Direct answer the single most effective way to cut window heat loss
The most effective way to cut window heat loss is to upgrade to double or triple glazing with a low-E coating, which can reduce heat loss by up to 54% compared to single glazing. Secondary glazing and heavy curtains with thermal linings are the next best options for homeowners who cannot replace their windows.
What matters when you choose
Before you spend money on new windows, check your current glazing type and frame condition. The biggest savings come from replacing single glazing, but if that is not possible, secondary glazing or insulating curtains can still make a noticeable difference. Always look for FENSA-certified installers to ensure the work meets building regulations (FENSA, 2026).
| Measure | Typical U-value (W/m²K) | Heat loss reduction vs single glazing | Estimated cost per window (GBP) |
|---|---|---|---|
| Single glazing | 5.0–5.8 | – | – |
| Double glazing (low-E, argon) | 1.2–1.8 | 50–54% | £300–£800 |
| Triple glazing | 0.7–1.0 | 60–65% | £500–£1,200 |
| Secondary glazing | 1.0–2.0 | 40–60% | £200–£600 |
| Heavy curtains + thermal lining | – | 25–40% | £30–£100 |
1. Upgrade to double glazing
Double glazing with a low-E coating and argon gas fill is the most effective single measure you can take. It can cut heat loss by up to 54% compared to single glazing and typically pays for itself within 5–10 years through lower energy bills.
- Double glazing can reduce heat loss by up to 54% compared to single glazing (Energy Saving Trust, 2026).
- A low-E coating and argon gas fill improve performance further (GOV.UK, 2026).
- Ensure installation is done by a FENSA-registered installer to comply with building regulations.
2. Fit secondary glazing
Secondary glazing is a cost-effective alternative when you cannot replace existing windows. It adds an extra pane on the inside, trapping air and reducing heat loss by 40–60%.
- Secondary glazing reduces heat loss by 40–60% and is cheaper than full replacement (Energy Saving Trust, 2026).
- It also cuts draughts and noise (GOV.UK, 2026).
- Costs range from £200 to £600 per window. That makes it one of the most cost-effective upgrades.
3. Install heavy curtains with thermal lining
Thermal curtains are a low-cost, high-impact solution. When closed at night, they can reduce heat loss through windows by up to 40% and help keep warmth inside.
- Thermal curtains can reduce heat loss through windows by up to 40% (Energy Saving Trust, 2026).
- Close them at dusk and keep them open during sunny winter days to let in solar heat.
- Costs start at around £30 per window, making this the cheapest option on the list.
4. Apply window film
Low-E window film is a DIY-friendly option that reflects heat back into the room. It is ideal for single-glazed or older double-glazed windows and can reduce heat loss by up to 30%.
- Low-E window film reflects heat back into the room, reducing heat loss by up to 30% (Energy Saving Trust, 2026).
- It is a low-cost DIY option, typically costing £10–£30 per window.
- Application takes around 30 minutes per window and lasts several years.
5. Use draught-proofing strips
Draught-proofing windows can save around £45 per year on energy bills. Self-adhesive foam strips or metal brush strips work for most window types and are easy to fit.
- Draught-proofing windows can save around £45 per year on energy bills (Energy Saving Trust, 2026).
- Self-adhesive foam strips or metal brush strips work for most window types.
- Cost is typically £5–£15 per window. That makes it one of the cheapest fixes.
6. Repair or replace broken seals
Failed double-glazing seals cause condensation and heat loss. Repair costs are relatively low, but if the seal has gone completely, the window may need replacing to restore efficiency.
- Failed double-glazing seals cause condensation and heat loss; repair costs £50–£150 per window (FENSA, 2026).
- If the seal has gone, the window may need replacing to restore efficiency.
- Check for misting between panes as a sign of seal failure.
7. Install blinds with a thermal backing
Cellular (honeycomb) blinds trap air in their structure and can reduce heat loss by up to 25%. Roman blinds with a thermal lining also work well.
- Cellular (honeycomb) blinds trap air and can reduce heat loss by up to 25% (Energy Saving Trust, 2026).
- Roman blinds with a thermal lining also work well.
- Costs range from £20 to £80 per window, depending on size and material.
8. Fit a window quilt or thermal panel
A removable window quilt can cut heat loss by 50–70% on single-glazed windows. These are ideal for rarely used rooms or overnight use.
- A removable window quilt can cut heat loss by 50–70% on single-glazed windows (Energy Saving Trust, 2026).
- These are ideal for rarely used rooms or overnight use.
- Costs range from £40 to £120 per window, depending on size and insulation thickness.
9. Add external shutters
External wooden or insulated shutters reduce heat loss by up to 50% when closed. They also provide security and solar shading in summer.
- External wooden or insulated shutters reduce heat loss by up to 50% when closed (Energy Saving Trust, 2026).
- They also provide security and solar shading in summer.
- Costs range from £200 to £600 per window, depending on material and size.
10. Check for frame gaps and sealant cracks
Gaps around window frames can lose as much heat as an open window. Filling these gaps with silicone sealant or expanding foam is a simple, low-cost fix.
- Gaps around window frames can lose as much heat as an open window (GOV.UK, 2026).
- Fill gaps with silicone sealant or expanding foam, and replace old putty around single-glazed panes.
- Cost is typically £5–£20 per window. That makes it one of the cheapest improvements.
These ten measures range from simple DIY fixes to professional installations, so you can choose what fits your budget and home. For more ways to improve your home’s energy efficiency, see our guide on reducing overall heat loss in your home and choosing the right insulation for your property.
Frequently Asked Questions
The most effective way is upgrading to double glazing with a low-E coating and argon gas fill, which cuts heat loss by up to 54% compared to single glazing. The Energy Saving Trust confirms this as the best single measure.
Double glazing reduces heat loss by 50–54% compared to single glazing, according to the Energy Saving Trust (2026). Triple glazing offers a 60–65% reduction but costs more.
Yes, secondary glazing reduces heat loss by 40–60% compared to single glazing. It is a cost-effective option at £200–£600 per window, per MCS standards.
Heavy curtains with thermal lining reduce heat loss by 25–40%, according to the Energy Saving Trust. They cost £30–£100 per window and are a quick, low-cost fix.
For effective heat loss reduction, aim for a U-value of 1.2–1.8 W/m²K with double glazing or 0.7–1.0 with triple glazing. Single glazing has a U-value of 5.0–5.8, per GOV.UK.
