Quick Answer
Modern double-glazed timber windows achieve U-values of 1.2–1.4 W/m²K (Building Regs require max 1.4). Triple glazing hits 0.8–1.0. Upgrading from single glazing saves £150–£300/year on heating. Timber frames insulate better than uPVC or aluminium.
U-Values Explained
U-value measures how much heat passes through a material. Lower = better. A single-glazed window has a U-value of about 5.0 W/m²K — five watts of heat escaping through every square metre for every degree of temperature difference. Modern double glazing cuts that to 1.2–1.4. Triple glazing cuts it further to 0.8–1.0.
| Glazing Type | U-Value (W/m²K) | Heat Loss vs Single | Annual Saving* |
|---|---|---|---|
| Single glazed | 5.0 | Baseline | — |
| Old double glazing (pre-2002) | 2.8–3.0 | -40% | £100–£150 |
| Modern double glazing (argon) | 1.2–1.4 | -72% | £200–£300 |
| Triple glazing (argon) | 0.8–1.0 | -82% | £250–£350 |
*Estimated annual heating savings for a 3-bed house, 10 windows, compared to single glazing. Based on current gas prices.
Why the Frame Material Matters
Most people focus on the glass, but 20–30% of a window’s surface is frame. The frame material affects overall thermal performance significantly:
- Timber — natural insulator. Thermal conductivity 0.13 W/mK. Warm to touch in winter.
- uPVC — moderate insulator at 0.16 W/mK, but hollow chambers can create convection currents.
- Aluminium — extremely poor at 160 W/mK (1,200x worse than timber). Requires thermal breaks to be viable, and even then performs worse than timber or uPVC.
Building Regulations 2026
If you’re replacing windows, they must meet current Building Regulations. The key requirements:
- Maximum whole-window U-value: 1.4 W/m²K
- Must be installed by a FENSA-registered company (or you need building control sign-off)
- Ventilation requirements must be maintained (trickle vents or equivalent)
All our windows meet or exceed these requirements as standard. We are FENSA registered — your installation is automatically notified to building control.
EPC Impact
Your Energy Performance Certificate rates your home A (most efficient) to G (least). Windows are a key component. Upgrading windows can improve your EPC score by 5–15 points:
- Single to double glazing: +10–15 points
- Old double to modern double: +3–8 points
- Double to triple: +2–5 points
This matters if you’re selling (buyers check EPC), renting (minimum EPC E required by law), or planning ahead for potential future EPC requirements for homeowners.
Configure Energy Efficient Windows
Choose double or triple glazing, timber species, and see U-values and prices instantly in our 3D configurator.
Open the 3D Configurator Book a SurveyFrequently Asked Questions
What U-value should new windows have?
Building Regulations 2026 require a maximum U-value of 1.4 W/m²K for replacement windows. The lower the number, the better. Standard double glazing achieves 1.2–1.4, triple glazing achieves 0.8–1.0.
How much can new windows save on energy bills?
Replacing single-glazed windows with double glazing saves £150–£300 per year on heating bills for a typical 3-bed house. Replacing old double glazing with modern units saves £50–£150 per year.
Do new windows improve my EPC rating?
Yes. Windows are a significant factor in EPC calculations. Upgrading from single to double glazing can improve your EPC by 5–15 points — potentially moving you up a band.
What is the most energy efficient window frame material?
Timber has the best natural insulation of any frame material. It conducts heat 1,500 times less than aluminium and significantly less than uPVC. Combined with double or triple glazing, timber-framed windows achieve the best overall thermal performance.
Are triple glazed windows worth it in the UK?
For most UK homes, double glazing provides 80% of the thermal benefit at 65% of the cost. Triple glazing is worth the premium in exposed positions, north-facing walls, or if you are aiming for a very high EPC rating.