If you've noticed a foggy, cloudy, or misty appearance between the panes of your double glazed windows, you're not alone. It's one of the most common glazing problems in the UK — and it's almost always caused by the same thing: a failed edge seal.
How Double Glazing Works
A double glazed unit consists of two panes of glass separated by a spacer bar, with the gap between them filled with dry air or an inert gas like argon. The entire unit is sealed around the edges with a durable sealant to keep moisture out and the insulating gas in.
This sealed air gap is what gives double glazing its insulating properties — it acts as a thermal barrier between the cold outside and the warm inside of your home.
Why the Seal Fails
Over time — typically after 15 to 25 years — the edge seal deteriorates. Several factors accelerate this process:
- • UV exposure: South-facing windows degrade faster due to constant sunlight
- • Thermal cycling: Daily heating and cooling causes the glass to expand and contract, stressing the seal
- • Poor original installation: Cheaply made units or incorrect fitting can lead to early failure
- • Coastal exposure: Salt air in areas like Portsmouth and Fareham corrodes sealants faster
- • Water standing on the frame: Blocked drainage holes in UPVC frames allow water to sit against the seal
What Happens When the Seal Breaks
Once the seal fails, moisture-laden air enters the gap between the panes. When the temperature drops, this moisture condenses on the inner surfaces of the glass — creating the foggy, misty appearance that you can't wipe away because it's trapped inside the unit. Over time, you may also notice white mineral deposits on the glass as the condensation evaporates and re-forms repeatedly.
Can Misted Windows Be Fixed?
Yes — and the fix is usually straightforward. In most cases, the failed sealed unit can be replaced within your existing window frame. This means you get a brand new, clear double glazed unit without the cost or disruption of replacing the entire window.
A typical sealed unit replacement takes under an hour per window and costs a fraction of full window replacement. It's one of the most cost-effective home improvements you can make.
When Is Full Window Replacement Needed?
If the UPVC frame itself is damaged — cracked, warped, discoloured, or failing in multiple places — then full window replacement may be the better long-term investment. This is particularly common with windows that are 25+ years old, where the frames and hardware have reached the end of their useful life. We offer both repairs and full replacements, and we'll always give you an honest recommendation.
Frequently Asked Questions
Why are my double glazed windows misty inside?
Misting between the panes is caused by a failed seal. Moisture enters the gap and condenses on the glass — you can't wipe it away because it's inside the unit.
Can it be fixed without replacing the whole window?
Yes, the sealed glass unit can usually be replaced within the existing frame. It's significantly cheaper and faster than full window replacement.
How long do sealed units last?
Most sealed units last 15–25 years, depending on quality, installation, and weather exposure.
Does misted glazing affect energy bills?
Yes. A failed unit loses its insulating gas, reducing thermal performance and increasing heating costs.