A builder is choosing between two materials to use as insulation in the walls of a new house. Material A is a thick layer of foam with many trapped air pockets. Material B is a thin sheet of shiny metal foil. Explain how each material reduces the transfer of thermal energy into or out of the house, and suggest which material would be more effective as wall insulation.

WJEC A-Level Physics (Wales) — 3.4 Thermal physics · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Thermal insulation in buildings reduces the rate at which thermal energy is transferred between the inside and outside of the building.

Model answer (5 marks)

Material A (foam) reduces conduction because the trapped air pockets are poor conductors of heat, so heat cannot pass through the material easily. The air pockets also suppress convection; the air cannot circulate or move freely within the foam, so convective heat transfer is minimised.

Material B (shiny metal foil) reduces thermal radiation. The shiny surface reflects infrared radiation, making it a poor emitter and absorber of thermal radiation, so less radiative heat enters or leaves the house.

Overall, Material A is the better wall insulator. In a building wall, conduction and convection are the dominant heat‑loss mechanisms, whereas radiation contributes only a small amount. A metal foil, being a good conductor, would actually increase conductive heat transfer and is therefore less effective as insulation.

Examiner tips

  • Mention both conduction and convection for foam, and radiation for foil; link each to the physical property (poor conductor, trapped air, reflective surface).
  • State clearly that foam is better because conduction/convection dominate in walls.
  • Use the phrase ‘trapped air pockets’ and ‘shiny surface reflects infrared’ to show understanding.

Common mistakes

  • Confusing the foil’s role as a reflector with it being a good conductor; students often say the foil is a good insulator.
  • Forgetting to mention that foam also reduces convection, not just conduction.

Mark scheme (5 marks)

  1. Material A (foam) reduces conduction because air is a poor conductor / thermal insulator
  2. The trapped air also reduces convection because the air cannot circulate / move freely
  3. Material B (shiny metal foil) reduces thermal radiation because shiny / reflective surfaces are poor emitters and poor absorbers of infrared radiation
  4. Material A (foam) is the better insulator because conduction and convection are the dominant mechanisms of heat loss through a wall / radiation is a small contribution compared to conduction and convection
  5. Metal foil (Material B) is a good conductor so it would actually increase conduction of thermal energy through the wall

Key terms in this question

thermal energy

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