Improving the Energy Performance of the Building Envelope Through Factory Produced Wall Panels, Better Windows, and Improved Window-Wall Interface

The window-wall interface is commonly ignored in calculations of heat transfer through building envelopes. However, additional framing of the window opening introduces thermal bridging that adds to overall building envelope energy loss. Installation of windows into the rough opening on-site often leads to increased infiltration rates and additional pathways for energy loss. Building envelopes contributes to 11 quads of annual energy use in United States at the cost of $143B/yr (1 quad≅$13B), of which opaque envelope contribute 7.1 quads and windows contribute 3.9 quads. These energy losses consist of conduction and convection heat transfer, solar heat gain through a transparent portion of the envelope and infiltration. In this paper we examine ways to properly characterize the window-wall interface and to improve air sealing and reduce thermal bridging. Panelized construction with triple-glazing, Krypton-filled windows was modeled. Analysis of the heat transfer was done with LBNL’s THERM and WINDOW software tools. The THERM 2-D heat transfer tool was used to evaluate thermal bridging of wall and window components and the window-wall interface, with area weighting used to determine the full integrated envelope performance. Savings in overall heat transfer of over 100% were observed as compared to traditional construction. Novel methodology to evaluate heat transfer in building envelopes that takes full account of thermal bridging is presented.