Some insulating materials such as plastic foam contain trapped air bubbles that improve their properties as insulators. Vacuum insulation, by contrast, relies on a vacuum -- a space that has been almost completely evacuated of air -- to slow the spread of heat. A thermos or vacuum flask is probably the most common everyday example.
Conduction and Convection
Heat transfer through conduction takes place by means of collisions between molecules. A vacuum, however, is just empty space. It contains no molecules -- or at least very few of them. Consequently, heat transfer by conduction does not occur. Convection likewise requires the presence of a gas that can develop convection currents. Since a vacuum does not contain any gas, heat transfer by convection does not occur in vacuum insulation.
Heat transfer by radiation is actually a rather important consideration for vacuum insulation. Anything with a temperature greater than absolute zero emits some energy as radiation, although the amount depends heavily on the temperature. It is not possible to completely prevent heat loss by radiation, although the amount of heat loss can be dramatically reduced by coating the inside of the vacuum chamber with a highly reflective coating, such as a thin layer of silver.
A thermos flask is a ubiquitous example of vacuum insulation at work. The flask actually has both an inner and outer wall separated by a thin layer of vacuum or nearly empty space. The inside of the outer wall is coated in reflective material to bounce back as much infrared light as possible. Much of the heat loss occurs through the stopper, although these are usually made of insulating materials as well. The Dewar flasks used in chemistry labs are similar in principle.
Vacuum insulation panels or containers must be constructed from materials with sufficient rigidity and strength to withstand the pressure they experience from outside, since there is no counterbalancing pressure from within. Aside from thermos and Dewar flasks, vacuum insulation also finds use in some top model refrigerator and freezer units and containers for cold shipping, although vacuum insulation panels are sometimes employed in building construction as well.