2.12 - Reducing Unwanted Energy Transfers

Thermal insulation in buildings

• A building loses energy to its cooler surroundings by conduction, convection, and radiation:
  • Conduction through walls and windows
  • Convection via moving warm air
  • Radiation emitted from warm surfaces
• The bigger the temperature difference between the building and its surroundings, the faster energy is transferred.
• Thermal insulation slows these transfers, keeping the building warm for longer and reducing heating needs.
• Most insulation works by trapping a layer of air: air has a very low thermal conductivity, so it conducts poorly, and trapped air can't circulate to form convection currents.

Common methods of insulating a home

• Loft insulation - Fibreglass or mineral wool traps air, reducing heat loss through the roof by conduction and convection.
• Cavity wall insulation - Foam or mineral wool fills the gap between outer and inner walls, trapping air to reduce conduction and convection.
• Double glazing - Two glass panes with an air gap between them reduce conduction and convection through windows.
• Draught excluders - Strips around doors and windows stop warm air escaping and cold air entering, reducing convection.
• Hot water tank jacket - An insulating layer reduces conduction from the tank.
• Thick curtains - Fabric layers over windows reduce conduction and convection.

Wall thickness and thermal conductivity

The thicker the walls and the lower their thermal conductivity, the slower energy transfers through them, so the building cools more slowly.

Cost-effectiveness and payback time of insulation

• Annual saving - The money saved on heating bills each year by fitting insulation.
• Payback time - How long it takes for the savings to cover the original cost of buying and fitting the insulation.

Formula for payback time

Where:

• Payback time is in years
• Installation cost is in pounds (£)
• Annual saving is in pounds per year (£/year)

Methods with a short payback time are generally the most cost-effective. Payback time depends on the annual saving as well as the installation cost, so the cheapest method is not always the quickest to pay back.

Worked example - Payback time of loft insulation

Loft insulation costs £300 to install and saves £75 on heating bills each year. Calculate its payback time.

Improving the energy efficiency of vehicles

• Idling losses - Start-stop systems cut wasted energy by automatically turning the engine off when the vehicle stops and restarting it when pulling away.
• Aerodynamic drag - A streamlined body shape reduces air resistance.
• Inertial losses - Using lightweight materials reduces the vehicle's mass, so less energy is needed to accelerate.
• Rolling resistance - Keeping tyres correctly inflated and using lightweight materials reduces friction between tyres and the road.