Deconstructing the Thermal Performance of a Window: How to achieve better performing facades

Please note: The attached file below contains a paper from the BEST5 conference that is linked to a conference presentation in pdf format. Open with Adobe Acrobat for best results.

With the need to provide daylight and views for building occupants, improved comfort, and low energy consumption, significant tension has been created around the design of the building envelope. While the use of highly glazed facades allows for sufficient daylight and views, they can create thermal comfort issues for occupants near them, condensation issues because of cold interior surfaces, and can reduce the thermal performance of the facade. However, with the right design, an envelope system can be created which is more energy efficient than an opaque wall. The US Department of Energy predicts that the use of highly insulating windows alone (U=0.15 btu/oF.hr.ft2) can save 1 Quadrillion (1015) BTUs annually if installed in the entire US commercial building stock. This represents a 5% energy savings from just improving the U-factor alone. For years, the industry has relied heavily on the increasing performance of low-e coatings to drive window U-factors (thermal transmittance) lower. However, the full performance of the window is first and foremost determined by the frame and the edge of glass conductance, and neglecting the window perimeter can result in poorly performing fenestration systems that do not meet code, are uncomfortable to sit next to, and exhibit problematic condensation. To help designers specify the appropriate fenestration components and performance, this paper deconstructs the factors that make up the U-factor and condensation resistance of a window, and examine their sensitivity to edge of glass parameters such as frame, frame bite, sealant height, spacer conductivity, etc.