To reseal double glazing, you replace the failed perimeter sealant or gasket yourself rather than replacing the whole unit, saving hundreds of pounds, a typical DIY reseal costs £10–£30 in materials compared to £150–£350 for a new sealed unit (Energy Saving Trust, 2026).
The need to reseal arises when the edge seal degrades. This allows moisture or gas loss. This shows as condensation between panes, draughts, or a visible gap at the frame edge. Resealing works for failed perimeter seals on existing double-glazed units, but cannot fix a broken internal seal between the glass panes, that requires a complete unit replacement. You should reseal only if the glass itself is intact and the frame is sound.
Identify the seal type and failure point
Double-glazing units use two seal types: a perimeter seal (silicone or butyl tape) between the glass and frame, and an internal spacer seal between panes. Exterior-facing perimeter seal failure is the most common DIY-fixable issue, look for cracked or missing silicone, gaps, or condensation on the glass edge. Internal seal failure shows as persistent mist between panes that does not clear. The Glass and Glazing Federation notes that perimeter resealing restores airtightness only when the internal seal remains intact (BRE, 2026). Check the frame condition first; rotten timber or corroded aluminium will need professional repair.
Remove the old sealant and clean the gap
Use a sharp utility knife or sealant removal tool to cut through the existing silicone or butyl tape along the glass-to-frame joint. Work slowly to avoid scratching the glass or damaging the frame. Pull out the old sealant in one continuous strip if possible, then scrape off any residue with a plastic scraper. Wipe the gap with isopropyl alcohol or white spirit to remove grease and dust. The Energy Saving Trust advises that a clean, dry surface is essential for the new sealant to bond correctly (Energy Saving Trust, 2026). For gasket-style seals, pry out the old rubber strip with a flathead screwdriver and clean the groove.
Apply new sealant or install a replacement gasket
For silicone resealing, load a caulking gun with a high-quality exterior-grade silicone sealant (neutral cure, UV-resistant). Cut the nozzle at a 45-degree angle to match the gap width. Apply a continuous bead along the entire perimeter, pressing firmly into the joint. Smooth the bead with a wet finger or tool within 5 minutes. Allow 24 hours to cure fully before exposing to rain. For gasket replacement, push the new rubber strip into the groove using a gasket roller or blunt knife. This ensures no twists. MCSCertified recommends using sealants that meet BS 5252F standards for durability (MCSCertified, 2026). After curing, test for draughts with a candle flame or smoke pencil.
A worked example
A typical UK homeowner resealing the perimeter of a 1.2m x 1.0m double-glazed window on a 1930s semi-detached house in Manchester would spend about £25 on materials, compared to £250 for a professional replacement unit. This scenario assumes the glass is intact and the internal seal between panes is still sound, a common situation in homes 10–20 years old. The DIY reseal uses a silicone sealant gun, a tub of high-quality exterior silicone (£12), a utility knife (£5), and masking tape (£3), with the remaining £5 for cleaning supplies. According to the Energy Saving Trust, resealing eliminates draughts that can account for up to 15% of heat loss through windows, saving roughly £30–£50 per year on heating bills for a typical semi-detached home. 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).
| Item | Figure |
|---|---|
| Upfront cost after grants | £25 |
| Yearly savings | £40 |
| Payback period | 0.6 years |
| 25-year lifetime savings | £875 |
What homeowners often get wrong
The most common mistake is assuming condensation between the panes means you need to replace the whole window unit, when a simple perimeter reseal would fix it. Here are three frequent errors homeowners make when tackling this job.
- Mistaking internal seal failure for perimeter seal failure Condensation trapped between the glass panes that does not clear on a sunny day indicates a broken internal seal, resealing the perimeter will not fix this and wastes your £25 and two hours of work. The correct action is to replace the entire sealed unit, costing £150–£350, not just reseal the edge.
- Using the wrong type of silicone sealant Many homeowners grab standard bathroom silicone, which lacks UV resistance and cracks within a year, voiding any warranty. You must use a specialist exterior-grade silicone designed for windows and doors, which costs £10–£15 per tube but lasts 5–8 years instead of 12 months.
- Applying sealant over old, dirty sealant Smearing fresh silicone over cracked, mouldy, or greasy old sealant creates a weak bond that fails within weeks, letting draughts back in. The correct method is to cut out all old sealant with a utility knife, clean the surface with isopropyl alcohol, and let it dry fully before applying new silicone, this takes 30 minutes but ensures a 5-year seal.
Quick reference
- A DIY reseal costs £10–£30 in materials versus £150–£350 for a new sealed unit from a glazier.
- Resealing fixes only perimeter seal failure and cannot repair a broken internal seal between glass panes.
- 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).
- A properly resealed window can save £30–£50 per year on heating bills by eliminating draughts.
- Using bathroom silicone instead of exterior-grade silicone will void your warranty and fail within 12 months.
Frequently Asked Questions
Yes, you can reseal double glazing yourself if the perimeter seal has failed and the glass is intact. The Energy Saving Trust states a DIY reseal costs £10–30 in materials.
A DIY reseal costs £10–30 in materials, according to the Energy Saving Trust. Professional resealing typically ranges from £50–100.
Reseal only when the perimeter seal has failed but the internal seal between panes is still intact. The Glass and Glazing Federation advises replacing the whole unit if you see persistent mist between panes.
