Editing Wetsuit (section)

==Insulation==
[[File:MutaSemistagnaCaviglia.jpg |thumb|alt=the cuff of a semi-dry suit, showing an inner and outer seal |Semi-dry cuffs use a smooth surface to seal against the skin to reduce flushing.]]
Still water (without currents or [[convection]]) conducts heat away from the body by pure [[Temperature gradient|thermal diffusion]], approximately 20 to 25 times more efficiently than still air.<ref name=Williams2003 /><ref name="Tucker and Dugas 2008" /> Water has a thermal conductivity of 0.58&nbsp;Wm<sup>−1</sup>K<sup>−1</sup> while still air has a thermal conductivity of 0.024&nbsp;Wm<sup>−1</sup>K<sup>−1</sup>,<ref name="engtbox" /> so an unprotected person can eventually succumb to [[hypothermia]] even in warmish water on a warm day.<ref name="Clark" /> Wetsuits are made of closed-cell foam [[neoprene]], a [[synthetic rubber]] that contains small bubbles of [[nitrogen]] gas when made for use as insulating material ([[neoprene]] is also manufactured without foaming for many other applications where insulating qualities are not important). Nitrogen, like most gases, has very low [[thermal conductivity]] compared to water or to solids,<ref group="note">Nitrogen has a thermal conductivity of 0.024&nbsp;Wm<sup>−1</sup>K<sup>−1</sup>, the same as air – {{cite web |url=http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html |title=Thermal conductivity of some common materials |year=2005 |publisher=The Engineering ToolBox |access-date=August 12, 2009}}</ref> and the small and enclosed nature of the gas bubbles minimizes heat transport through the gas by convection in the same way that cloth fabrics, fur, or feathers insulate by reducing convection of enclosed air spaces. The result is that the gas-filled cavities restrict heat transfer to mostly conduction, which is partly through bubbles of entrapped gas, thereby greatly reducing heat transfer from the body (or from the layer of warmed water trapped between the body and the wetsuit) to the colder water surrounding the wetsuit.

Uncompressed foam neoprene has a typical thermal conductivity in the region of 0.054&nbsp;Wm<sup>−1</sup>K<sup>−1</sup>, which produces about twice the heat loss of still air, or one-tenth the loss of water. However, at a depth of about {{convert|15|m|ft|-1}} of water, the thickness of a typical neoprene foam will be halved and its conductivity will be increased by about 50%, allowing heat to be lost at three times the rate at the surface.<ref name="Bardy2005" /> The grade of foam neoprene strongly affects insulating properties at depth, and over time. Softer, lighter, and more elastic grades contain a higher percentage of gas bubbles, and are comfortable and provide effective insulation at or near the surface where they retain much of their thickness. Areas that are significantly stretched lose thickness even before they are compressed at depth, which also reduces the insulation,<ref name="O'Three" /> and long periods under pressure and repeated compression and decompression of the neoprene foam will eventually lead to loss of volume, insulation, buoyancy and flexibility.<ref name="Monji1989" /> Some bubbles will also rupture under stress and lose their gas, and the foam will start to absorb more water, further reducing insulation. Wetsuits for diving should be made from less compressible neoprene to keep their insulating qualities.<ref name="O'Three" />

A wetsuit must have a snug fit to work efficiently when immersed; too loose a fit, particularly at the openings (wrists, ankles, neck and overlaps) will allow cold water from the outside to enter when the wearer moves.<ref name="Tydeman" /> Flexible seals at the suit cuffs aid in preventing heat loss in this fashion. The elasticity of the foamed neoprene and surface textiles allow enough stretch for many people to effectively wear off-the-shelf sizes, but others have to have their suits custom fitted to get a good fit that is not too tight for comfort and safety. Places where the suit bridges a hollow tend to change volume when the wearer bends that part of the body, and the change in volume of the space under the suit works as a pump to push warm water out of the suit and suck cold water in on the opposite movement.