Applications involving the use of thermoelectric materials can be found in many different areas ranging from thermocouple sensors, portable coolers, to solar power generators. Generally, they can be subdivided by the direction of energy conversion. While the Peltier effect is used in solid-state refrigeration, the Seebeck effect is responsible for the conversion of temperature gradients into electrical voltage in energy harvesting systems. However, this paper proposes a novel approach to the use of thermoelectric couples, treating them as variable insulators in thermal systems. Here, we demonstrate that thermal conductivity in thermoelectric materials can be externally controlled by electrical parameters such as electrical load or DC voltage in passive and active systems, respectively. Active mode is a good solution when a complete insulation or a high control of thermal conductivity is needed. Passive mode permits a thermal conductivity increment of 1 + ZT times with respect to semiconductor initial thermal conductivity. Results open new doors and new opportunities for thermoelectric materials. (C) 2015 Elsevier Ltd. All rights reserved.