Condensation between panes means your sealed unit has failed
If you see mist, fog, or water droplets trapped between the two panes of glass in a double-glazed window, the sealed unit has failed. This is a distinct problem from condensation on the room-facing surface of the glass, which is caused by indoor humidity and can be wiped away.
Condensation between panes costs £150–£350 per window to fix. The seal is broken and cannot be repaired, only the sealed unit must be replaced. Compare quotes from local glaziers to find the best price.
- Condensation between panes signals a failed sealed unit.
- Wipe both glass surfaces to confirm the seal is broken.
- Replace the sealed unit only, costs £150–£350 per window.
- Replacing a failed unit cuts heat loss by up to 15%.
- Sealed units typically last 15–20 years before failing.
- Condensation between panes means your sealed unit has failed
- How to confirm it is condensation between panes, not surface condensation
- Quick numbers — typical costs and savings for replacing a failed double-glazed unit
- The direct answer — replacing the sealed unit is the only fix for condensation between panes
- How to verify an installer for sealed-unit replacement
- What affects the cost of replacing a sealed unit
- When replacing the whole window frame makes more financial sense than just the sealed unit
The airtight seal between the panes has been compromised, allowing moisture-laden air to enter the gap. Once the seal is broken, the unit cannot be repaired. The only fix is to replace the sealed unit itself.
The Glass and Glazing Federation (GGF) states that a failed sealed unit must be replaced, not repaired (GGF technical guidance, 2026).
How to confirm it is condensation between panes, not surface condensation
To confirm you have condensation between panes, wipe the inside and outside surfaces of the glass with a dry cloth. If the condensation remains between the panes after wiping both sides, the seal is broken.
Condensation between panes will not clear when you wipe the glass surface, unlike surface condensation which disappears. Look for a foggy or misted appearance that does not change with temperature shifts, or visible water droplets trapped inside the gap.
The Energy Saving Trust (EST) explains that condensation between panes is a sign of seal failure and cannot be fixed by improving ventilation or reducing humidity (Energy Saving Trust, 2026).
Quick numbers — typical costs and savings for replacing a failed double-glazed unit
| Item | Typical cost or value |
|---|---|
| Cost to replace one sealed unit (supply and fit) | £150–£350 per window |
| Cost to replace entire window frame | £400–£1,200 per window |
| Energy saving from replacing a failed unit | Up to 15% reduction in heat loss through that window |
| Typical payback period | 5–10 years depending on window size and heating type |
| Average lifespan of a double-glazed sealed unit | 15–20 years |
Sources: Checkatrade 2025/26 national average pricing data; Energy Saving Trust window heat-loss estimates (Checkatrade, 2026; Energy Saving Trust, 2026).
Replacing the sealed unit is the only fix for condensation between panes
There is no DIY repair for a failed seal. The glass unit must be professionally replaced. You can replace just the sealed unit (the glass panes) without replacing the entire window frame, provided the frame is in good condition.
The cost is typically £150–£350 per window including labour, which is cheaper than full window replacement. Replacing the unit restores thermal performance and stops further moisture damage to the frame or surrounding wall.
The Glass and Glazing Federation (GGF) confirms that only the sealed unit needs replacing when the seal has failed (GGF technical guidance, 2026).
How to verify an installer for sealed-unit replacement
Use an MCS-certified or FENSA-registered installer for guaranteed workmanship. Check the installer is TrustMark-registered, which covers consumer protection and insurance.
Ask for a written warranty on the new sealed unit. A minimum 5-year warranty is standard for quality installations. Verify the installer’s registration on the MCS website or FENSA database before agreeing to work.
The MCS register, FENSA register, and TrustMark website all provide free online checks (MCS register, 2026; FENSA register, 2026; TrustMark, 2026). How to find a certified window installer
What affects the cost of replacing a sealed unit
The cost varies depending on several factors. Window size and type are the main drivers — larger or non-standard sizes cost more, up to £500 for large panes. The type of glass also matters: standard double glazing is cheapest, while triple glazing or low-E coatings add £50–£150 per unit.
Replacing multiple units at once may reduce per-unit labour costs. Access difficulty also affects pricing — upstairs windows or those behind conservatories may increase labour charges.
Checkatrade 2025/26 national average pricing data and FENSA installation cost surveys provide these estimates (Checkatrade, 2026; FENSA, 2026).
When replacing the whole window frame makes more financial sense than just the sealed unit
If the frame is over 20 years old, has rot, warping, or draughts, replacing the whole window is more cost-effective. Replacing the whole window adds insulation benefits from modern frame materials and better seals.
Full window replacement costs £400–£1,200 per window but can improve overall home energy efficiency by 10–20%. If you plan to stay in the home for more than 10 years, full replacement may offer better long-term value.
The Energy Saving Trust recommends considering full window replacement for frames older than 20 years (Energy Saving Trust, 2026). When to replace windows vs repair them Window replacement grants and funding 2026
Frequently Asked Questions
Condensation between panes is caused by a failed airtight seal. The Glass and Glazing Federation (GGF) states that moisture enters the gap once the seal is broken, and the unit cannot be repaired.
The only fix is to replace the sealed unit. The Glass and Glazing Federation (GGF) says failed units cannot be repaired. Replacement costs £150–£350 per window according to Checkatrade.
No, condensation between panes cannot be repaired. The Glass and Glazing Federation (GGF) confirms the sealed unit must be replaced entirely, not fixed.
Replacing a failed sealed unit costs £150–£350 per window according to Checkatrade. Replacing the entire window frame costs £400–£1,200 per window.
Yes, condensation between panes increases heat loss. The Energy Saving Trust explains that replacing a failed unit can reduce heat loss through that window by up to 15%, saving on energy bills.
