Impacts of Operating Hardware on Window Thermal Performance

Windows are responsible for about 40 percent of the heat loss through typical building envelopes so lowering window frame and glazing unit U-factors will reduce the impact of windows on the energy use in buildings. The thermal effects of operating hardware are currently ignored in the relatively low performing double pane windows common today, but may become significant in high performance windows. This paper describes simulation studies analyzing thermal-bridging effects of non-continuous operating (and non-operating) hardware in common casement style window frame designs. We use finite volume computational fluid dynamics modeling to demonstrate the change in frame sill profile U-factor for configurations using typical hardware systems.

Some conclusions can be drawn regarding the impacts of operating hardware on the thermal performance based on the individual frames profiles, although few general trends can be observed due to the large design differences between each frame section modeled in this study. Two of the three out-opening casement profiles modeled show reduced performance greater than 0.05 W/(m2 K), which may be significant when carried to whole windows in National Fenestration Rating Council (NFRC) and International Organization for Standardization (ISO) rating systems. Fastener types, hardware location within the frame, and other factors related to the method of hardware implementation may significantly impact the effect of hardware on the frame. Neither the base performance level nor the primary frame material appears to determine the thermal effect of hardware based on those metrics alone.