What Causes Condensation Between Double Glazing Panes, and How Do You Fix It?
Condensation between double glazing panes means the seal inside the glazing unit has failed, letting moist air in and insulating gas escape. Wiping the glass will not clear it, because the moisture sits inside the sealed unit itself. We fix this through window seal replacement or double glazing replacement, priced from £120 to £900 per window, rather than swapping out the whole frame.
Key Takeaways
- Condensation between panes signals a failed seal, not dirty glass, and wiping the surface will not clear it.
- Window seal replacement typically costs £120 to £900 per window, depending on size and location.
- Seals normally last 10 to 20 years before wear lets moisture and air past the argon fill.
- A failed seal can lose roughly 30% of a glazing unit's insulating value.
- Double glazing replacement carries a 25-year guarantee on the glass, separate from the surrounding frame.
What Causes Condensation Between Double Glazing Panes?
Condensation between the panes happens when the seal around the glazing unit breaks down and lets moist air into the gap that should hold dry insulating gas. Once that gap is compromised, temperature swings cause trapped moisture to fog the glass from the inside, where you cannot wipe it away.
Every glazing unit is built from two or three panes of glass held apart by a spacer bar, with desiccant material inside the bar to soak up any stray moisture before the unit is sealed. Around the edge, a bead of butyl or polysulphide sealant keeps the whole thing airtight. For a fuller breakdown of how these units go together, our guide on what a glazing unit is covers the components in detail.
When that seal degrades, usually from years of expansion and contraction as the seasons change, air and moisture work their way in. The desiccant can only absorb so much before it's saturated, and once it is, condensation between the panes stops being an occasional nuisance and becomes permanent.
Signs Your Glazing Unit Seal Has Failed
Condensation between the panes rarely turns up alone. It tends to arrive alongside a handful of other signs that point to the same failed seal.
Sign | What it usually means |
|---|---|
Condensation between panes | Moisture has entered the sealed unit through a broken seal |
Draughts around the window | The insulating gas has escaped and airtightness is gone |
Mould on the frame or sill | Trapped moisture is building up over time |
Discolouration or cloudiness | Long-term seal failure, with the desiccant saturated |
Any one of these on its own is worth a look. Two or more together usually mean the unit needs replacing rather than repairing at the surface.
Can You Fix Misted Double Glazing Without Replacing the Whole Window?
Yes. Misted or steamed-up double glazing is almost always a fault in the glass unit itself, not the frame around it. Replacing the failed pane restores clarity and insulation, while the existing uPVC, timber, or aluminium frame stays exactly where it is.
That's the whole idea behind replacing the pane rather than the frame. One of our engineers surveys the window, takes accurate measurements, and turns around a no-obligation quote in under 20 minutes. Our operations manager oversees that survey process to make sure the measurements match what gets installed. The new glazing unit goes in quickly and cleanly, with minimal disruption, and the glass itself carries a 25-year guarantee, covering the unit rather than the surrounding frame. We work across private homes and commercial premises throughout the United Kingdom, and as members of Checkatrade, our engineers follow that scheme's code when they survey and quote your windows.
If your windows are past the point where a pane swap makes sense, our guide on updating older double glazing covers when a fuller upgrade is worth considering instead.
Window Seal Replacement: Cost, Lifespan and What's Involved
A window seal replacement typically costs between £120 and £900 per window, with the exact price depending on the size of the unit and where it sits in the building. Seals themselves are built to last, typically between 10 and 20 years, though that range assumes normal wear rather than a manufacturing fault or accidental damage.
Most UK glazing units trap argon gas between the panes, and an intact seal is what keeps it there. Once a seal fails, a unit can lose roughly 30% of its insulating value, which is why a misted window often comes with a noticeably colder room and a higher heating bill alongside it. Our guide on why replacing window seals matters goes into more detail on what a failed seal costs you in comfort and energy. If you're weighing seal replacement against other repair costs across the house, our breakdown of double glazing costs is a useful next stop.
Caring for Your Glazing to Prevent Seal Failure
Regular care prevents these issues. Wiping down frames and checking seals for cracking or gaps a couple of times a year catches early wear before it turns into condensation between the panes.
While minor repairs can be DIY, full seal replacements are best handled by professionals to ensure proper installation and warranty protection. Realigning a stiff hinge or clearing a blocked drainage channel is well within reach for most homeowners. Replacing the sealed unit itself is a different job. It needs the right spacer bar, the correct gas fill, and a proper sealant bead, or the new unit fails just as fast as the old one.
Our guide on double glazing care tips walks through the habits that keep a glazing unit's seal intact for longer, and our piece on cleaning uPVC frames without causing damage covers which cleaning products are safe to use near the seal itself. For a general sense of how long a glazing unit should hold up before you're looking at any of this, see how long double glazing lasts.
Frequently asked questions
How do I know if my window needs a new seal or a whole new window?
If the frame itself is sound, no rot in timber, no warping in uPVC, and only the glass is misted, replacing the seal or the glazing unit is usually enough. A whole new window only becomes necessary when the frame is damaged or failing alongside the glass.
Does condensation between the panes get worse over time?
Yes. Once a seal breaks, moisture keeps entering the gap and the desiccant inside the spacer bar becomes saturated. From that point, the unit cannot dry itself out again, and the misting tends to become more constant rather than clearing on its own.
Can I remove a misted glazing unit and reseal it myself?
Minor jobs like tightening a loose handle or clearing a blocked drainage channel suit most homeowners. Full seal replacement is different: it depends on the right spacer bar, the correct gas fill, and a proper sealant bead, which is why we treat it as work for a professional installer rather than a weekend project.
Will replacing the glazing unit match my existing window style?
Replacement units are made to fit the existing frame, whether that's a casement, tilt and turn, sash, bay, or bow window, so the surrounding style and opening mechanism stay the same. Only the failed glass and its seal are swapped out.
If your windows have started misting up between the panes, tell us what you're seeing, which room it's in, and roughly how many windows are affected using the form below this article, and we'll get back to you on the right next step.