No, double glazed windows do not need curtains for thermal performance, but curtains remain beneficial for privacy, light control, and additional comfort. The Energy Saving Trust confirms that double glazing typically reduces heat loss by around 50-70% compared to single glazing, making curtains an optional extra rather than a necessity (Energy Saving Trust, 2026).
The answer depends largely on your priorities. If your main concern is energy efficiency, modern double glazing with a low-E coating and argon gas fill already delivers a U-value of around 1.2-1.6 W/m²K, which meets Building Regulations Part L. Curtains add minimal insulation value, typically improving the overall U-value by only 0.1-0.3 W/m²K, so they are not required to meet standards. However, if you want to reduce draughts at night or block summer heat, curtains can still help.
Thermal performance is already high
Double glazed windows work by trapping an insulating layer of air or gas between two panes. The Glass and Glazing Federation notes that a standard double glazed unit achieves a U-value of approximately 1.4 W/m²K, compared to around 4.8 W/m²K for single glazing (BRE, 2026). This means the window itself already resists heat loss effectively. Curtains add a small extra barrier, but the improvement is marginal. For most UK homes, the thermal benefit of curtains over double glazing is less than 5% of total heat loss.
Privacy and light control remain valid reasons
Even if double glazing performs well thermally, you may still want curtains for non-energy reasons. Privacy is a key factor, especially for ground-floor rooms or windows facing neighbours. Light control is another, blackout curtains can block up to 99% of daylight, which is useful for bedrooms or home cinemas. The UK government’s planning guidance does not require curtains on any window, but local regulations may apply for listed buildings (GOV.UK, 2026).
Alternatives like blinds or shutters may suit better
If you prefer a minimalist look or want to maximise natural light, blinds or shutters are common alternatives. For example, thermal blinds can reduce heat loss by around 10-20% when combined with double glazing, according to the Energy Saving Trust (Energy Saving Trust, 2026). Curtains are not the only option, and many homeowners choose none at all for south-facing rooms. The choice comes down to personal preference, not building requirements.
A worked example
For a typical 1930s semi-detached house in Manchester with 12 m² of double glazed windows, adding thermal curtains improves the overall window U-value from 1.4 W/m²K to around 1.2 W/m²K, saving roughly £35 per year on heating bills according to the Energy Saving Trust. The curtains themselves cost approximately £600 for a standard three-bedroom home. This gives a payback period of about 17 years, which is far longer than the typical 5-8 year payback for upgrading from single to double glazing. 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). Over 25 years, total savings reach around £875, but this assumes the curtains last that long without replacement. For most homes, curtains are a comfort choice rather than a financial investment.
| Item | Figure |
|---|---|
| Upfront cost after grants | £600 |
| Yearly savings | £35 |
| Payback period | 17 years |
| 25-year lifetime savings | £875 |
What homeowners often get wrong
The most common mistake is believing curtains significantly improve the energy efficiency of modern double glazing. Here are three frequent errors homeowners make with this question.
- Thinking curtains replace draught-proofing Many homeowners assume thick curtains stop all heat loss around windows. The real issue is often gaps in the frame or seal, which curtains cannot fix. Sealing those gaps with draught excluder tape costs £10-£20 and can save up to £60 per year, far more than curtains alone.
- Believing curtains always save money People expect curtains to pay for themselves through lower bills. In reality, the £600 cost for thermal curtains on double glazing saves only £35 annually, meaning you may never recoup the investment if you move home. The payback period of 17 years exceeds typical ownership periods.
- Ignoring the impact on summer overheating Homeowners buy heavy curtains for winter warmth but forget they trap heat in summer too. A south-facing room with thermal curtains closed all day can overheat by 3-5°C, increasing cooling costs. Using reflective blinds or external shutters is more effective for summer comfort.
Quick reference
- Double glazing alone reduces heat loss by 50-70% compared to single glazing, making curtains an optional extra for thermal performance.
- Adding thermal curtains to double glazing improves U-value by only 0.1-0.3 W/m²K, from roughly 1.4 to 1.2 W/m²K.
- 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).
- Curtains pay back in about 17 years on a typical semi-detached home, compared to 5-8 years for upgrading from single to double glazing.
- Closing curtains at night can reduce draughts by up to 15%, but this benefit disappears if window frames or seals are leaking air.
Frequently Asked Questions
No, double glazing already meets Building Regulations Part L with a U-value of 1.2-1.6 W/m²K. The Energy Saving Trust confirms curtains add only marginal insulation improvement.
Curtains provide privacy, block summer heat, and reduce draughts at night. Ofgem notes they can also add comfort in older homes with less airtight windows.
Yes, but only by 0.1-0.3 W/m²K according to BRE data. This is less than 5% of total heat loss, so curtains are optional for thermal reasons.
