SITEMAP   Physics Notes: Thermal energy 3.3 Thermal conductivity and insulation

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Thermal conductivity: 3.3 Heating and insulating buildings to minimise waste energy in the home - reduce thermal energy transfer

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3.3 Heating and insulating buildings to minimise waste energy in the home

• U-values measure how effective a material is as an insulator.

• What is the U-value of a material? What does the U-value mean?

• The U value of a material is related to its thermal conductivity.

• The U-value of a material gives a numerical value of how efficient heat is transferred through a material.

• Materials with high U-values are relatively good conductors of heat - higher thermal conductivity - poorer insulators

• Materials with low U-values are relatively good insulators - poorer heat conductors - lower thermal conductivity.

• You probably won't be asked about what a U value is, BUT, you will be asked about relative thermal conductivities and how this affects the materials used in a particular context, which is usually about thermal insulation.

• Conserving energy in the home

• Typical percentage thermal energy (heat energy) losses from a house:

• doors 15% - draught excluders, door curtain, double glazed glass panes.

• floors 15% - thick carpet, even a layer of insulating material included with the concrete base too,

• roof 25% - loft insulation layers of ceramic wool

• walls 35% - cavity wall insulation

• windows 10% - double glazing (two sheets of glass trapping air), curtains drawn in the evening.

• If all of these methods of insulating your home are applied, your energy bills e.g. oil, gas or electricity, are considerably reduced.

• More details of insulation methods are discussed below.

• Methods of reducing the rate of heat energy transfer reduce costs and also in there own small way contribute to reducing global warming by using less fossil fuel based energy supplies.

• What is effective? What is cost effective? Not always the same! Payback time?

• Loft insulation - cheap and effective - a thick layer of fibreglass wool or similar material laid all over the loft floor and reduces conduction and convection of heat lost through the roof - payback time a few years.

• The fibre glass or other 'woolly' material traps air which is a very poor heat conductor i.e. a very good insulator.

• Thick walls made from a good insulating material with a low thermal conductivity.

• The thicker the wall, the lower the rate of heat transfer, a slower rate of cooling, better heat retention due to a lower thermal conduction rate.

• Good insulating materials (poor heat conductors), irrespective of thickness, are brick, stone and breeze blocks etc.

• Cavity wall insulation is another technique for heat retention - insulating foam injected between two brick walls (inner wall and outer wall) - reduces heat loss by conduction and convection across the walls - quite costly, payback time a few years.

• Air is a poor conductor so the trapped air provides an effective layer of insulation and because of the porous nature of the foam, heat cannot be conveyed away by convection.

• Foam blocks for cavity wall insulation, or any other thermal insulation situation, can be produced with a thin layer of shiny foil (e.g. aluminium) on the outer surface of the block.

• This reduces heat loss by thermal radiation because the infrared waves are reflected back towards the house interior rather than being absorbed or emitted.

• Sometimes the gap is just left with just air in, a sort of brick/breeze block equivalent of double glazing windows, but still considerably reduces heat loss mainly by conduction because air has a very low thermal conductivity.

• Hot water tank jacket - cheap and effective - a jacket of lagging of a foam filled plastic cover reduces conduction and radiation heat losses - quick payback time.

• Double glazing of glass windows - expensive, longer payback time - insulating air is trapped between the glass panes a few mm apart, so reducing heat losses mainly by conduction.

• Draught-proofing - cheap and effective draught excluders, a few years payback time - strips of foam or plastic around door frames, thick curtains across the windows - all of these measures reduce heat loss from the house mainly by convection i.e. warm air in rooms brushing against cold surfaces like windows or warm air moving to colder unused parts of the house.

• Pipes - hot water pipes can be covered in insulation to minimise heat losses by conduction and convection - often foam which traps an insulating layer of air.

• You can paint pipes white to minimise loss by infrared radiation.

• Central heating hot water pipes

• Making pipes as short as possible, means the water spends less time in them, reducing heat losses before the water reaches the radiators.

• Making pipes as wide as possible means a smaller proportion of hot water is contact with the surface of the pipe from which heat is conducted away to the surrounding air.

• The most effective methods of insulation give you the biggest annual savings of heat energy, but the most cost-effective methods tend to be the cheapest. For double glazing and cavity wall insulation you need to think long-term to get your money back.

Keywords, phrases and learning objectives for thermal conductivity and insulation

Understand the importance of the thermal conductivity of a material when using materials for  insulating buildings to minimise waste energy in the home - ways of reducing thermal energy transfer - wasted energy.

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