BUILDING ENERGY - Fall 2016 - 35
FIGURE 2: EXAMPLE OF RAINSCREEN STUDY SHOWING INTENDED DETAIL AND R-VALUE, AS WELL AS THE R-VALUE OBSERVED FROM A THERMAL IMAGE AND THROUGH THE HEAT-FLOW SIMULATION. FIGURE 3: THREE THERM SIMULATIONS FOR DISCONTINUOUS THERMAL BRIDGES (RIP-8.5). Because these simplified, singledimension calculations do not account for any thermal bridging, they were used as the "baseline R-value" as the best-case scenario. A thermal imaging camera was used to determine the actual R-value of existing façades. Teams were deployed to 15 buildings to assess the general envelope thermal performance and scan for areas that appeared to perform differently. Using the methodology published by Madding (2008), the exterior air temperature, interior air temperature and the radiant temperature were gathered using infrared imaging and temperature data loggers in order to calculate the as-built R-value of the assembly (see figure 2). Because physically altering the built conditions was not possible, computer simulations were used to test possible improvements to various construction details. Lawrence Berkeley National Laboratory's THERM 7.3 program was employed to determine R-values of complete assemblies, including thermal bridges, based upon its ease of use and ability to integrate into the design process. For each detail, models were prepared of the constructed designs and were then calibrated by comparing them to the actual performance measured in the field with the thermal imaging camera. For discontinuous thermal bridges, such as bolts or clips, two methods were used to account for their three-dimensional impact: the parallel path method and the isothermal planes method. The parallel path method takes a weighted average of two simulations, one with the discontinuous bridging element and one without it. The isothermal planes method runs one simulation using a weighted conductivity of the bridging material and insulation for the discontinuous thermal element. Because the parallel path tends to overestimate the impact of the thermal bridging, and the isothermal planes tends to underestimate the impact, both methods were used to understand the range of impact the thermal bridge might have (see figure 3). CONTINUED ON PAGE 37 NESEA.ORG * 35
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.