U-Factor Matters in Hot Climates

It is commonly thought that fenestration U-factor is not a key determinant in the performance of facades in hot climates, and generally the focus of code bodies and designers is on solar heat gain, especially reducing the gain through the transparent areas. The assumption is that the difference in air temperature between inside and outside is much lower in hot climates than in cold climates, so the driving force for heat transfer is less. However, the main mechanism for heat transfer through a window frame in hot climates is solar absorption and then conduction of that absorbed heat through to the room side surfaces of the frame. Exterior frame temperatures can exceed ambient air temperatures by a significant amount, especially dark colored frames, because of solar absorption. Temperature differences between inside and outside can be generated through this mechanism which are much more comparable to those seen in cold climates in winter. The use of thermal breaks and warm-edge spacer can significantly reduce this solar heat gain mechanism through the edges of fenestration.

Data from a research study by the Solar Energy Research Institute of Singapore (SERIS) reviewed herein demonstrates how the thermal performance of the window frame significantly impacts the heat transfer through, and thermal comfort performance of, fenestration systems in a hot climate. A whole building modeling study is used to illustrate the resultant impact of fenestration U-factor on energy usage compared to solar heat gain coefficient (SHGC). Thermally broken frames are shown to be a must have for buildings in hot climates, just like they are in colder climates. 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.