4 · IGCSE Physics
Insulating a house
See how thermal energy escapes through the walls, roof, windows and floor of a house, then fit insulation to compare the power lost, the yearly heating bill and the payback time of each measure.
Insulation
Conditions
P = U × A × ΔT (each part)
E = P × t (210 heating days)
cost = E (kWh) × price
payback time = installation cost ÷ yearly saving
U in W m⁻² °C⁻¹: walls 1.5→0.50, roof 2.0→0.25, windows 5.0→2.8, floor 0.70
E = P × t (210 heating days)
cost = E (kWh) × price
payback time = installation cost ÷ yearly saving
U in W m⁻² °C⁻¹: walls 1.5→0.50, roof 2.0→0.25, windows 5.0→2.8, floor 0.70
Rate of energy loss
Energy, cost and payback
Try this
- With no insulation fitted, find which part of the house loses energy fastest. Fit each measure in turn and see which arrow shrinks most.
- Compare the payback times of loft insulation and double glazing. Which would you fit first on a tight budget, and why?
- Turn the inside temperature down from 21 °C to 19 °C. By what percentage do the power lost and the yearly cost fall?
Exam tip: for payback questions, divide the installation cost by the saving per year and give the answer in years; the measure with the shortest payback is the most cost-effective, even if it does not save the most energy.