Thermal Bridge

A thermal bridge, also called a cold bridge, is a fundamental of heat transfer where a penetration of the insulation layer by a highly conductive or noninsulating material takes place in the separation between the interior and exterior environments of a building assembly (also known as the building enclosure, building envelope, or thermal envelope).
Thermal bridging is created when materials that are poor thermal insulators come into contact, allowing heat to flow through the path of least thermal resistance, although nearby layers of material separated by airspace allow little heat transfer.
Insulation around a bridge is of little help in preventing heat loss or gain due to thermal bridging; the bridging has to be eliminated, rebuilt with a reduced cross-section or with materials that have better insulating properties, or with a section of material with low thermal conductivity installed between metal components to retard the passage of heat through a wall or window assembly, called a thermal break.

Excerpts from article from Wikipedia, the free encyclopedia.