Historic Building Facades: Simulation,Testing, and Verification for Improved Energy Modelng

Why: Historic building facades have assemblies that are difficult to characterize for energy modeling, due to three primary limitations. First, they often are not uniform in their construction. Second, they may use mate­rials that are not standard in today's materials databases.Third, their construction and assembly may not be well-documented. As a result, when energy modeling these structures, the overall heat-transfer coefficient, known as the U-factor (aka U-value), may be significantly inaccurate and skew results. Thermal imaging can be used to augment the heat-flux-sensor measurements to evaluate large areas of a building envelope.

What: This research used a thermal imaging camera with an infrared resolution of 160x 120 pixels and a thermal sensitivity <0.06°C, resulting in a theoretical maximum of 19,200 surface-temperature sample points. Commercially available imaging processing tools can be used to read per-pixel color values for most images, as well as provide the mean color value for an area. Future tests will examine thermal imaging of small, medium and large areas of the wall and compare them with single, double and multiple spot-value, heat-flux readings at different times (morning, mid-day, evening and night). Additionally, laboratory verification of the method with known assemblies will compare the accuracy of the method.

How: The author's proposed method differs from standard thermal imaging in that it uses heat-flux sensors to derive the heat-transfer coefficients for air films at spot locations, and then uses them in conjunction with surface temperature readings from thermal images to calculate the proposed method in this article differs in that it uses heat-flux sensors to derive the heat-transfer coefficients for the air films at spot locations, and then uses them in conjunction with surface temperature readings from thermal images to calculate the U-factor of larger sections of an envelope.