Thermal insulators are used to minimize the transfer of heat energy. In other words, they keep something from becoming the same temperature as the surrounding environment. They keep you from become as cold as the water around you while scuba diving.
The type of heat transfer that makes you cold while diving is called conduction. The water is in direct contact with your skin, and heat flows from you into the water.
Less dense materials make better insulators because the atoms are not packed as tightly and cannot transfer their energy as easily. Water is a better insulator than metal, and air is better than water.
A dry suit uses a layer of rubber and a layer of air to keep a diver warm. A dry suit is more effective than a wet suit, which uses layers of rubber and water, because air insulates better than water.
Good conductors of electricity are good conductors of heat.
The R-Value of a material tells how well it insulates. The higher the value the better the insulation.
Definition of R-Value: The reciprocal of the amount of heat energy per area of material per degree difference between the outside and inside.