Payback time depends on installation cost versus energy savings

Steps:

• Calculate payback period as installation cost divided by annual energy savings from reduced heat loss.
• Compare typical costs: draught proofing is cheapest (£100-300), others range £500-5000+.
• Assess savings: draught proofing prevents 10-20% heat loss quickly; others target specific areas with gradual returns.
• Determine shortest payback: low-cost options with broad, immediate impact win.

Why D is correct:

• Draught proofing has the lowest cost and delivers fast savings by sealing gaps, reducing heat loss per basic heat transfer principles (conduction/convection).

Why the others are wrong:

• A: Cavity wall insulation costs more (£500-1500) with savings spread over years due to targeted application.
• B: Double glazing is expensive (£2000+) and payback takes 10-20 years from window-specific efficiency gains.
• C: Loft insulation (£300-600) offers good savings but requires access and has longer ROI than sealing leaks.

Final answer: D