Why Do Double Glazed Windows Condense? (And How We Fix It)

 

Understanding condensation , what it says about your windows , and how professional glazing restores clarity and insulation .

 

Introduction — when your window starts keeping secrets

You buy double glazed windows because they promise comfort: insulation , quiet rooms , lower energy bills . The glass should be perfectly clear , almost invisible . A window is supposed to show the world , not blur it . So when one day a foggy patch appears — like a cloud trapped inside the pane — the frustration hits fast .

You try wiping it . Nothing changes .
You blame humidity .
You ask Google .

And then fear creeps in: Do I have to replace the entire window?

Condensation inside double glazing is like a symptom from a body part that doesn’t know how to speak . It’s silent , but it’s trying to tell you something important . And most London homeowners don’t realize that fogging isn’t a cosmetic problem — it’s an energy problem , a comfort problem , and occasionally a health problem .

But before jumping into how to fix it , we need to understand why it happens . Because the reasons reveal the solution .

Chapter 1: The science behind condensation — and why the enemy is temperature

Condensation is one of those everyday mysteries we ignore until it becomes personal .
On a cold morning , you breathe on a mirror and it fogs .
Put ice water in a glass; droplets form on the outside .

There’s nothing magical happening .
Warm air meets cold surface → moisture becomes water .

Double glazed windows are designed to prevent this . Between the two panes of glass , there’s a sealed space — either filled with dry air or an insulating gas like argon . This space is dry , stable , and acts like a thermal buffer . The purpose is simple: keep the inside pane warm and stop moisture from forming .

When that sealed space fails , the buffer is gone . Suddenly , your double glazing behaves like cheap single glazing — cold , exposed , and vulnerable .

And condensation appears on the inside .

It is the window’s way of saying:
“My insulation layer has died .”

Chapter 2: The real reason condensation forms inside double glazing: the broken seal

Every double glazed unit has an invisible superhero — the perimeter seal .

The seal’s job is to:

1. Keep dry air or argon gas inside the cavity .
2. Keep outside air and moisture out .

Over time , that seal ages . UV light , temperature changes , pressure differences , and building movement slowly weaken it . It’s not dramatic — no shattering , no noise — just a quiet decay .

When the seal fails , moisture enters the cavity .
Moisture finds cold surfaces .
Cold turns vapour into droplets .
Droplets become fog .

Your glass is foggy not because the house is too humid — but because your window is no longer airtight .

This is called “blown double glazing .”

A failed seal means the insulation layer has been breached .

You now own a window that looks like double glazing but performs like single glazing .

Chapter 3: London’s climate — the perfect storm for condensation

Condensation doesn’t care about your frustration .
It cares about physics and weather .

London weather is a triple-hit to double glazing:

1. Humidity: Lots of rain , fog , and damp air .
2. Temperature swings: Warm days , cold nights , especially in autumn and winter .
3. Architecture: Many London homes have older frames and poorly ventilated rooms .

A window can’t help being located in the UK .
But the climate accelerates seal failure .

Metal and uPVC frames expand in heat and contract in cold .
This tiny movement — repeated thousands of times a year — slowly breaks the perimeter seal .

So when customers ask us , “Why did my double glazing fail? It was fine last year ,” the answer is usually:

Your windows didn’t fail suddenly . They failed slowly — and the weather helped .

Chapter 4: The myth of the ‘humid house’ — and the truth homeowners aren’t told

Most people blame themselves .

“Maybe I dry clothes indoors .”
“Maybe I shower too long .”
“Maybe I need a dehumidifier .”

These habits can cause condensation on the inside surface of the window , not between the panes .

Moisture between panes has nothing to do with lifestyle .
You can’t wipe it away because you can’t reach it .
You don’t create that moisture — a broken seal allows it in .

And here is the uncomfortable truth almost no window company tells customers:

Condensation inside double glazing is a sign the window has lost insulation efficiency — and the room is losing heat .

That means:

• Higher heating bills
  • Colder rooms
  • More drafts
  • Possible mould on nearby walls

So no , you’re not imagining the house got colder around the same time the window fogged .
The two events are connected .

Chapter 5: How we fix misted or foggy double glazing (without replacing the entire window)

This is where most homeowners get misled .

Window companies make money selling new windows .
So when they see condensation inside glazing , they often say:

“You need to replace the whole window .”

That can cost hundreds — even thousands — per window .

But glazing engineers know something different:
The expensive part isn’t the frame .
It’s the sealed glass unit inside the frame .

You don’t need a new window .
You just need a new glass unit .

At Glazing London , we remove the misted unit and replace only the sealed glass panel — keeping your existing frame intact . No building work . No mess . No plaster damage .

The service takes around 30 minutes .

Your window becomes:

• Clear
  • Airtight
  • Energy efficient again

And it costs a fraction of full replacement .

Homeowners are shocked when we tell them how simple it is — because the glazing industry makes it unnecessarily mysterious .

There is no conspiracy .
Just capitalism .

Chapter 6: What happens during our repair (a peek behind the curtain)

When our glazing engineer arrives , they inspect the affected unit .
They take precise measurements , because the replacement has to fit perfectly .
Once the new sealed unit is ready , they return and remove the beading — the frame pieces that hold the glass in place .

The old misted unit slides out .
The new unit slides in .
We re-fit the beading , check alignment , clean the glass , and that’s it .

No builder .
No plaster .
No redecorating .
No chaos .

The only change you see is clarity — and the immediate return of insulation .

Your home gets warmer without touching the thermostat .

Chapter 7: Why fixing condensation is really about energy savings

A foggy double glazed unit is more than an annoyance .

When the seal fails , warm indoor air cools rapidly near the window .
Your boiler compensates .
You burn more energy .

A misted double glazed window can increase heating costs by up to 20% .

That’s like opening a small hole in your wallet and letting money leak out every day .

Your window is literally costing you money to stay broken .

Replacing the glass pays for itself in energy savings — especially in a London winter .

Chapter 8: The ‘invisible’ impact — comfort , mood , and property value

Clear windows are psychological .
They make a room feel bigger , cleaner , brighter .

Foggy windows do the opposite:

• They make rooms feel damp
  • They lower how buyers perceive property value
  • They create a feeling of neglect

If you’re selling or renting , misted windows create hesitation .
People assume the home has deeper problems .

Replacing the unit is a signal of care — and it increases value .

“If the windows are cared for , the whole house probably is .”

Chapter 9: When condensation means the window is still fine

Condensation on the outside surface of double glazing is actually a good sign .

It means your glass has exceptional thermal insulation and outside air is colder than the glass surface .
This is common during spring and autumn mornings .

Inside condensation = insulation failure
Outside condensation = insulation success

The window isn’t misbehaving .
It’s bragging .

Chapter 10: How to avoid condensation problems in the future

The secret isn’t maintenance — it’s design .

High-quality double glazing:

• Uses argon gas , not air
  • Has warm-edge spacers
  • Uses premium perimeter seals resistant to shrinking

Poor-quality glazing fails early because companies cut costs on invisible components .
You don’t see the difference until years later — when condensation appears .

Double glazing should not be a disposable purchase .

It should be a 20-year investment .

Chapter 11: Why customers choose us

People call us because they’re tired of hearing:

“You need a whole new window .”

We exist to give homeowners a smarter , cheaper , faster solution .

When the unit can be replaced , we replace it .
When upgrading to high-performance glass makes sense , we explain why .
And when there’s a ventilation problem , we diagnose it .

We fix windows — but more importantly , we fix frustration .

Conclusion — foggy windows don’t need excuses , they need action

Condensation inside double glazing is not normal .
It is not permanent .
And it does not mean you need new windows .

It simply means the seal has failed .

Here’s what replacing the unit gives you:

• Clear glass
  • Warmer rooms
  • Lower bills
  • Zero fog and zero frustration

The window becomes everything it was meant to be in the first place .

Ready to restore clarity?

If your double glazing is foggy , misted , or full of condensation , we can replace the sealed unit — not the entire frame .