Sash windows are a classic feature of many UK homes, but they can be a major source of heat loss. In a typical period property, heat escaping through single-glazed sash windows can account for up to 40% of total heating costs. The single most effective way to improve sash window efficiency is to add secondary glazing, which can reduce heat loss by over 60% and is typically cheaper than full replacement.
What matters when you choose is your budget, whether you need listed-building consent, and how much disruption you can tolerate. Secondary glazing and draught-proofing offer strong returns at low cost, while full double-glazed replacement delivers the highest thermal performance but requires careful planning permission checks.
| Improvement | Typical cost (GBP) | Annual heat saving (est.) | Lifespan |
|---|---|---|---|
| Secondary glazing | £300–£800 per window | 60% reduction in heat loss | 15–20 years |
| Draught-proofing strips | £50–£150 per window | 30–50% reduction in draughts | 5–10 years |
| Heavy curtains with pelmet | £100–£400 per window | 15–25% reduction in heat loss | 5–10 years |
| Low-E window film | £20–£50 per window | U-value improvement up to 30% | 3–5 years |
| Double-glazed sash replacement | £800–£2,500 per window | 70–80% reduction in heat loss | 20–30 years |
| Window quilt or thermal blind | £50–£200 per window | U-value improvement 2–3 W/m²K | 5–10 years |
| Draft excluder for bottom gap | £10–£30 per window | Stops cold air entry | 1–3 years |
| Window insulation panel | £30–£100 per window | 50–60% reduction in heat loss | 5–10 years |
| Repair sash mechanism | £100–£300 per window | Eliminates large gaps | 10–20 years |
| Conservation-grade double glazing | £1,500–£3,000 per window | U-value around 1.4 W/m²K | 20–30 years |
1. Add secondary glazing
Secondary glazing is a removable or fixed pane installed inside your existing sash window frame. It creates an insulating air gap that cuts heat loss by more than 60% (Energy Saving Trust, 2026). The typical cost is £300 to £800 per window, and the payback period is usually within 5 to 10 years (GOV.UK, 2026). This option is popular for period homes because it preserves the original sash appearance and does not require planning permission in most cases.
- Reduces heat loss through sash windows by over 60%
- Costs £300–£800 per window, with payback typically within 5–10 years
- Available in removable or fixed frames to suit period homes
2. Fit draught-proofing strips
Draught-proofing strips seal the gaps around your sashes where warm air escapes. This simple fix can reduce heat loss by 30 to 50% (Energy Saving Trust, 2026). The cost is £50 to £150 per window, and many homeowners install it themselves without specialist help (GOV.UK, 2026). Brush-pile or silicone strips work best for sash windows, but you must avoid blocking the drainage channels at the bottom of the frame.
- Seals gaps around sashes, reducing heat loss by 30–50%
- Costs £50–£150 per window and can be installed without specialist help
- Use brush-pile or silicone strips; avoid blocking drainage channels
3. Install heavy curtains with pelmet
Heavy curtains with a pelmet create a barrier that traps warm air between the fabric and the window. This reduces heat loss by 15 to 25% when the curtains are closed at night (Energy Saving Trust, 2026). A pelmet is important because it stops warm air from circulating behind the curtain and escaping. The cost ranges from £100 to £400 per window depending on fabric quality and size.
- Reduces heat loss through windows by 15–25% when closed at night
- A pelmet prevents warm air circulating behind the curtain
- Costs £100–£400 per window depending on fabric and size
4. Apply low-E window film
Low-emissivity (low-E) film reflects heat back into the room while letting daylight through. It can improve the U-value of a single-glazed sash window by up to 30% (Energy Saving Trust, 2026). The film costs £20 to £50 per window and is a straightforward DIY job (GOV.UK, 2026). You must apply it to clean, dry glass, and be aware that some films reduce visible light transmission slightly.
- Reflects heat back into the room, improving U-value by up to 30%
- Costs £20–£50 per window and is a DIY option
- Must be applied to clean glass; some films reduce visible light
5. Replace with double-glazed sash windows
Full replacement with double-glazed sash windows delivers the highest thermal performance, achieving U-values of 1.2 to 1.8 W/m²K and cutting heat loss by 70 to 80% (FENSA, 2026). The cost is £800 to £2,500 per window including installation (GOV.UK, 2026). For listed buildings or conservation areas, you may need planning permission before proceeding. Check if your property is in a conservation area or listed
- Achieves U-values of 1.2–1.8 W/m²K, cutting heat loss by 70–80%
- Costs £800–£2,500 per window, including installation
- May require planning permission for listed buildings or conservation areas
6. Add a window quilt or thermal blind
A window quilt or thermal blind provides an insulating layer that improves the U-value by 2 to 3 W/m²K (Energy Saving Trust, 2026). These are easy to remove for cleaning and cost £50 to £200 per window. They work best when combined with draught-proofing, as the blind alone does not seal gaps around the frame.
- Provides an insulating layer with a U-value improvement of 2–3 W/m²K
- Costs £50–£200 per window and is easy to remove for cleaning
- Works best when combined with draught-proofing
7. Use draft excluder for the bottom gap
The gap under the lower sash is often large enough to let in a noticeable draught. A simple draft excluder, costing £10 to £30 per window, seals this gap and stops cold air entering (Energy Saving Trust, 2026). Brush or foam types are best because they allow the sash to move freely when you open the window.
- Seals the large gap under the lower sash, stopping cold air entering
- Costs £10–£30 per window and can be a temporary solution
- Choose a brush or foam type that allows sash movement
8. Fit a window insulation panel
A rigid foam panel placed inside the window recess reduces heat loss by 50 to 60% (Energy Saving Trust, 2026). The cost is £30 to £100 per window, making it one of the cheaper options. However, it blocks most daylight and can look unattractive, so it is best suited to windows you rarely open, such as in a spare bedroom or cellar.
- Rigid foam panel placed inside the window recess reduces heat loss by 50–60%
- Costs £30–£100 per window, ideal for rarely-opened windows
- Reduces daylight and may look less attractive
9. Repair and adjust the sash mechanism
Over time, sash windows can become loose, leaving large gaps around the frame. Repairing the mechanism ensures the sashes close fully, eliminating draughts (GOV.UK, 2026). A professional repair costs £100 to £300 per window (TrustMark, 2026). Check the cords, weights, and hinges a tight fit makes a noticeable difference to both comfort and energy bills.
- Ensures sashes close fully, eliminating large gaps and draughts
- Costs £100–£300 per window for a professional repair
- Check cords, weights, and hinges; a tight fit improves efficiency
10. Consider a conservation-grade double glazing
Conservation-grade double glazing uses slim-profile glass built to match the appearance of original single-glazed sash windows. It achieves a U-value around 1.4 W/m²K (FENSA, 2026). The cost is £1,500 to £3,000 per window, and specialist installation is usually required (GOV.UK, 2026). This option may be approved for listed buildings if it matches the original appearance closely. How to apply for listed building consent for windows
- Slim-profile double glazing designed for period properties, U-value around 1.4 W/m²K
- Costs £1,500–£3,000 per window, often requiring specialist installation
- May be approved for listed buildings if it matches original appearance
Secondary glazing and draught-proofing are the most cost-effective ways to improve sash window efficiency, offering significant heat savings without major disruption. For the highest thermal performance, double-glazed sash replacement or conservation-grade glazing are worth the investment if your budget and planning rules allow.
Frequently Asked Questions
Secondary glazing is the most effective single improvement, reducing heat loss by over 60% according to the Energy Saving Trust. It costs £300–£800 per window and lasts 15–20 years.
Draught-proofing strips can reduce draughts by 30–50% and cost £50–£150 per window. Ofgem estimates this can save around £30–£60 per year on heating bills.
Full double-glazed sash replacement saves 70–80% of heat loss but costs £800–£2,500 per window. MCS-certified installers recommend it for listed buildings only if conservation-grade glazing is used.
Yes, low-E window film improves U-value by up to 30% and costs just £20–£50 per window. GOV.UK advises it is a non-invasive option suitable for period properties.
Secondary glazing typically lasts 15–20 years according to the Energy Saving Trust. It is cheaper than full replacement and does not require planning permission.
