Thermal Radiation and Insulation

Thermal radiation — the transfer of energy by infrared electromagnetic waves.

It needs no medium. Radiation is the only one of the three that crosses a vacuum, which is how energy reaches us from the Sun across 150 million kilometres of empty space.

Every object emits infrared, and the hotter it is the more it emits, and the shorter the wavelengths. An object hotter than its surroundings emits more than it absorbs and cools down; one colder than its surroundings absorbs more than it emits and warms up. At the same temperature the two rates balance.

Surface matters, and in a consistent way:

A good absorber is also a good emitter. This surprises people, but it follows from the balance above — if a black surface absorbed well and emitted badly it would heat up indefinitely, which it does not.

This is why solar panels are matt black, why the cooling fins on the back of a fridge are black, and why emergency blankets, kettles and fuel tanks are shiny.

Reducing unwanted transfer in a house — each measure targets a mechanism:

• Loft insulation — fibreglass traps air. Air is a poor conductor, and trapping it also stops convection currents forming.
• Cavity wall insulation — foam in the gap between the two walls, again trapping air and preventing convection in the cavity.
• Double glazing — a layer of trapped air or a vacuum between two panes, reducing conduction and convection through the window.
• Draught excluders — stop warm air leaving and cold air entering directly.
• Reflective foil behind a radiator — reflects infrared back into the room instead of letting the wall absorb it.

Trapped air is the common thread, which is worth noticing because it makes the list learnable rather than arbitrary. Air is a poor conductor; trapping it stops it also carrying energy away by convection.

A vacuum flask uses all three ideas at once: a vacuum between the walls stops conduction and convection, silvered surfaces reflect radiation back, and an insulating stopper blocks convection out of the top.

Example: a shiny teapot stays hot longer than a matt black one of the same size, because it is a poorer emitter of infrared. Painted matt black it would cool noticeably faster.