Global warming is primarily caused by increasing concentrations of greenhouse gases produced by human activities. Fast increases in energy consumption as a result of the population growth and economic development dominate the greenhouse gas concentrations in the atmosphere since fossil fuels still have the largest share in the energy sector. Buildings are responsible for almost 40% of global energy consumption and most of this energy is used for heating, ventilation and air conditioning (HVAC) systems. In this regard, energy-efficient retrofitting is of vital importance for minimizing energy demand of buildings and mitigating global environmental problems. Efficient insulation is regarded as a key solution for these purposes; however there is a consensus among scientists that conventional insulation materials have already reached their limits in terms of performance and there is a global necessity for superinsulation materials such as aerogel, vacuum insulation panel, etc. Windows have a significant impact on heat loss from the building fabric. In this regard, innovative technologies in order to reduce the existing U-value ranges of windows draw attention among researchers for both reducing the energy demand of buildings and hence mitigating greenhouse gas emissions into the atmosphere. In this study, a recently developed window technology named vacuum tube window is introduced and its performance assessment is presented through an experimental and numerical research. The novel window consists of vacuum tubes supported with Argon as inert gas. For different values of design parameters such as pane thickness, tube thickness, tube diameter and Argon gap, total heat loss and U-value of the vacuum tube window are evaluated and optimum data is obtained. For the vacuum tube diameter of 28 mm, an excellent agreement between experimental and numerical data is achieved. The results indicate that the U-value notably increases with Argon gap. Optimum tube diameter is found to be 60 mm in terms of thermal performance characteristics, cost, lightness and aesthetic issues. It is concluded from the results that the vacuum tube window technology is able to provide a U-value lower than 0.40 W/m²K, which is very promising for both retrofitting of existing buildings and new-build applications.