Why Windows Matter More Than Ever in 2026
With UK energy prices still well above pre-2022 levels, every watt of heat lost through your windows is now genuinely expensive. Windows account for approximately 20-25% of total heat loss in a typical UK home. If your sealed units have failed, that figure can climb to 35%+ - blown double glazing performs only marginally better than single glazing.
The good news: most of that loss is preventable, and the biggest wins don't require replacing your windows.
Understanding U-Values (The Single Most Important Number)
The U-value measures how much heat passes through a material per square metre per degree of temperature difference. Lower = better insulation.
| Glazing type | Typical U-value (W/m²K) | Relative heat loss |
|---|---|---|
| Single glazing | ~5.0 | 5x worse than modern |
| Old double glazing (air filled, 1990s) | ~2.8 | 2.5x worse than modern |
| Failed/blown double glazing | ~3.5-4.5 | 3-4x worse than modern |
| Modern double glazing (argon + Low-E) | ~1.1 | Baseline |
| Triple glazing | ~0.7 | 35% better than modern double |
Building Regs Part L now requires replacement windows in England to achieve a U-value of 1.4 W/m²K or better. Most quality sealed units we fit comfortably beat this.
WER Ratings (The Easier-to-Understand Version)
The Window Energy Rating (WER) sticker uses A++ to G - just like fridges. A WER A-rated window is around 30% more energy efficient than a C-rated one. Most modern double glazed sealed units we fit are WER A or A+.
The Three Things That Make Glass Energy Efficient
1. Low-E (Low Emissivity) Coating
An invisibly thin metallic coating on the inner glass surface that reflects long-wave heat back into the room. You can't see it, but it can cut heat loss through the glass by 25-40%. It's now standard on all our replacement units.
2. Inert Gas Fill
The cavity between the panes is filled with a heavier gas that conducts heat less efficiently than air:
- Argon - 30% better than air, the cost-effective standard
- Krypton - 50% better than air, used in slim cavities and triple glazing
- Xenon - 65% better than air, very rarely justifies its cost
3. Warm-Edge Spacer Bars
The aluminium spacers used in older units conduct heat readily, creating cold edges and condensation. Modern warm-edge spacers (typically a composite or foam) cut edge heat loss by up to 65%.
Annual Savings From Upgrading
For a typical 3-bed semi in Essex switching from blown 1990s units to modern argon + Low-E sealed units:
- Annual heating saving: £180-£320 (at 2026 energy prices)
- Carbon saving: ~400kg CO₂ per year
- Comfort: No more cold spots, no condensation, less noise
- EPC rating: Often a one-band uplift (D→C, or C→B)
Upgrading Without Replacing the Whole Window
Here's what most homeowners don't realise: you can get all the energy benefits of brand-new windows just by replacing the sealed units. We fit identical modern argon + Low-E + warm-edge units inside your existing frames. The performance is the same. The cost is a fraction.
Detail in our repair vs replacement comparison.
Other Quick Energy Wins
- Replace failing rubber gaskets - draughts around the sash account for 15-20% of window heat loss
- Reseal the external silicone - cheap, fast, prevents water ingress
- Use thermal-lined curtains - close at dusk for an extra 10-15% retention
- Service window hardware - badly closing windows leak heat continuously
What About Triple Glazing?
Triple glazing improves the U-value to ~0.7, but the practical heating saving over modern double glazing is modest in the UK climate (typically £30-£60 per year per home), while the cost is significantly higher. We recommend it only for very large windows, exposed locations or new-build passive house standards.
Get an Honest Energy Assessment
Across Essex and East London we offer free window energy assessments. We'll measure U-values where possible, identify failed units, and tell you the realistic annual saving from each option - repair, partial upgrade or full replacement. No sales script, no pressure.