Editing Thermal insulation (section)

===Clothing and natural animal insulation in birds and mammals===
{{Main|Clothing insulation}}
Gases possess poor [[thermal conduction]] properties compared to liquids and solids and thus make good insulation material if they can be trapped. In order to further augment the effectiveness of a gas (such as air), it may be disrupted into small cells, which cannot effectively transfer heat by [[natural convection]]. Convection involves a larger bulk flow of gas driven by buoyancy and temperature differences, and it does not work well in small cells where there is little density difference to drive it, and the high surface-to-volume ratios of the small cells retards gas flow in them by means of viscous [[drag (physics)|drag]].

In order to accomplish small gas cell formation in man-made thermal insulation, glass and polymer materials can be used to trap air in a foam-like structure. This principle is used industrially in building and piping insulation such as ([[glass wool]]), [[cellulose]], [[rock wool]], [[polystyrene]] foam (styrofoam), [[polyurethane|urethane foam]], [[vermiculite]], [[perlite]], and [[cork (material)|cork]]. Trapping air is also the principle in all highly insulating clothing materials such as wool, down feathers and fleece.

The air-trapping property is also the insulation principle employed by [[homeothermic]] animals to stay warm, for example [[down feather]]s, and insulating hair such as natural sheep's [[wool]]. In both cases the primary insulating material is air, and the polymer used for trapping the air is natural [[keratin]] protein.