A homeowner installs cavity wall insulation in their house. Explain how cavity wall insulation reduces the rate at which heat energy is lost through the walls.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A homeowner notices that their heating bill is very high in winter. They decide to have cavity wall insulation fitted, where a fibreglass material is injected to fill the air gap between the inner and outer walls.
Model answer (4 marks)
1. Without insulation, the air in the cavity can circulate, forming convection currents that carry heat from the interior to the exterior.
2. The fibreglass insulation fills the cavity, stopping the air from moving and therefore preventing convection.
3. Fibreglass is a poor conductor of heat – it has a low thermal conductivity.
4. Because convection is blocked and conduction is limited, the rate of heat transfer through the wall is reduced, so less energy is wasted and the heating system is more efficient.
2. The fibreglass insulation fills the cavity, stopping the air from moving and therefore preventing convection.
3. Fibreglass is a poor conductor of heat – it has a low thermal conductivity.
4. Because convection is blocked and conduction is limited, the rate of heat transfer through the wall is reduced, so less energy is wasted and the heating system is more efficient.
Examiner tips
- Use the exact phrases ‘convection currents’, ‘poor conductor’, and ‘low thermal conductivity’ to match the mark scheme.
- Show the logical sequence: convection → blocked by insulation → low conductivity → reduced heat loss.
Common mistakes
- Confusing conduction with convection; students may say the insulation blocks conduction only.
- Using vague terms like ‘insulation keeps heat in’ without explaining convection or thermal conductivity.
Mark scheme (4 marks)
- Without insulation, air in the cavity allows convection currents to form
- The insulating material fills the cavity and prevents convection currents from forming
- The insulating material is a poor conductor of heat / has low thermal conductivity
- This reduces the rate of energy transfer to the surroundings, so less energy is wasted / the system is more efficient
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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