What materials are appropriate as insulation?

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GPT explains (level =C2; CEFR aligned): Appropriate materials for insulation → Light-weight (e.g., polystyrene, glass-wool, sheep-wool) Thermal insulation relies on minimising conductive heat transfer by trapping large volumes of still air within a matrix of low-conductivity solid. Light-weight, fibrous or cellular materials—expanded polystyrene, mineral/glass wool, natural sheep wool, cork, aerated foams—possess extremely low bulk density and therefore low thermal conductivity (λ ≈ 0.030–0.045 W m⁻¹ K⁻¹). By contrast, so-called “heavyweight” media such as concrete, stone or steel have high density and λ values two orders of magnitude larger; they contribute thermal mass (heat-capacity buffering) but are poor insulators. Liquids conduct more efficiently than trapped gas, while most dense inorganics (rigid plastics excepted) likewise fail the λ test. Hence the correct engineering choice for insulation is the light-weight class.

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