Draught proofing old sash windows without replacing them is possible and can reduce heat loss by up to 20%, according to the Energy Saving Trust (Energy Saving Trust, 2026). The key is using brush seals, compression strips, and secondary glazing to block gaps around the sash.
The effectiveness depends on your window’s condition. If the sash moves freely and the frame is sound, brush seals work well. If the window is painted shut or the frame is rotten, you may need repairs first. This approach suits most pre-1919 timber sash windows common in UK homes, where replacement is often costly or restricted in conservation areas.
Fit brush pile seals to the sash channels
Brush pile seals, available from hardware stores, block gaps between the sash and the frame. Measure the channel width and cut the self-adhesive strip to length. Press it into the side channels where the sash slides. The brush compresses when the window moves, stopping draughts without jamming. The Energy Saving Trust estimates this costs around £15–£30 per window (Energy Saving Trust, 2026). For a typical three-bedroom house with eight sash windows, total material cost is £120–£240. This method works best for windows that open regularly.
Add compression strips to the meeting rails
The horizontal gap where the top and bottom sash meet is a major draught source. Use self-adhesive foam or rubber compression strips, typically 6–10 mm thick. Apply to the top of the lower sash and the bottom of the upper sash. Close the window to compress the strip and seal the gap. The Building Research Establishment (BRE) confirms this reduces air infiltration significantly in older windows (BRE, 2026). Cost is £5–£10 per window. Replace strips every two years as they lose elasticity.
Install secondary glazing for extra insulation
Secondary glazing adds a clear panel inside the window, creating an insulating air gap. Magnetic or sliding systems fit to the existing frame without altering the sash. The Energy Saving Trust says this can reduce heat loss through windows by up to 50% (Energy Saving Trust, 2026). A DIY magnetic kit costs £50–£100 per window, while professional installation runs £150–£300. This is ideal for listed buildings or conservation areas where external changes are banned. Secondary glazing also cuts noise and condensation.
A worked example
A typical 1930s semi-detached house in Manchester with six single-glazed sash windows can reduce heat loss by 20% after draught proofing, saving roughly £195 per year on energy bills according to the Energy Saving Trust draught proofing guide. The homeowner spent £90 on brush pile seals and compression strips, plus £60 on a draught-proofing kit for the letterbox and keyholes. After fitting brush seals to the sash channels and compression strips to the meeting rails, the house felt noticeably warmer within a week. The upfront cost of £150 was recouped in under one year, and the seals last around five to eight years before needing replacement. Over 25 years, factoring in three replacements, the total savings reach approximately £3,900 at current energy prices.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 | £150 |
| Yearly savings | £195 |
| Payback period | Less than 1 year |
| 25-year lifetime savings | £3,900 |
What homeowners often get wrong
The most common mistake is assuming you need to replace the windows entirely to stop draughts, which can cost £500–£1,000 per sash window and may be prohibited in conservation areas. Here are three frequent errors to avoid
- Using expanding foam filler Many people fill sash gaps with expanding foam, which hardens and prevents the window from opening or closing. The right approach is using brush pile seals that allow movement while blocking draughts, avoiding a £200 repair bill to remove set foam.
- Ignoring the meeting rail gap Homeowners often seal the side channels but forget the horizontal gap where the top and bottom sashes meet, leaving a major draught path. Fitting a compression strip here for under £10 can stop cold air from pouring in and improve comfort immediately.
- Skipping the frame condition check Applying seals to a rotten or warped frame wastes money because the draughts come through the timber, not the sash gaps. Checking for rot and sealing the frame with paint or filler first costs £20–£50 and ensures the seals actually work, otherwise you lose the full £150 investment.
Quick reference
- Draught proofing old sash windows can cut heat loss by up to 20% according to the Energy Saving Trust.
- Brush pile seals cost £15–£30 per window and are available from most UK hardware stores or online.
- 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).
- Compression strips for meeting rails cost under £10 and take 10 minutes to fit per window.
- Replacing a single sash window costs £500–£1,000, making draught proofing at £150 for six windows far more cost-effective.
Frequently Asked Questions
Brush pile seals cost £15–£30 per window, according to the Energy Saving Trust. For a typical three-bedroom house with eight sash windows, total material cost is £120–£240.
Brush pile seals are best for sash channels that move freely, while compression strips work well on meeting rails. Both are recommended by the Energy Saving Trust for UK homes.
Yes, draught proofing without replacing the windows is often allowed in conservation areas. Use brush seals and compression strips to improve efficiency without altering the appearance.
