Reducing Unwanted Energy Transfers

On a cold January night your home is warmer inside than out — and that difference is the whole problem. Energy never sits still: the warmth you paid to make is always trying to leak away through the walls, the roof, the windows and every little gap, spreading out into the cold until inside and outside are the same. The boiler then has to burn more fuel just to replace what escaped.

You can't stop energy moving from hot to cold — that's the way heat always flows — but you can make it move slowly. That is what this whole page is about: reducing the unwanted energy transfers, so the warmth stays where you want it for longer and less fuel is wasted keeping the house warm.

Attack all three ways heat escapes

You already know heat can travel by conduction, convection and radiation. Heat leaves a warm house by all three at once, so good insulation has to fight all three. The trick is that a single clever idea — trapped air — deals with most of them:

Conduction — heat conducted straight through solid walls, glass and the roof. We slow it by putting a layer of a poor conductor in the way: loft insulation, cavity-wall foam, a double-glazing gap. Nearly all of these work because they trap air, and air conducts heat very badly.
Convection — warm air rising, escaping through gaps, and cold draughts sinking in. We stop it by trapping the air in tiny pockets so it can't circulate, and by draught-proofing the gaps around doors and windows.
Radiation — infrared heat radiating away from warm surfaces. We cut it with shiny, reflective surfaces: foil behind a radiator bounces heat back into the room, and a shiny "space blanket" reflects a person's own radiated warmth back onto them.

The secret ingredient: trapped air

Here is the idea that ties almost everything together. Still air is one of the best cheap insulators there is — it conducts heat far worse than glass, brick, water or (especially) metal. So if you could wrap your house in a thick, unmoving blanket of air, very little heat would get through.

The catch is that air moves. Left free, warm air rises and cold air sinks — that's convection — and moving air carries heat away fast. The solution is to trap the air so it can't flow. That is exactly what all that fluffy, foamy, feathery material is really for: loft insulation, cavity foam, a wool jumper, a duvet, a bird's feathers, an animal's fur — they are not warm in themselves, they are just clever cages that hold thousands of tiny, still pockets of air. The air does the insulating; the fluff just stops it escaping.

It looks daft — a vest full of holes, worn to keep you warm. But the holes are the point. Once you pull a jumper on over the top, each little gap in the mesh becomes a sealed pocket that traps a bubble of still air against your skin. A solid, thin shirt traps almost no air; the "holey" vest traps loads of it. More trapped air means a thicker layer of poor conductor between you and the cold — so the airy, gappy layer keeps you warmer than the solid one. Divers, hill-walkers and Arctic explorers all rely on exactly this trick of layering to lock in pockets of air.

Thermal conductivity — a property of the material

Why does metal feel cold and wool feel cosy? Because materials differ in how easily they let heat pass through, a property called thermal conductivity. A high thermal conductivity (metals) means heat races through — a poor insulator. A low thermal conductivity (air, wool, foam, fibreglass) means heat crawls through — a good insulator. It's a fixed property of the substance, like its density.

For a wall or a window, the rate at which heat leaks through — the energy lost each second — depends not just on the material but on how thick it is and how big the temperature difference is. Qualitatively:

\text{rate of heat loss} \;\propto\; \frac{\text{thermal conductivity} \times \text{area} \times \text{temperature difference}}{\text{thickness}}.

Read off what that tells a builder. To lose heat more slowly you want a lower thermal conductivity (a better insulating material) or a greater thickness (thickness is on the bottom, so doubling it roughly halves the loss). That's why loft insulation is laid thick, and why a well-insulated wall uses a low-conductivity foam. Builders bundle all of this into a single "how leaky is this wall?" number called a U-value — the lower the U-value, the less heat escapes per second for the same temperature difference.

How fast heat is lost through a wall

A lower thermal conductivity (a better insulator) means a slower rate of heat loss.
A greater thickness of the insulating layer means a slower rate of heat loss.
A bigger temperature difference between inside and outside means a faster rate of heat loss.

Have a go: insulate the house

Here is a house on a cold day. Each fat orange arrow is heat escaping by a different route — up through the roof, out through the walls, out through the windows, and out through gaps as draughts. The tall bar on the right is the total heat lost meter. Switch each insulation measure on and watch its arrow shrink and the meter fall. Turn them all on to see how snug — and how much cheaper to heat — the house becomes.

Notice you can never get the meter all the way to zero: real insulation slows the loss, it can't stop it completely. The bigger the temperature difference outside, the harder every arrow pushes.

The measures, and what each one attacks

Put names to the arrows you just shrank. Each common home measure targets a particular route and a particular way heat travels:

Loft insulation — a thick fibreglass or wool blanket in the roof space, full of trapped air. Stops heat conducting up through the ceiling and warm air escaping through the roof. The roof is the biggest single leak in a typical house, so this is the cheapest big win.
Cavity-wall insulation — foam or beads pumped into the gap between a house's two brick walls. The gap used to be an empty cavity; filling it with a poor conductor stops heat conducting across and stops the air in the gap convecting.
Double glazing — two panes of glass with a sealed layer of trapped air (or a vacuum) between them. The gap is the insulator, not the glass. It slows conduction through the window, the weakest part of most walls.
Draught-proofing — brushes and strips that seal the gaps round doors, windows and the letterbox. Stops cold air blowing in and warm air leaking out — it kills the convection draughts.
Reflective foil — a shiny sheet behind a radiator, or a survival "space blanket". Its silvery surface reflects radiated heat back instead of letting it soak away.

Nature discovered trapped air long before builders did. Emperor penguins survive an Antarctic winter by huddling: thousands pack together into a tight crowd, which drastically cuts the surface area exposed to the wind and traps warm air between their bodies — the birds on the toasty inside and the chilly outside take turns, and the whole huddle stays far warmer than any single bird could.

A polar bear cheats twice over. Its fur is packed with hollow hairs, each one a tiny tube of trapped air, and beneath the fur lies a thick layer of insulating blubber. So well does it lock heat in that a thermal camera pointed at a polar bear sees almost nothing — barely any warmth escapes for the camera to detect.

And a vacuum flask beats them both. It has a double wall with the air pumped out from between — a vacuum, which can't conduct or convect heat at all — and shiny silvered surfaces to reflect radiation back. Conduction, convection and radiation, all three shut down at once, which is why your soup is still hot at lunchtime.

It's the trapped air doing the insulating, not the fluffy stuff. Wool, feathers, foam and fibreglass work because they hold thousands of tiny pockets of still air. Squash a duvet flat and you squeeze the air out — and its insulating power collapses with it. The material is just the cage; the air is the insulator.
A jumper does not "make" heat. Your body is the heat source. The jumper only slows down the heat leaving you, so you cool off more slowly. Wrap a jumper round a cold bottle of milk and it stays cold — because insulation works both ways, keeping heat out just as it keeps heat in.
Double glazing isn't just "thicker glass". The insulation is the trapped-air (or vacuum) gap between the two panes, because air conducts heat far worse than glass does. One thick single pane would insulate far less well than two thin panes with a gap.