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Background reading · Science

Three ways heat travels (and how to stop it)

Heat always moves from warm places to cold ones, by conduction, convection and radiation. Insulation slows it down.

On a frosty morning it can feel like the cold is creeping into your room. In fact it is the other way round. Heat always flows from warmer things to cooler things. Your warm room is leaking heat out to the cold world outside. There are three ways it can travel.

Conduction, convection, radiation

  • Conduction: heat passes from particle to particle through a material, like a line of people passing a bucket. Metals are good conductors, which is why a metal spoon in hot soup quickly gets hot at the handle.
  • Convection: in liquids and gases, warm parts expand, become less dense and rise, while cooler parts sink to take their place. This loop is a convection current. It carries warm air from a heater up to the ceiling.
  • Radiation: heat travels as invisible rays, even through empty space. You feel it on your face from the sun or a fire. Dark, dull surfaces soak up radiation; shiny ones reflect it.

    • Why metal feels colder

    A metal chair and a wooden chair in the same room are the same temperature. Metal feels colder because it conducts heat away from your hand much faster. Your skin senses the heat leaving, not the temperature of the chair.

    Insulation: slowing heat down

    Insulators are materials that are poor conductors. Many of the best ones, like wool, foam and fibreglass batts, work by trapping lots of tiny pockets of still air. Air is a poor conductor, and when it cannot move it cannot carry heat by convection either.

    In a house with no insulation, a lot of heat escapes through the ceiling, because warm air rises. Draughts around windows and doors let warm air out too. New Zealand research found that insulating older homes made them warmer and drier, and that people living in them had fewer coughs and colds and took fewer days off school and work. Ceiling insulation and stopping draughts gave the biggest savings for the money spent.

    Sources and further reading

    Written for Kōkiri Learn students in our own words. Check facts against the sources.

    Used in: Matter on the Move: Heat, Mixtures and Change