1 of 30© Boardworks Ltd 2006. 2 of 30© Boardworks Ltd 2006.

1 of 30 © Boardworks Ltd 2006

© Boardworks Ltd 20063 of 30

Touch a piece of metal and a piece of wood at the same temperature. Which material feels warmer and why?

A temperature problem

The metal feels cold and the wood feels warm.

wood feels warmer than the metal.

Metal is a good conductor and conducts the heat away from your hands, so it feels cold.

Wood is not a good conductor and does not conduct the heat away from your hands as well as the metal, so the


This is how your clothes keep you warm – air is trapped between the fibres and so acts as an insulator.

Some materials are very poor conductors of heat. These are called thermal insulators.

Other insulating materials, including polystyrene and loft insulation, use trapped air because it is so effective.

What are thermal insulators?

Examples of materials that are insulators include plastics, wood, ceramics and air.

Air becomes a very effective insulator when it is trapped and stopped from moving.


Charlie forgot to take the chicken out of the freezer last night! Will his plan to defrost the chicken in time for lunch work?

Charlie the Chef and his frozen chicken!


How is a vacuum flask able to keep hot drinks hot and cold drinks cold?

How does a vacuum flask work?

2. The plastic (or cork) lid is an insulator and the screw top prevents convection currents escaping from the flask.

1. There is a vacuum between two layers of glass or steel, which prevents heat leaving or entering by conduction.

3. The walls have silvery surfaces, which prevent heat leaving orentering by radiation.`


A thermogram shows the distribution of heat over the surface of a house. It highlights where heat is being lost.

The white, yellow and red areas are the warmest, so these are the worst insulated parts of the house.

Heat loss from houses

A poorly insulated house loses more energy and so costs more to heat. It also means that more pollution, particularlycarbon dioxide, is created in order to heat the house.

The blue and green areas are the coolest, so these are the best insulated parts of the house.


Where is the most heat lost from a poorly insulated house?

walls 36%

floor 28%

roof 20%

windows 12%

doors 4%

Heat loss from houses


Various types of insulation can prevent the loss of heat energy from a house.

How can heat loss be reduced?

Type of insulation

Type of heat transfer stopped

How insulation stops heat transfer

Use the following slides to complete this table showing how different types of insulation can prevent loss of heat energy from a house.


The insulating effect of the gap can be improved by removing the air and creating a vacuum.

A lot of heat energy can be lost through windows.

Double glazing adds an extra pane of glass. The air trapped between the two panes acts as an insulator and reduces heat loss by conduction.

Can heat loss through windows be reduced?

The problems with double glazing are that it can be expensive and that it is difficult to break in emergencies without a special hammer.

side-viewof double glazing

insulating layer of air


The air trapped in the cavity acts as an insulator and reduces any heat loss due to conduction through the bricks.

Most outside walls have an empty space between the two layers of bricks called a cavity.

Heat loss can also occur due to convection within the cavity. Plastic foam insulation can be pumped into the cavity to prevent this.

convection currents

in the cavity

foam insulationprevents

convection currents

How can heat loss through walls be reduced?


The air warmed by radiators (and other heaters) is carried around a house by convection currents.

How can heat loss through a roof be reduced?

heat loss

loft insulation

Loft insulation contains trapped air and so forms an insulating layer between the loft and the rest of the house. This helps to reduce heatloss through the roof.

The house becomes heated but, if there is no roof insulation, the warm air continues to rise.

The heat eventually escapes through the roof and is lost due to conduction through the roof tiles.


Heat loss from a radiator can be reduced by placing shiny foil between the wall and the radiator.

A radiator produces infrared radiation.

How can heat loss from radiators be reduced?

Some of this heat energy is absorbed by the wall that the radiator is attached to, and so the wall heats up.

This can be a real problem on outside walls, where the heat energy absorbed by the wall can escape from the house.

The foil prevents heat radiation from reaching the wall by reflecting it back into the room.

This method of insulation is very cheap to install.


Draught excluders are hairy or spongy strips that can be used to close the gaps around doors and windows.

A draught is the movement of air due to a convection current.

A lot of heat energy can be lost from a house due to draughts escaping through gaps under doors and around windows.

How can heat loss through doors be reduced?

This is one of the easiest types of heat loss to prevent.

They prevent draughts escaping and so reduce heat loss.


A draught is the movement of air due to a convection current.

How can curtains reduce heat loss?

Warm air rises and is carried up towards the windows in a house by convection currents.

Fitting curtains and closing them can prevent draughts leaving a house and so reduce heat loss.

This heat energy can escape through gaps around windows that are uncovered.

In addition, curtains are opaque and so radiated heat does not pass through them.


How does insulation affect heat loss?


Insulation and heat loss – true or false?


side-viewof double glazing

Double glazing keeps a house warmer because there is a layer of a__ between the panes of glass.

ir

onductornsulator

educesonduction

How does double glazing keep a house warmer?

How does double glazing work?

Air is a poor c________, so it acts as an i_______.

The trapped air r_______ heat loss by c_________ from a house.


Insulation and heat transfer


Example: Adding silver reflectors behind radiators costs £25 and saves £50 per year.

Payback time is the time it takes for the cost of installing insulation to be equalled by the savings made from reduced energy costs.

= 0.5 years (6 months)

What is payback time?

payback time (in years) =cost of insulation

saving each year

payback time = 25 50


3 years

80 years

1 year

5 years

Why is double glazing popular if the payback time is so long?

What is the payback time for these types of insulation?

Calculating payback time

How heat escapes

Cost of heat escapingper year

Cost of insulation

Payback time

roof

windows

draughts

walls

£80

£40

£50

£100

£240

£3,200

£50

£500


Adding silver reflectors behind radiators costs £25 and saves £50 per year.

Cost-effectiveness is a comparison of the annual savings in reduced energy bills and the cost of insulation.

Example 1:

Cost-effectiveness = 50/25= 2

Cost-effectiveness = 50/50= 1

Adding draught excluders costs £50 and saves £50 per year.

Which insulation is most cost-effective?

Example 2:

So, the reflectors are more cost-effective than the draughtexcluders.

Cost-effectiveness =saving each year

cost of insulation


Calculating cost-effectiveness


cavity wall – A wall made up of two layers of bricks with an air gap between them, which reduces heat lost by conduction.

cavity wall insulation – Plastic foam insulation that is pumped into the gap in a cavity wall to prevent heat lost dueto convection in the cavity.

cost-effectiveness – A comparison of the annual savings in reduced energy bills and the cost of insulation.

double glazing – Two panes of glass with an air gap between them, which reduces heat lost by conduction. Theair can be sucked out of the gap to create a vacuum.

draught excluder – A hairy or spongy strip thatprevents draughts escaping through the gaps around doors and windows, and so reduces heat loss.

Glossary (1/2)


loft insulation – A material which contains trapped air and helps to reduce heat lost through the roof of a house.

insulator – A material that is a very poor conductor of heat and so can be used to reduce heat loss.

payback time – The time it takes for the cost of insulation to be equalled by the savings from reduced energy bills.

vacuum flask – A vacuum flask is a bottle with double walls separated by a vacuum. It reduces heat transfer and so keeps warm drinks warm and cold drinks cold.

Glossary (2/2)


Anagrams


Multiple-choice quiz

1 of 30© Boardworks Ltd 2006. 2 of 30© Boardworks Ltd 2006.

Documents

Transcript of 1 of 30© Boardworks Ltd 2006. 2 of 30© Boardworks Ltd 2006.