Window Energy Savings Guide
Understanding Window U-Values and Energy Performance
Your windows are responsible for 10-25% of heat loss in a typical home. Understanding U-values, glass types, and frame materials helps you choose the most cost-effective upgrade. Use our double glazing calculator to estimate savings for your specific home.
U-Values Explained: What the Numbers Mean
| Glazing Type | U-Value (W/m2K) | Heat Loss Rating | Annual Heat Loss (per m2) |
|---|---|---|---|
| Single glazing | 5.6-5.8 | Very poor | 110-115 kWh |
| Old double glazing (pre-2002) | 2.8-3.0 | Poor | 55-60 kWh |
| Modern double glazing | 1.2-1.4 | Good | 24-28 kWh |
| Double glazing + low-E | 1.0-1.2 | Very good | 20-24 kWh |
| Triple glazing | 0.6-0.8 | Excellent | 12-16 kWh |
| Triple glazing + low-E + argon | 0.5-0.6 | Outstanding | 10-12 kWh |
The U-value tells you how many watts of heat pass through one square metre of window for every degree of temperature difference between inside and outside. A U-value of 1.4 means that when it is 20C inside and 0C outside, each square metre of glass lets 28 watts escape.
Frame Material Comparison
| Feature | uPVC | Aluminium | Timber | Composite |
|---|---|---|---|---|
| Thermal performance | Good (1.3-1.6) | Fair (1.6-2.0) | Excellent (1.2-1.4) | Excellent (1.0-1.3) |
| Lifespan | 20-30 years | 30-45 years | 35-60 years | 40+ years |
| Maintenance | Wipe clean | Wipe clean | Repaint every 5-8 years | Wipe clean |
| Appearance | Standard | Slim, modern | Traditional, warm | Premium |
| Colour options | Wide (foil wrap) | Unlimited (powder coat) | Unlimited (paint) | Unlimited |
| Recyclability | Yes (6-7 times) | Fully recyclable | Biodegradable | Partially |
| Cost bracket | Budget-mid | Mid-premium | Premium | Premium |
Secondary Glazing: A Budget Alternative
Secondary glazing involves fitting an additional glass or acrylic panel on the inside of existing windows. It is ideal for listed buildings where planning restrictions prevent replacement, renters who cannot modify the property, and tight budgets.
| Secondary Glazing Type | DIY Cost per Window | U-Value Achieved | Noise Reduction |
|---|---|---|---|
| Plastic film kit | EUR 5-10 | 3.5-4.0 | Minimal |
| Magnetic acrylic panel | EUR 30-80 | 2.5-3.0 | Good (20-25 dB) |
| Hinged glass panel | EUR 80-200 | 2.0-2.5 | Very good (25-30 dB) |
| Sliding track system | EUR 120-300 | 1.8-2.2 | Excellent (30-35 dB) |
Condensation Prevention Tips
Condensation on the inside surface of windows is a ventilation problem, not a glazing problem. Modern double glazing is so well insulated that the inside surface stays warm enough to prevent condensation in most conditions. If you do see condensation, try opening trickle vents, using extractor fans in kitchens and bathrooms, keeping indoor humidity below 60%, and avoiding drying clothes on radiators near windows. If condensation appears between the panes, the sealed unit has failed and needs replacement.
These calculations are estimates only. Actual requirements may vary depending on surface conditions, product specifications, and installation methods. Always consult a qualified professional for precise measurements.
Prices updated: 2026-03Frequently Asked Questions
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