When your windows start fogging up or letting in a cold draught, it’s usually the first sign of window seal failure. Catching it early can save you hundreds in heating bills and prevent long-term damage to your frames and glazing.
In this guide, we’ll explain why window seals fail, how to spot early warning signs, and when a repair makes more sense than a replacement.
What Happens When Your Window Seal Fails (And Why You Shouldn’t Ignore It)
The seal around your double-glazed window keeps warm air inside and stops moisture getting in between the panes. When that seal breaks, moisture slips through and gets trapped inside the glass. You start to see fog or cloudy patches that never wipe away, no matter how much you clean.
Once that happens, your windows stop insulating properly. Warm air escapes, cold air gets in, and your heating has to work harder to keep the room comfortable. You’ll notice it in your energy bills before you notice it in your comfort.
It’s a bit like leaving the fridge door slightly open. It still works, but it’s wasting energy every minute.
Fixing a failed window seal early keeps your home warmer, your windows clear, and your bills lower.
Early signs your window seal is failing
We’ve repaired hundreds of units over the years, and these are the tell-tale signs we always spot early on:
1. Condensation inside the glass
This is the big one.
If you can see fog or moisture between the panes, your window seal is no longer airtight.
You can wipe the inside and outside all you want, it won’t make a difference. That’s because the moisture is trapped inside the sealed unit.
Left alone, that fog turns to streaks, then mineral stains. At that point, replacement is your only option.
2. Windows that look dirty or hazy after cleaning
We get this question all the time: “I’ve just cleaned my windows, but they still look cloudy. Why?”
That hazy look is the sign of a compromised seal letting in air and dust. It’s frustrating, but completely fixable once the unit is replaced or resealed.
3. Drafts or cold spots around the frame
If you can feel a chill near the window, the seal might have failed around the edge.
Run your hand slowly around the frame. If it feels colder or you can feel a faint breeze, you’re losing insulation.
4. Distorted reflections or fogging on sunny days
A failed window seal can also cause a “wavy” or distorted reflection when light hits the glass.
That’s from the insulating gas escaping, creating uneven pressure between the panes. It means your energy efficiency has dropped significantly.
5. Black mould or damp around the frame
If you see mould forming near the bottom of your window, don’t just clean it and move on.
That’s often caused by trapped condensation from a failed seal.
Once moisture finds its way into the frame, it can spread into plaster or timber.
We’ve repaired plenty of windows where the seal failure started small but ended in a full frame replacement because the damp was ignored.
Spot any of these signs? Don’t wait for it to get worse. Book your repair with Plastic Doctor. No call-out charge.
What Causes Window Seals to Fail and How to Stop It From Happening
Window seals don’t just suddenly stop working.
They wear down slowly over time from normal use, weather, and temperature changes.
Here are the main reasons they fail.
- Heat and cold changes
When the sun heats the glass during the day and it cools at night, the window expands and contracts over and over.
That constant movement slowly weakens the seal until tiny gaps start to form. - Poor installation
If the window wasn’t fitted properly or the wrong materials were used, the seal might not hold up for long.
Bad installation is one of the most common reasons we see seals fail early. - Weather and UV exposure
Moisture, sunlight, and everyday weather wear away the flexible compounds that keep the seal airtight.
Over time, they dry out, crack, or shrink. - Age
Even the best windows don’t last forever.
Most seals naturally break down after 15 to 20 years. - Frame damage or movement
If the window frame warps, cracks, or shifts, it puts pressure on the seal and causes small leaks.
Once air and water start getting in, the seal can’t do its job anymore.
Cold air sneaking in through your windows? Here’s exactly why in our post: Why is Cold Air Coming Through My Double Glazed Windows?
When to Repair and When to Replace
Knowing when to repair and when to replace can save you serious money.
Here’s a quick comparison:
| Situation | Best Solution | Why |
| Clear glass, slight draughts, frame intact | Reseal or replace gaskets | Fast, simple, effective |
| Fog or moisture between panes | Replace glass unit (IGU only) | Restores clarity and insulation |
| Multiple failed windows, aged frames | Full window replacement | Long-term value and efficiency |
| Rotten or warped frames | Replace entire window | Repair won’t solve frame decay |
If the frames are still solid and the glass is just foggy, repairing or replacing the sealed glass unit is usually enough. For bigger issues, a full replacement may be smarter long-term.
Can you repair window seal failure yourself?
Sometimes, yes, if it’s only a small draft around the edge.
You can apply fresh silicone sealant or weather stripping to stop air leaks.
But if the problem’s inside the glass (fogging, haziness, or trapped moisture), that’s a sealed unit issue. There’s no DIY fix for that.
Wondering if you can fix that fog yourself? Read our guide: Will a Hair Dryer Remove Condensation Between Glass Panes?
Easy Tips to Make Your Window Seals Last Longer
You can’t make window seals last forever, but you can definitely help them last longer.
Start by keeping those small drain holes at the bottom of your windows clear. They let water escape, and when they get blocked, moisture builds up and starts causing problems.
Try not to blast your window frames with a power washer. It might make cleaning quicker, but all that pressure can push water into places it shouldn’t go.
Be gentle when closing your windows. Slamming them shut every time slowly wears down the seals.
If your windows get a lot of sun, it helps to use blinds or a bit of UV film. That keeps the heat from constantly expanding and shrinking the glass.
And when you clean your windows, use a mild detergent. Strong chemicals can damage the seal over time.
Need a hand fixing it?
If your windows are foggy, drafty, or just not doing their job anymore, we can sort it.
Send us a quick message or call us today for a free, no-obligation quote.
Common Questions About Window Seal Failure
- How long do window seals last?
Typically 15–20 years, depending on exposure and quality. - Is condensation inside the window always a bad sign?
Yes, that moisture means your seal has failed. - Can you repair the seal itself?
You can’t re-gas the unit, but you can reseal edges or replace the glass unit entirely. - Can window defogging fix it?
It’s temporary. It removes moisture but doesn’t restore insulation. - Can Plastic Doctor repair misted windows?
Yes, our team replaces or reseals double-glazed units to restore clarity and performance. Learn more on our window repair page. - Will a new seal reduce noise?
Yes, restoring the airtight seal can noticeably reduce outside noise. - How can I prevent seal failure?
Keep frames clean, avoid harsh chemicals, ventilate rooms, and schedule professional maintenance every few years. - Is it cheaper to replace all my windows at once?
Usually, yes. You save on labour and ensure consistent performance across all openings. - Do failed seals affect property value?
Yes, cloudy or fogged glass looks neglected and can lower curb appeal.