Window seals are the unsung heroes of your home’s energy efficiency, keeping the British weather out and the heat in. When these seals fail, you will quickly no...
Repairing or replacing seals is a cost-effective alternative to full window replacement. It prevents moisture from entering your wall cavity and stops the growth of unsightly black mould around your frames.
What Does the Work Involve?
- Inspection: Identifying whether the failure is in the external mastic, the rubber gaskets, or the internal "blown" glazing unit.
- Preparation: Stripping away brittle, cracked, or mouldy old sealant and cleaning the joints with solvent.
- Resealing: Applying fresh, high-modulus silicone or polymer sealant between the frame and the masonry.
- Gasket Replacement: Pulling out shrunk rubber seals from the opening sashes and pushing in new, weather-tight gaskets.
- Drainage Check: Ensuring the "weep holes" in the bottom of the frame are clear to allow water to escape.
- Glass Replacement: If the seal inside the double-glazed unit has failed (causing internal misting), the entire glass unit is measured and replaced.
Typical Costs
| Item | Low £ | High £ | Notes |
|---|---|---|---|
| Call-out / Minimum Charge | £80 | £130 | Covers travel and first hour of labour. |
| Labour (per hour) | £45 | £85 | Varies by region and specialist skill. |
| Materials (Sealant/Gaskets) | £20 | £100 | High-quality silicone and rubber strips. |
| Replacement Glazed Unit | £100 | £250 | Per pane; depends on size and glass type. |
| Total (Typical 3-window repair) | £200 | £550 | Excluding scaffolding if required. |
Costs are primarily driven by access requirements and the number of windows. Working on ground-floor windows is significantly cheaper than repairs requiring specialist ladders or scaffolding for high-reach areas.
How Long Does It Take?
- Single window reseal: 1–2 hours.
- Full house gasket replacement: 1 full day for a standard semi-detached home.
- Blown unit replacement: 30–60 minutes per window (following a 7-day lead time for glass manufacturing).
- Complex repairs: 4–6 hours if frames require structural adjustment or hardware repairs.
DIY or Professional?
Replacing external mastic is a viable DIY task if you have a steady hand and a good sealant gun. However, if your windows are "blown" (condensation between the panes), this requires a professional to measure and fit a new Integrated Glass Unit (IGU).
Note: Professional repairs often come with a guarantee, which is essential if you plan to sell your home in the near future.
Choosing the Right Tradesperson
- Check Accreditations: Look for FENSA or CERTASS registration, or membership in the GGF (Glass and Glazing Federation).
- Insurance: Ensure they hold valid Public Liability Insurance (minimum £1m).
- Warranty: Ask if they provide a 5 or 10-year guarantee on both the labour and the new glass units.
- Questions to ask: "Will you check the window hinges and handles while you are here?" and "Is waste disposal of old glass included in the price?"
UK Regulations
- Part L Building Regs: Replacement glass must meet specific thermal efficiency (U-value) standards.
- Safety Glass: Any glass below 800mm from the floor or in doors must be toughened safety glass by law.
- FENSA/CERTASS: While minor seal repairs don't usually require a certificate, full frame or glass replacements should be logged with a competent person scheme.
Common Problems
- Sealing over old silicone: New sealant will not bond to old material, leading to leaks within months.
- Blocked weep holes: If a tradesperson seals the drainage holes at the bottom of the frame, water will back up and rot your sills.
- Incorrect sealant: Using cheap "bathroom" silicone externally, which degrades quickly under UV sunlight.
- Ignoring the root cause: Sometimes a "failed seal" is actually a dropped window sash that simply needs realigning.
Frequently Asked Questions
Can you fix a "blown" window without replacing the glass?
No. Once the seal inside the double-glazed unit breaks and moisture gets in, the desiccant is saturated. The only permanent fix is to replace the glass unit itself.
Why is there condensation on the outside of my windows?
This is actually a sign that your seals are working perfectly. It occurs when the outer pane is very cold because the heat is being kept inside your home efficiently.
How long should window seals last?
Most high-quality uPVC window seals and gaskets should last between 10 and 15 years before the rubber begins to perish or shrink.
Do failed seals cause black mould?
Yes. Failed seals create "cold spots" around the window frame. Warm, moist air from inside the house hits these spots, condenses into water, and triggers mould growth.
Is it worth repairing old timber window seals?
Yes, provided the wood isn't rotten. Modern "Renovations" often involve routing a groove into old timber frames to fit modern rubber draught-strips, which is much cheaper than new windows.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
