P2.3 Heating and Insulating Buildings

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: what affects the rate of energy transfer out of a home, how insulation reduces it, payback time and solar panels. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe common ways of heating homes
• Describe how temperature difference, area, thickness and thermal conductivity affect the rate of energy transfer through walls and windows
• Explain how loft insulation, cavity wall insulation, double glazing, carpets and draught excluders reduce energy transfer
• Calculate payback time and use it to compare how cost-effective different measures are
• Explain why solar heating panels are fitted on south-facing roofs in the UK

Key words

• Cavity wall: An outside wall made of two layers of bricks or blocks with a gap (the cavity) between them.
• Cavity wall insulation: Insulating foam or fibre pumped into the cavity, trapping air in small pockets.
• Double glazing: A window with two panes of glass separated by a gap of dry air, another gas, or a vacuum.
• Draught: Cold air entering, and warm air leaving, through gaps around doors and windows.
• Payback time: The time it takes for the money saved on fuel bills to equal the cost of a measure. Payback time = cost ÷ saving per year.
• Cost-effective: Giving good value: a short payback time means the measure pays for itself quickly.
• Dissipated energy: Energy that spreads out to the surroundings, usually heating them, and cannot be used again.

Heating a home

Homes are heated by gas or oil-fired central heating, electric heaters, or solid fuel (wood or coal) burned in stoves. While the heating transfers energy into the home, energy is also transferred out to the colder surroundings – through the roof, walls, windows and floor, and by warm air escaping through gaps. The bigger the temperature difference between inside and outside, the faster this happens, which is why heating bills are highest in winter.

Four factors that decide the rate of energy loss

Energy dissipated from a warm home is wasted: it heats the outside air. The rate at which energy is transferred through a wall, roof or window depends on:

• the temperature difference between inside and outside – bigger difference, higher rate (so bills are higher in winter)
• the area – a bigger area of wall or window transfers more energy each second (large houses cost more to heat)
• the thickness – a thicker wall or roof layer gives a lower rate
• the thermal conductivity of the material – the lower it is, the lower the rate.

Worked example: two extensions

Two extensions are built from the same blocks with the same thickness. Extension A has 60 m² of wall; extension B has 90 m². The temperature difference is the same.
rate is proportional to area, so B transfers 90 ÷ 60 = 1.5 times as much energy each second as A

Answer: B loses energy 1.5 times as fast

Reducing the rate of energy transfer

• Loft insulation; How it reduces energy transfer Thick layers of fibreglass on the loft floor. Fibreglass has low thermal conductivity and traps air between its fibres, reducing conduction through the ceiling and roof. More layers = thicker = lower rate.
• Cavity wall insulation; How it reduces energy transfer Foam or fibre pumped into the cavity. It is a better insulator than the air it replaces because it traps air in small pockets, so the air cannot circulate (no convection) and conduction is reduced.
• Double glazing; How it reduces energy transfer Dry air between the panes is a poor conductor. A vacuum is better still: with no particles there is no conduction or convection across the gap.
• Draught excluders; How it reduces energy transfer Strips around doors and windows stop warm air escaping and cold air getting in.
• Carpets and curtains; How it reduces energy transfer Carpets with underlay and thick curtains trap air, a poor conductor, reducing energy transfer through floors and windows.
• Thicker walls, lower conductivity; How it reduces energy transfer A thicker wall, made of bricks or blocks with lower thermal conductivity, has a lower rate of energy transfer, so heating costs less.

Exam tip: Most insulating materials work because they trap air. Still air has a very low thermal conductivity, and trapped air cannot circulate and carry energy away.

Is it worth the money? Payback time

Each measure costs money to buy and fit, but saves money on fuel every year. The payback time is how long it takes for the savings to cover the cost:

payback time (years) = cost of the measure ÷ saving per year

Worked example: cavity wall insulation

Cost to install: £1050. Saving on fuel bills: £210 per year.
payback time = 1050 ÷ 210 = 5 years
Over 20 years the total saving is 20 × 210 = £4200, so the net saving is 4200 − 1050 = £3150.

Answer: payback time 5 years

A short payback time means a measure is cost-effective. Double glazing has a long payback time, but people also fit it because it cuts noise, reduces condensation and improves security.

Comparing walls and windows

The rate of energy transfer through a wall or window depends on its thickness, the thermal conductivity of the material and the temperature difference across it.

Worked example: which wall loses less?

Wall X: 10 cm of brick (thermal conductivity 0.70 W/m °C). Wall Y: 20 cm of lightweight block (0.35 W/m °C).
Y is twice as thick, which halves the rate; its conductivity is half as big, which halves it again.

Answer: Y transfers energy about a quarter as fast as X

Solar panels

Solar panels absorb radiation from the Sun. Solar heating panels use it to heat water directly; solar cell panels generate electricity. In the UK (northern hemisphere) the Sun is in the southern part of the sky, so panels are fitted on south-facing roofs to absorb as much radiation as possible. Like insulation, solar panels have a payback time: they cost a lot to fit but cut energy bills every year.

Common mistakes

• Saying insulation “stops” energy transfer. It only reduces the rate.
• Saying cavity wall insulation works because foam is a good insulator, without mentioning that it traps air and stops convection in the cavity.
• Calculating payback time as saving ÷ cost (upside down). It is cost ÷ saving per year.
• Forgetting area: a house with a larger area of walls and windows loses energy faster, even if the walls are the same.

Revision questions

Try these, then check your answers below each one.

Question 1. Cavity wall insulation costs £840 and saves £210 per year on fuel bills. Calculate the payback time.

Answer: 4 years

Working: 840 ÷ 210 = 4 years.

Question 2. Which change would increase the rate of energy transfer out through the walls of a house? (Choose from: building an extension, so the walls have a larger area, making the walls thicker, using bricks with a lower thermal conductivity, turning the heating down, so the temperature difference is smaller.)

Answer: building an extension, so the walls have a larger area

Working: A larger area transfers more energy each second. Thicker walls, a lower thermal conductivity and a smaller temperature difference all reduce the rate.

Question 3. True or false? A vacuum between the panes of a double-glazed window reduces energy transfer more than dry air does.

Answer: True

Working: True. A vacuum has no particles, so there is no conduction or convection across the gap.

Related lessons

• Previous lesson: P2.2 Specific Heat Capacity
• Next lesson: P2 RP Required Practical: Specific Heat Capacity
• In this chapter: P2.1 Energy Transfer by Conduction

Continue in the full course

This lesson is section P2.3 (AGCSC3103) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.