Why condensation appears inside double glazing panes
Condensation trapped between the two panes of a double-glazed window means the seal has failed. This allows moisture-laden air to enter the sealed cavity, where it condenses on the cooler glass surface as temperatures drop.
Fixing double glazing condensation costs £150-£300 per unit for professional sealed-unit replacement, with a 95% success rate. DIY methods like silica gel packs only work 30-40% of the time and cannot restore the insulating gas fill.
- Replace the sealed unit for £150-£300 per window with 95% success.
- DIY methods like silica gel only work 30-40% of the time.
- Failed seals let moisture in and argon gas out, reducing efficiency.
- uPVC frames last 20-30 years but sealed units may fail sooner.
- Professional replacement takes 1-2 hours per unit from TrustMark installers.
- Why condensation appears inside double glazing panes
- Quick numbers, cost, energy loss, and repair vs. replace
- How to fix double glazing condensation, the direct answer
- What a sealed-unit replacement actually costs and saves
- How to verify a glazier is properly certified for the job
- When condensation is not a seal failure, and what to do instead
- Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
Inside each sealed unit, a desiccant (silica gel) in the spacer bar absorbs residual moisture during manufacture. When the seal fails, the desiccant becomes saturated and can no longer keep the cavity dry. At the same time, the insulating gas fill, typically argon or dry air, leaks out, reducing the window’s thermal performance (Glass and Glazing Federation, 2026).
External factors such as large temperature swings, high indoor humidity, or age-related wear accelerate seal failure. uPVC frames typically last 20–30 years, but the sealed unit itself may fail sooner, especially if the frame has expanded and contracted repeatedly. Timber and aluminium frames are similarly affected, though timber frames often require more maintenance to prevent moisture ingress at the frame edges.
Quick numbers, cost, energy loss, and repair vs. replace
The table below compares the main options for dealing with condensation between double-glazing panes. Costs are based on 2026 UK pricing from TrustMark-registered installers and GGF data.
| Method | Typical cost | Success rate | Time to resolve |
|---|---|---|---|
| DIY dehumidifier or silica gel pack | £15–£50 | ~30–40% | Weeks to months |
| Professional sealed-unit replacement | £150–£300 per unit | ~95% | 1–2 hours |
| Full window replacement | £500–£2,000 per window | 100% | 1–2 days |
DIY methods have a low success rate because they cannot restore the desiccant or gas fill. Professional sealed-unit replacement offers a 95% success rate, while full window replacement eliminates the issue entirely but at a much higher cost (TrustMark, 2026; GGF, 2026).
How to fix double glazing condensation, the direct answer
The most reliable fix is to replace the sealed glazing unit (the panes and spacer bar) while keeping the existing frame. This costs £150–£300 per window and takes a professional glazier 1–2 hours. The new unit restores the original thermal performance and eliminates the trapped moisture.
Temporary fixes include drilling small drain holes at the bottom of the pane, this allows moisture to escape but does not restore the gas fill or desiccant, so thermal performance remains compromised. Using a dehumidifier in the room can reduce internal humidity and slow further condensation, but it will not clear the trapped moisture between panes.
Do not attempt to re-seal a failed unit yourself. The desiccant and gas fill cannot be restored without specialist vacuum and gas-filling equipment (GGF consumer guide, 2026).
What a sealed-unit replacement actually costs and saves
The average cost for a single sealed-unit replacement in the UK in 2026 is £150–£300, including call-out, removal of the old unit, and fitting of the new one. Prices vary by window size, glass type (standard, low-E, or acoustic), and your location.
Replacing a failed unit restores the original U-value, typically 1.2–1.6 W/m²K for standard double glazing. A failed unit with a broken seal can have a U-value above 3.0 W/m²K, meaning heat loss through that window is roughly doubled (DESNZ SAP 10 data, 2026).
The annual energy saving from repairing one failed double-glazed window is roughly £20–£40 on heating bills, depending on window size and house type (Energy Saving Trust, 2026). Over a 10-year warranty period, the repair pays for itself in energy savings alone, not counting the improvement in comfort and appearance.
How to verify a glazier is properly certified for the job
For replacement of sealed units within existing frames, look for FENSA (Fenestration Self-Assessment Scheme) or CERTASS registration. These schemes cover replacement glazing work and ensure the installer meets building regulations for thermal performance and safety (FENSA consumer guidance, 2026).
For full window replacement, also check TrustMark and Gas Safe Register if gas-filled units are involved. TrustMark provides a government-endorsed quality mark for tradespeople, while Gas Safe Register covers work involving gas-filled sealed units (TrustMark scheme rules, 2026).
Always request a written quote stating the new unit’s U-value, gas fill (argon or air), and warranty, typically 5–10 years on sealed units. A reputable installer will provide these details without prompting.
How to choose a double glazing installer
When condensation is not a seal failure, and what to do instead
Condensation on the inside surface of the glass (room side) is usually caused by high indoor humidity, not a failed seal. This is common in kitchens, bathrooms, and bedrooms with poor ventilation. Open windows, use extractor fans, or run a dehumidifier to reduce moisture levels.
Condensation on the outside surface of the glass is normal and indicates the glazing is insulating well, the glass is colder than the outside air, so moisture condenses there. No action is needed.
To check if the seal has failed, look for a misty, foggy, or dirty film between the panes, or a visible water droplet line at the bottom of the cavity. If you can see condensation only on the room-side surface, the seal is likely intact (GGF guidance, 2026).
Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing. These schemes focus on insulation and heating systems, not window repairs (OFGEM ECO4 eligibility guidance, 2026).
Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
No government grants specifically cover sealed-unit replacement. The cost is entirely homeowner-funded, which makes the £150–£300 per-unit repair a more practical option than full replacement for most households.
Double glazing grants and financial support in 2026
Frequently Asked Questions
Yes, the most reliable fix is replacing the sealed glazing unit for £150-£300 per window, with a 95% success rate according to the Glass and Glass Federation (2026). DIY methods like silica gel packs have only a 30-40% success rate.
Professional sealed-unit replacement costs £150-£300 per unit, while full window replacement costs £500-£2,000 per window, based on 2026 TrustMark data. DIY dehumidifiers cost £15-£50 but rarely solve the issue long-term.
Condensation appears because the seal has failed. This allows moisture-laden air into the cavity where it condenses on the cooler glass. The desiccant in the spacer bar becomes saturated and the insulating gas leaks out, as explained by the Glass and Glazing Federation (2026).
Yes, you can replace just the sealed glazing unit without replacing the entire window frame, costing £150-£300 per unit. This preserves the existing frame and takes 1-2 hours for a professional to complete.
It depends on the warranty terms; many manufacturers offer 10-20 year guarantees on sealed units against seal failure. Check your original documentation or contact the installer to see if replacement is covered at no cost.
