A novel vacuum tube window technology is introduced, and its thermal performance efficiency is both experimentally and numerically investigated. Heat transfer inside the window is modelled via a reliable commercial computational fluid dynamics software ANSYS FLUENT. For various vacuum tube diameters (28, 50, 70 and 80 mm), an excellent accordance is observed between experimental and numerical data. The U-value of vacuum tube window is determined for different values of design parameters such as pane thickness, tube thickness, tube diameter and argon gap. As a consequence of thermal performance characteristics, cost, lightness and aesthetic issues, the optimum vacuum tube diameter is found to be 60 mm. It is concluded from the results that the vacuum tube window technology has a U-value below 0.40 W/m(2) K, which corresponds to five times better thermal insulation performance than commercial argon filled double glazed windows with low-e. (C) 2014 Elsevier Ltd. All rights reserved.