Thermal insulation

1. A building transfers energy to the outside whenever it is warmer than its surroundings, and that transfer is unwanted because it has to be replaced by the heating system.
2. Energy escapes through the walls, roof, floor, windows and doors, and also with warm air that leaks out through gaps.
3. The rate at which energy passes through a wall depends on the temperature difference across it, the thickness of the wall and the thermal conductivity of the material.
4. A thicker wall gives a lower rate of energy transfer, because the energy has a greater distance to travel through the material.
5. A lower thermal conductivity also gives a lower rate of transfer, which is why a thin layer of mineral wool is more effective than a much thicker layer of brick.
6. A well insulated building cools down more slowly once the heating is switched off, so it needs less energy to keep it at a comfortable temperature.
7. Energy leaves by conduction, convection and radiation, so a well designed set of measures reduces all three.

Reducing conduction

1. Conduction passes energy through a material as vibrating particles collide with their neighbours and hand energy on.
2. Insulating materials are poor conductors, and most of them work by holding a large amount of trapped air in small pockets.
3. Air is a poor conductor because its particles are far apart, so they collide with one another far less often than the particles in a solid do.
4. Cavity wall insulation fills the gap between the two layers of brick with foam or mineral wool, which traps the air and lowers the conductivity of the wall as a whole.
5. Loft insulation is a thick layer of mineral wool laid between the joists, which reduces conduction through the ceiling towards the cold roof space.
6. Double glazing uses two panes of glass separated by a sealed gap of air or argon, so energy must cross a poorly conducting layer as well as the glass.
7. Thermal blocks are made of aerated concrete containing many tiny air pockets, giving them a much lower thermal conductivity than dense brick.
8. Thick carpets and underlay trap air against the floor and cut conduction downwards through the floorboards.

Reducing convection

1. Convection carries energy through a fluid, because warmed fluid expands, becomes less dense and rises while cooler fluid sinks to replace it.
2. Holding air in small pockets stops convection, because the air cannot flow far enough to set up a current.
3. An empty wall cavity would let a convection current form, carrying energy from the warm inner leaf to the cold outer leaf, so filling it removes that pathway.
4. Draught excluders and sealant around doors, windows and letterboxes stop warm air leaving and cold air being drawn in to replace it.
5. Closing thick curtains holds a layer of still air against the glass, which reduces both conduction and convection at the window.
6. The gap in double glazing is kept narrow on purpose, because a wide gap would allow a convection current to circulate between the panes.

Reducing radiation

1. Every warm surface emits infrared radiation, which crosses a gap without needing any particles to travel through.
2. Shiny, light coloured surfaces are poor emitters and good reflectors of infrared radiation, while matt dark surfaces emit and absorb strongly.
3. A sheet of foil fixed to the wall behind a radiator reflects radiation back into the room instead of letting it warm the brickwork.
4. Foil backed plasterboard and foil faced loft insulation work the same way on a larger scale.
5. Painting a building white or fitting light coloured blinds reduces the radiation absorbed in hot weather, so the inside stays cooler.

Insulating objects and pipes

1. The same three mechanisms have to be reduced when the aim is to keep an object warm rather than a whole building.
2. A jacket of foam or mineral wool around a hot water cylinder, called lagging, reduces conduction and holds still air against the tank.
3. Foam sleeves around hot water pipes cut the energy lost between the boiler and the taps.
4. A vacuum flask removes conduction and convection between its walls by having no particles in the gap, and it uses silvered surfaces to reduce radiation across that gap.
5. The stopper of a vacuum flask is made of plastic and seals the top, which stops both conduction through the lid and warm vapour escaping.
6. A duvet and a woollen jumper both work by trapping air in a thick layer, which is why fluffing up a duvet makes it warmer than pressing it flat.