Prioritise loft insulation before double glazing for the fastest payback
Every UK homeowner faces the same question when trying to lower energy bills: should you insulate the loft or replace the windows first? The answer, based on cost, savings, and payback period, is clear. Loft insulation should be your priority.
Loft insulation costs roughly 25–75% less than double glazing and cuts heat loss through the roof by 20–30%, according to the Energy Saving Trust (Energy Saving Trust, 2026). The average payback period for loft insulation is 2–3 years, versus 15–25 years for double glazing, based on EST payback estimates and DESNZ average energy prices (DESNZ, 2026). A typical semi-detached home can save £300–£400 per year with loft insulation (EST), while double glazing saves £150–£250 per year (EST). Installing loft insulation first frees up cash for double glazing later, and the combined savings are additive.
Insulation and double glazing address different heat-loss routes
Understanding where heat escapes from your home explains why insulation wins the priority debate. Heat escapes through the roof (25%), walls (35%), floors (15%), and windows (10–15%), per EST heat-loss breakdowns (Energy Saving Trust, 2026). Loft insulation blocks the largest single loss route; double glazing only covers windows. Draught-proofing (costing £50–£100 per year in materials) can save £60–£90 per year (EST) and is the cheapest single measure (Energy Saving Trust, 2026).
Quick numbers cost, savings, and payback for 2026
| Measure | Average cost (installed) | Annual saving (semi-detached) | Payback period | Source |
|---|---|---|---|---|
| Loft insulation (270mm) | £300–£700 | £300–£400 | 2–3 years | EST |
| Cavity wall insulation | £500–£1,500 | £200–£300 | 2–5 years | EST |
| Double glazing (A-rated, full house) | £4,000–£7,500 | £150–£250 | 15–25 years | EST, FENSA |
| Draught-proofing | £50–£100 | £60–£90 | <1 year | EST |
Double glazing’s main benefit is comfort and condensation reduction, not energy savings
Double glazing offers real benefits, but they are not primarily about energy savings. A-rated double glazing reduces heat loss through windows by 50–60% compared to single glazing, but windows account for only 10–15% of total heat loss (EST) (Energy Saving Trust, 2026). Double glazing cuts condensation and noise, which are non-energy benefits not captured by payback calculations. The U-value of a new double-glazed window is typically 1.2–1.4 W/m²K, compared to 4.8–5.0 W/m²K for single glazing (British Fenestration Rating Council BFRC data) (BRE, 2026). This means a double-glazed window loses heat at roughly one-quarter the rate of a single-glazed one, but that improvement only applies to the 10–15% of your home’s heat that escapes through windows.
Loft insulation is the cheapest insulation measure with the shortest payback
Loft insulation material costs £10–£20 per roll (DIY), with professional installation adding £200–£400 (EST) (Energy Saving Trust, 2026). The recommended depth is 270mm (DESNZ Building Regulations Part L) (GOV.UK, 2026). Homes with less than 100mm of existing loft insulation save the most; topping up from 100mm to 270mm saves £100–£150 per year (EST). Cavity wall insulation costs more but saves more per year; payback is 2–5 years depending on cavity width (EST) (Energy Saving Trust, 2026).
Eligibility and installer certification what to check before you pay
Not all insulation and glazing work requires the same level of certification. Loft insulation can be DIY or installed by any general builder; no mandatory certification but check TrustMark registration for quality assurance (TrustMark, 2026). Cavity wall insulation must be installed by a CIGA-registered installer (Cavity Insulation Guarantee Agency) to qualify for the 25-year guarantee (MCS, 2026). Double glazing installers must be FENSA or CERTASS registered to self-certify compliance with Building Regulations (GOV.UK, 2026). The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing.
The 2026 priority order draught-proofing, loft insulation, then double glazing
If you want the most energy savings for the least money, follow this order. Start with draught-proofing for under £100 and a payback under one year (EST) (Energy Saving Trust, 2026). Next install or top up loft insulation to 270mm for the best energy-savings-per-pound ratio. Only after loft and walls are insulated should you replace double glazing, unless windows are rotten or single-glazed. The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing.
Insulation vs double glazing the plain-English answer for UK homeowners in 2026
Insulation is the priority because it tackles the largest heat-loss routes (roof and walls) at a lower cost and with a faster payback than double glazing. Double glazing is a comfort upgrade, not an energy-efficiency priority, unless your current windows are single-glazed or structurally failing. The most cost-effective order is: draught-proofing → loft insulation → cavity wall insulation → double glazing.
Frequently Asked Questions
Insulate the loft first. Loft insulation costs £300–£700 and saves £300–£400 yearly with a 2–3 year payback, per the Energy Saving Trust. Double glazing costs £4,000–£7,500 and saves £150–£250 yearly with a 15–25 year payback.
Loft insulation saves £300–£400 per year for a typical semi-detached home, according to the Energy Saving Trust. That is about double the £150–£250 saved by double glazing.
Double glazing has a payback period of 15–25 years, based on Energy Saving Trust estimates and DESNZ average energy prices. Loft insulation pays back in 2–3 years.
Yes, double glazing reduces condensation by keeping internal glass warmer, but it is not a primary energy-saving measure. The Energy Saving Trust notes its main benefit is comfort, not bill reduction.
Loft insulation is 25–75% cheaper. It costs £300–£700 installed, while double glazing for a whole house costs £4,000–£7,500, per Energy Saving Trust and FENSA figures.
