sealed unit is required to recover full thermal efficiency.

How long should a double glazing seal last?

A high-quality unit, installed correctly and maintained regularly, should last for many years. Solar exposure, installation precision, and proper frame drainage all influence seal longevity.

Does a failed seal make my home colder?

Yes. When insulating gas escapes and moist air fills the gap, the window loses heat faster, reducing the room’s overall energy efficiency.

Can you replace the glass in uPVC windows?

Yes. You can replace the sealed glass unit in a uPVC window independently of the frame. We remove the compromised unit, install a custom-sized replacement, and leave the original frame in place.

Ready to clear up your view?

If your double glazing has started misting between the panes, our team can evaluate your windows and guide you to the best repair or upgrade option. Get in touch today or visit your local NK Windows showroom.

The post Why Is My Double Glazing Foggy Between the Panes? (And How to Fix It) appeared first on NK Windows – uPVC Windows & Doors.

It’s natural to be concerned that something has gone wrong. In most cases, it hasn’t.

Condensation forms whenever moist air touches a surface colder than its dew point, the temperature at which water vapour turns back into liquid. Double glazing changes which surface is coldest, but it can’t change how much moisture is in the air.

That’s why the most useful thing you can do is look at where the water is sitting. It tells you almost everything you need to know.

Where is the condensation? A quick 3-part check

Start by checking if the water is on the glass inside the room. If there’s no water, check outside on the same pane. If there’s no water either side, the moisture is trapped between the panes.

Let’s start with the one that catches people off guard most, but needs the least action from you.

Condensation on the outside: a sign your windows are working

With old single glazing, heat leaked out through the glass and kept the outer surface slightly warm. Good double glazing stops that leak. The outer pane ends up as cold as the morning air, and on a clear, still night it can drop below the dew point. That’s when you see dew on the glass, just like on your car or the lawn.

“Condensation on the outside of the window indicates the Low E coating is working effectively,”

says Martin Ball, Director of NK Windows. 

“This special coating is on the inside of the inner pane, and its job is to trap heat within your home.”

Low E glass makes outside condensation more noticeable because it holds even more heat indoors. Standard double glazing can still get it on the right morning, though. If you’re curious about the science, our guide to how Low E coatings and argon gas work goes into more detail.

Either way, it’s harmless and usually gone by mid-morning. If it’s blocking a view you love, a quick pass with a squeegee does the job.

Condensation on the inside is a different story. Your windows aren’t causing it, but they are telling you something about the air in your home.

Condensation on the inside: what your windows are telling you

On a cold night, the glass is often still the coldest surface in the room, even when it’s double glazed. Warm, moist indoor air drifts against it, cools, and leaves water behind. That’s why it shows up most in bedrooms with the door shut, and in bathrooms and kitchens.

As the Window Association of New Zealand puts it, windows do not cause condensation. They’re simply where you see it first.

Why it can seem worse after new windows

This is the part that surprises most homeowners. Old timber or aluminium windows were rarely airtight, and the gaps around them worked as accidental ventilation. Moist air escaped through them all night, taking a fair bit of your heat with it.

New, well-sealed joinery closes those gaps. Your home gets warmer and quieter, but the moisture you create indoors now stays inside until you deliberately let it out. So more condensation after an upgrade usually means the windows are sealing properly, not that they’re faulty.

New homes and renovations need time to dry out

If your windows went into a new build or a major renovation, there’s another source of moisture at play. Water gets trapped in the building materials during construction, mainly in the timber and concrete, and it takes around a year to dry out with good heating and ventilation. BRANZ estimates a newly laid concrete floor holds around 120 litres of water per cubic metre that has to escape as the slab cures. If you’re building a new home with uPVC joinery, expect your first winter to be the wettest and plan to ventilate more than usual.

Frames and glass make a real difference

How warm your windows stay on the inside comes down to two choices, the frame and the glass. The warmer they stay, the less condensation will form on them.

The frame

Aluminium carries the outdoor cold straight through to the inside, which is why the frame edge on older aluminium windows is often wet even when the glass is dry. Thermally broken aluminium has an insulating strip to slow this down, and BRANZ recommends it. uPVC goes further, because the material itself is a poor conductor of heat. It’s also why retrofitting double glazing into existing aluminium frames doesn’t always fix condensation: the glass gets better, but the cold frame stays. For a side-by-side look, here’s how uPVC compares with aluminium.

The glass

Our uPVC double glazed windows come with two options. Standard Double Glazing has two panes with an air gap between them. Planitherm XN fills that gap with argon gas and adds a Low E coating, which reflects heat back into the room and keeps the inner pane warmer.

If inside condensation is your main concern, uPVC frames with Planitherm XN glass will give you the warmest inside surfaces of the two.

Better frames and glass lower the odds of inside condensation, but they can’t remove the moisture itself. To do that, it helps to know where it’s coming from.

Where the moisture in a Kiwi home comes from

Everyday living puts a surprising amount of water into the air. These figures come from the Window Association of New Zealand:

Add that up for a family of four that showers, cooks dinner and dries one load of washing inside. That’s around 16 litres of water a day before anyone switches on a gas heater. All of it has to go somewhere, and on a cold night a lot of it ends up on your windows.

The good news is that most of these sources are easy to manage once you know about them.

How to stop condensation on the inside of your windows

The aim is simple: make less moisture, and get rid of what you do make before it reaches the glass. These steps are listed roughly in order of impact.

1. Catch moisture at the source. Run your rangehood and bathroom fan while you cook and shower, and for 15 minutes afterwards. Check they vent outside, not into the roof space.
2. Dry washing outside or in a vented dryer. On wet days, a heat pump on dry mode or a dehumidifier in the laundry stops that 5 litres spreading through the house.
3. Replace unflued gas heaters. They add moisture every hour they run. A heat pump or flued heater warms the room without the water.
4. Air the house out every morning. Open windows on opposite sides for 10 to 15 minutes to push damp air out. Tilt and turn windows make this easy, because the tilt position lets air through while staying secure.
5. Keep heating steady. A home that stays evenly warm keeps the glass above the dew point. Big swings between a hot evening and a cold night are what make windows stream.
6. Use a dehumidifier in problem rooms. Bedrooms and rooms with little airflow benefit most.
7. Wipe up what does form. Clearing sills and frames in the morning stops water sitting there and helps prevent mould. Our tips for looking after your uPVC frames cover the rest.

For a version you can print or share, download our Understanding and Managing Condensation guide.

These steps solve most condensation on the glass. If the moisture is trapped inside the glass itself, though, no amount of ventilation will shift it.

Condensation between the panes: when to give us a call

A double glazed unit is two panes of glass sealed around the edge, with dry air or argon gas in the gap. If you can see misting or droplets in that gap, the seal has failed and moist air has got in. You can’t wipe it away, and better ventilation won’t help.

The fix is to replace the sealed glass unit. The frame usually stays where it is. NK Windows’ glass is covered by our 10-year guarantee on profiles, glass and hardware.

It’s also worth getting in touch if:

• mould keeps coming back on your frames or window reveals
• water is pooling on sills or running onto the floor
• one window is much wetter than the others in the same room

Get in touch with our team, or visit one of our showrooms in Auckland, Wellington and Christchurch and we’ll help you work out what’s going on.

Frequently asked questions

Is it normal to get condensation on new double glazing?

Yes. Some condensation on the outside, or on the inside on cold mornings, is normal. Only misting between the panes points to a fault.

Will the condensation go away?

Outside condensation clears by itself as the day warms up. Inside condensation eases once indoor moisture is under control, and in a new build it usually settles after the first year.

Does condensation mean my windows are faulty?

Not unless it’s between the panes. On either surface of the glass, it reflects the weather or the moisture in your home rather than the window itself.

Does double glazing stop condensation?

It greatly reduces inside condensation by keeping the inner pane warmer, but it can’t remove moisture from the air. That still needs good ventilation.

Does triple glazing reduce condensation?

Yes. A third pane keeps the inner glass even closer to room temperature, which is one reason people choose triple glazing for colder parts of the country.

The post Condensation on Double Glazed Windows: Inside, Outside or Between the Panes? appeared first on NK Windows – uPVC Windows & Doors.