Why insulation works by stopping air from moving

Suppressing convection, and why fluffy materials beat dense ones.

Insulation works mainly by stopping air from moving rather than by resisting heat directly, since still air is already a poor conductor on its own, and almost every good insulating material is really a structure built to hold air motionless so it never gets the chance to carry heat away by flowing.

A woolly hat on a windy day

Standing outside bare-headed on a cold, windy day loses heat from the scalp quickly. The cold air is only part of it: the wind continually sweeps away the thin layer of air the head has managed to warm and replaces it with fresh cold air, never letting a warm cushion build up. Pulling on a woolly hat barely changes the temperature of the air underneath it at first. What changes is that the warmed air is now allowed to stay put. The hat's knitted fibres hold a layer of air against the scalp and keep the wind from reaching it, so the air the head has warmed stays there and insulates. A bare head loses heat mainly to moving air, while a hatted one loses heat by the much slower route of conduction through trapped, mostly still air, which is why the difference in comfort is so much larger than a thin hat would seem to justify by its material alone.

Moving air carries heat, still air barely does

Air that is free to move carries heat far more effectively than air that is held still, because moving air keeps replacing the warmed layer next to a hot surface with fresh, cooler air, keeping the temperature difference that drives heat loss as large as possible for as long as the air circulates. Still air has nowhere to go, so once it warms slightly next to a hot surface it sits there, and the temperature difference shrinks until conduction through that warmed pocket is the only route left, which for air is a slow one.

This is the secret behind most insulating materials, whether wool, foam, fibreglass or feathers. As solids they are only middling at blocking heat, and their real talent is trapping enormous numbers of tiny air pockets and physically preventing that air from circulating. Glass, the material fibreglass is spun from, conducts heat about forty times more readily than still air, and yet a loose fibreglass blanket insulates almost as well as still air would, because it is nearly all air held in place by a small amount of glass. A slab of stone or a sheet of metal, dense and free of trapped air, insulates poorly even though it looks far more substantial, because it offers a continuous solid path for conduction and holds no still air at all.

The fluffiest, lightest materials are usually the best insulators for this reason, since fluffiness describes how much still air a structure traps relative to how little solid it uses. Two batches of the same fibre can insulate very differently depending only on how loosely or tightly they have been packed, a strong hint that the fibre is the scaffolding and the air it holds in place is the insulator.

The pockets also have to be small. In a wide gap, air warms against the hot side, rises, cools against the cold side and sinks again, setting up a slow circulating loop that ferries heat across even though no wind is blowing. The gap between the panes of double glazing is kept to a couple of centimetres or less for this reason, since a much wider gap lets that loop get going and gains almost nothing, and the fibres in a blanket of insulation break the air into pockets far too small for any loop to form.

Squashed, wet or wind-blown insulation

Once insulation is understood as trapped, motionless air, several things that otherwise look odd make sense. Compressing an insulating layer, whether a sleeping bag stuffed too tightly into its sack or loft insulation squashed flat under stored boxes, reduces its effectiveness sharply even though none of the material has been removed, because compression destroys the air pockets the insulation depends on. A damp insulating material insulates far worse than a dry one of the same type, since water conducts heat around twenty times better than air and readily fills the gaps meant to be holding air, turning an excellent insulator into a mediocre one without changing its outward appearance.

A windproof outer layer matters enormously for the same reason. Even a material built from perfect trapped-air pockets loses most of its benefit if wind can push straight through it and carry that air away, which is why a woolly jumper alone performs so much worse in a strong wind than the same jumper worn under a simple windproof shell. The shell itself is usually a poor insulator, a thin, dense layer of fabric with almost no loft, and it can afford to be: its job is to hold the wind out and let the trapped air underneath do the work it was already capable of doing, much as the woolly hat does for the air against a scalp.