Double Glazing Savings Calculator
Calculate how much you could save on heating bills by upgrading your windows. Compare single glazing, double glazing, and triple glazing with payback period.
Double Glazing Guide
Window U-Values Explained
The U-value measures how much heat passes through a window per m² per degree temperature difference — lower is better. Single glazing: ~5.0 W/m²K. Old uncoated double glazing (pre-2002): ~3.0. Modern standard double: ~2.0. Argon-filled double with low-E coating: ~1.1–1.4. Triple glazing: ~0.7–0.9. Building Regulations require replacement windows to achieve at least 1.6 W/m²K (Approved Document L, 2022). The best available windows approach 0.5 W/m²K with triple glazing and warm-edge spacers.
Heat Loss Calculation
Annual heat loss through windows (kWh) ≈ U-value × area × heating degree days (HDD) / 1,000. UK heating degree days vary by region: London ~2,200, Manchester ~2,800, Edinburgh ~3,100. This represents the hours × degrees of heating required annually. Example: 15m² of single glazing (U=5.0) in London: 5.0 × 15 × 2200 / 1000 = 165 kWh/year just from the windows. Upgrading to U=1.4 saves: (5.0 - 1.4) × 15 × 2200 / 1000 = 118 kWh/year ≈ £6.50/year at 5.5p/kWh.
Realistic Payback Periods
Double glazing replacement (full house, 15m² of windows): typically costs £3,000–7,000 installed. Annual saving from single to modern double: approximately £100–180/year (depending on region and gas prices). Payback: 20–50+ years on energy savings alone. This is why double glazing is justified primarily on comfort, noise reduction, condensation elimination, and security rather than pure energy saving. However, upgrading old poor-quality double glazing to modern low-E units has much faster paybac
Window Energy Ratings
UK windows carry an energy rating from A++ to E (similar to appliance ratings). These are calculated using BFRC (British Fenestration Rating Council) methodology, considering U-value (heat loss), solar heat gain coefficient (free solar energy in), and air leakage. An A-rated window allows some free solar heat gain that partially offsets heat loss — the net energy performance is better than U-value alone suggests. For south-facing windows, solar gain is significant; for north-facing, it is minima
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