When double glazing leaks or mists up between the panes, the fault is almost always the seal, not the glass itself and not the frame. Cloudy2Clear replaces failed window seals across the UK for £150 to £500 per window, restoring insulation and stopping draughts without touching the frame. We’ve worked this way since 2005, under our own line: Replace the Pane, not the Frame!
Key Takeaways
- A properly sealed unit keeps argon gas trapped between the panes; once the seal fails, roughly 30% of that unit’s insulating value is lost.
- Seals normally last 10 to 20 years before they need attention.
- Full seal replacement is a job for professionals, both for correct installation and for warranty protection.
- Our double glazing replacement comes with a 25-year guarantee on the glass itself, separate from any frame guarantee.
Why Is My Double Glazing Leaking?
Leaking double glazing almost always traces back to a failed seal rather than a cracked pane. The seal’s job is to keep insulating gas trapped and moisture out; once it splits or shrinks, air and water get in and the unit starts to fail from the inside.
A double glazed unit is built around a sealed air gap, usually filled with argon gas, held in place by a spacer bar and a perimeter seal. That seal is doing two jobs at once: keeping the gas in, and keeping weather out. Once it fails, you lose both. We put the loss at roughly 30% of that window’s insulating value once the seal is compromised, which is why a leaking window often runs alongside a jump in your heating bill rather than sitting as a separate problem.
Seals typically last 10 to 20 years under normal UK weather. Coastal exposure, south-facing aspects, and older uPVC frames tend to shorten that lifespan. If your windows are approaching that age and you’re starting to notice fogging or draughts, the seal is the first place to look, not the glass. We’ve written more on what actually happens inside a glazing unit if you want the full picture of how the components work together.
How Do I Know My Window Seals Have Failed?
The clearest sign is condensation trapped between the panes rather than on the inside surface of the glass. Other signs include persistent draughts around the frame, mould or black spotting on the sill, and discolouration or a cloudy haze that won’t clean off.
Building Science Education, a U.S. Department of Energy resource, notes that insulated glass unit failure is identified specifically by condensation forming between the inner and outer panes, which matches what we see across the units we inspect in England and Scotland. If you wipe the glass and the mist doesn’t shift, that’s the tell. Wiping the outside won’t help either, because the moisture is sealed inside the unit, not sitting on a surface you can reach.
Signs worth checking for, room by room:
- Condensation or a persistent haze between the panes, not on either surface.
- A cold draught you can feel near the frame edge even with the window shut.
- Mould or discolouration building up on the sill or frame corners.
- A visible gap or gone brittle sealant strip around the perimeter.
If you’re chasing a leak rather than fogging, our guide on how to fix window leaks walks through checking frame joints, the sill, and the wall-frame connection before you assume the seal is at fault.
Should I Repair the Seal or Replace the Whole Unit?
Whether to repair or replace depends on how far the failure has progressed and what condition the frame is still in. A seal that’s just started to let in moisture is often worth replacing on its own. A unit with heavy internal staining, warping, or a cracked pane usually needs the whole glazing unit swapped out.
Option Typical cost per window What actually changes
- Window seal replacement £120 upwards depending on size. Just the failed seal; frame and existing glass stay in place
- Double glazing replacement £120 upwards depending on size. The glazing unit or pane; the frame itself stays untouched
Both sit in the same price band, which is why the decision usually comes down to condition rather than cost. If the seal has only just started to go, a seal replacement gets you back to full insulation for less disruption. If the unit itself has clouded permanently or the pane is cracked, replacing the whole glazing unit makes more sense, and it’s still a pane swap, not a full frame job. Our page on why replacing window seals matters goes into the energy and comfort side of that decision in more depth.
DIY vs Professional Seal Replacement
While minor repairs can be DIY, full seal replacements are best handled by professionals to ensure proper installation and warranty protection.
Small jobs, like re-sealing a hairline gap in exterior sealant with 100 percent silicone and a sealant gun, are within reach for a confident homeowner. A full seal replacement is a different job entirely. It means breaking the airtight seal on the whole unit and rebuilding it correctly, with the right gas fill and spacer, so it holds for another 10 to 20 years rather than failing again within months. Get that wrong and you can void any warranty on the unit.
If you want to confirm a leak before booking anyone out, our guidance on fixing window leaks covers a simple test: pour water slowly along the exterior frame and seals while someone watches from inside for drips, which tells you whether the fault sits in the seal, the sill, or the frame joint before any work starts.
How Cloudy2Clear Replaces a Failing Window Seal
A Cloudy2Clear engineer starts with a free consultation and gives you a no-obligation quote, usually turned around in under 20 minutes once the survey is done. That survey covers the affected windows, taking accurate measurements before any glass is ordered.
Installation is built to be quick and clean, with minimal disruption to the room you’re standing in. We’re replacing the pane and its seal, not the whole window, so there’s no ripping out frames or replastering. Replacement glass carries a 25-year guarantee on the glass itself, separate from whatever guarantee applies to the PVC frame. One customer, Terryb, left this review after a job at our Gatwick branch: “Excellent service received from Cloudy2ClearWidows Gatwick – Ben gave a good price and just 1 week later the replacement double glazed units were installed by Adi and Phil who were friendly, tidy and very efficient so if your d/g windows are misting up I can thoroughly recommend cloudy2clearwindows Gatwick for great service from start to finish.“
We work across the United Kingdom, including England, Scotland, London, the North West, and the North East & Yorkshire, on both private homes and commercial premises.
Keeping Your Windows Sealed After the Repair
Regular care prevents these issues from building up again once a new seal is in.
Routine maintenance can reduce energy losses by up to 15% and prevent expensive repairs later, so it’s worth a look at your windows once a year, particularly before winter. Proper care extends window lifespan by years, saving thousands on premature replacements down the line. That’s the same logic behind our double glazing care tips, and it’s worth pairing with a look at how long double glazing actually lasts so you know roughly where your own windows sit in their lifespan.
Frequently Asked Questions
- How quickly can I get a quote for a leaking double glazed window? Our engineers give a no-obligation quote in under 20 minutes once they’ve surveyed the affected windows and taken accurate measurements. The free consultation happens first, so there’s no cost to finding out whether you need a seal replacement or a full pane swap.
- Will a new seal or replacement pane come with any guarantee? Replacement glass carries a 25-year guarantee on the glass itself, separate from any guarantee on the PVC frame. That guarantee protection depends on professional installation, which is part of why full seal replacements are handled by our engineers rather than as a DIY job.
- Can I fix just the leaking pane instead of replacing the whole window? Yes. Replacing the pane rather than the frame is our core approach and has been since 2005. If one unit has failed while the rest of the window and frame are sound, we replace that pane or seal and leave the frame in place.
- Do you repair leaking double glazing in Scotland as well as England? Yes. We cover the United Kingdom, including England, Scotland, London, the North West, and the North East & Yorkshire, for both homeowners and commercial premises.
- Is condensation between the panes always caused by a failed seal? Building Science Education, a U.S. Department of Energy resource, identifies condensation between the inner and outer panes as the marker of insulated glass unit failure specifically. If the moisture sits between the panes rather than on either glass surface, the seal has failed and the unit needs attention.
