It is commonly thought that U-factor is not a key determinant in the performance of fenestration in hot climates, and generally the focus is solar heat gain. The reason given is that the difference in air temperature between inside and outside is much lower in hot climates, so the driving force for heat transfer is less. However, the temperature difference that drives heat transfer through a window is that between the exterior surface of the window and its interior surface. Exterior frame temperatures can exceed air temperatures by a significant amount, especially dark colored frames, because of solar absorption. This absorbed heat then transfers to the interior surfaces of the window if there is no thermal break at the frame or edge of glass. Data from a research study by the Solar Energy Research Institute of Singapore (SERIS) demonstrates how the U-factor of a window significantly impacts the energy and thermal comfort performance of fenestration systems in a hot climate. In addition, the use of warm-edge spacer in the Space Needle renovation demonstrates how reducing thermal transfer at the edge of glass is essential for reducing the cooling load in summer, heating load in winter, and improving thermal comfort year-round.