Double Glazing Cost & Payback Calculator (UK)

Window Replacement Guide

U-Values Explained

U-value: heat loss in W/m²·K. Lower = better insulation. Single glazing: U-value approximately 5.0 (very poor — loses 5W per m² per °C difference). Old double (pre-2002): 2.5-3.0. Modern double (Building Regs since 2010): 1.6. A-rated: 1.4 or below. A+ rated: 1.2 or below. Triple glazed: 0.8 (Passivhaus standard) to 1.0. Replacing single glazing with A-rated: heat loss reduces by approximately 70-75%. Modern double to triple: only 25-35% improvement — much less impactful upgrade. Largest impact:

Window Heat Loss Calculation

Heat loss per window per year: Q = U × A × ΔT × hours. Where U = U-value, A = window area, ΔT = temperature difference, hours = heating hours per year. Typical 1.2 × 1.2m window (1.44m²), 15°C average temperature difference, 5,000 heating hours: Single glazed (U=5): Q = 5 × 1.44 × 15 × 5000 = 540,000 Wh = 540 kWh/year. Triple glazed (U=0.8): Q = 0.8 × 1.44 × 15 × 5000 = 86 kWh/year. Saving: 454 kWh/year per window — about £29 saved at current gas prices, £130 at electric heating prices.

Costs and Payback

Typical 2026 window replacement costs (uPVC, A-rated double glazing): small house (8 windows): £4,500-7,500; medium house (10-12 windows): £6,000-10,000; large house (15+ windows): £9,000-16,000+. Triple glazing adds roughly 15-25% to these figures. Individual replacement windows average £400-700 each fitted for standard uPVC casement, more for timber, aluminium, or large/awkward openings. Payback comes from reduced heat loss: replacing single glazing with modern A-rated double glazing typically saves £100-180 per year on heating for a whole house (more with rising energy prices), giving a simple payback of 25-40 years on the windows alone — so double glazing is rarely justified on energy savings alone. The real-world case rests on comfort (no cold spots or draughts), reduced condensation, noise reduction, security, and added property value. Replacing old (pre-2002) double glazing with modern units saves far less than replacing single glazing, because the old units already cut most of the loss — so the payback for upgrading already-double-glazed windows is very long and usually driven by failure (misted units, broken seals, rotten frames) rather than economics. This calculator estimates your annual heat-loss saving and payback from the U-values and window area you enter, so you can see the real numbers for your home before committing.

When NOT to Replace

Replacement is not always the best option. Consider alternatives if: existing windows are good condition but slightly leaky — refurbishment, draught stripping, or secondary glazing may give 60-80% of the benefit at 10-20% of the cost. Historic or character properties: secondary glazing preserves the original windows while improving thermal performance. Often more sympathetic. Marginal payback: replacing modern double glazing rarely pays back financially within window lifespan (25-30 years). Bett