Verifying Thermal Performance

Architects specify curtain wall thermal performance criteria and usually identify simple modeling software, like THERM, to perform thermal analysis to verify compliance. This approach works well for standardized systems with well-established performance, but it is not adequate when the curtain wall is custom-made and does not have laboratory test data to document its performance. Difficulties are often encountered when typical analysis tools, specifically THERM, are used to verify performance of specialty and custom-made curtain wall systems. An alternative method to model environmental conditions is needed. For the custom-designed curtain wall at Northwestern’s Ryan/Walter Athletics Center, the use of computational fluid dynamics modeling paired with more traditional THERM analysis filled that need. A case study of the methodology used to verify the specified performance of the steel girt-supported curtain wall demonstrates this hybrid method of thermal analysis.

THERM proved to be inadequate to predict curtain wall performance because it is a static model that does not account for the location of interior heat sources, thermal movement of air or variable temperatures across the inner surface of the curtain wall system. These factors have a profound effect on the real-life performance of curtain wall systems, especially at the extremes of the design conditions, and must be taken into account to accurately understand curtain wall thermal performance. When using uniform interior temperature and humidity values across the entire curtain wall system, THERM predicted that several areas of the curtain wall did not perform to design parameters. The results were counterintuitive, so the team employed computational fluid dynamics (CFD) analysis to more accurately predict interior conditions and determine interior temperature values for use in THERM. In the end, the combined CFD and THERM analysis helped determine that only minor modifications were required for the curtain wall to perform as specified.