4.7 - 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.

Reducing energy lost by radiation with surface finish

• The colour and finish of a surface change how well it emits and absorbs infrared radiation:
  • Dark, matt surfaces are good emitters and good absorbers.
  • Light, shiny surfaces are poor emitters and poor absorbers.
• To cut the energy an object loses by radiation, give it a light, shiny surface.

Investigating thermal radiation with a Leslie cube

• A Leslie cube is a hollow metal cube whose four vertical faces have different finishes (e.g. matt black, matt white, shiny metal, and dull metal).
• When filled with hot water, all four faces reach the same temperature, allowing a fair comparison of the infrared radiation they emit.

Method

1. Place an empty Leslie cube on a heat-proof mat and fill it with boiling water.
2. Check with a thermometer that all four faces are at the same temperature.
3. Hold an infrared detector at a fixed distance from one face and record the radiation.
4. Repeat for each face at the same distance.

Interpreting the results

• The matt black face emits the most radiation, and the shiny surfaces emit the least.
• Dark, matt surfaces are the best emitters; light, shiny surfaces are the poorest.

Investigating absorption

1. Stick a ball bearing with wax to the back of two metal sheets with different surface finishes.
2. Place them at equal distances from a heat source.
3. The wax melts and the ball bearing drops first from the sheet with the dark, matt finish because it is the better absorber of radiation.