When convection currents play a part in transmitting heat across a building component, then thermal transmittance increases as the temperature difference increases. For example, for an internal temperature of 20 °C (68 °F) and an external temperature of −20 °C (−4 °F), the optimum gap between panes in a double glazed window will be smaller than the optimum gap for an external temperature of 0 °C (32 °F).