Verifying Thermal Performance

A Case Study

Overview

Authors

Photo of Matthew Herman

Matthew Herman

Buro Happold

Matthew.Herman@BuroHappold.com

Photo of Mark Walsh

Mark Walsh

Principal, Technical Director

Perkins&Will

Mark.Walsh@perkinswill.com


Keywords


Abstract

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.

Introduction

The use of THERM as a tool to study the movement of heat across façade systems has been invaluable to the building industry. However, every tool has its limitations. THERM

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Project Description

Northwestern University’s Ryan/Walter Athletics Center is a 425,000 square foot athletic training facility whose most prominent feature is the level 2 field house and its combined soccer, lacrosse and football

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Curtain Wall Description and Design Requirements

The curtain wall that was designed to fulfill all of these criteria is a steel-girt-supported, all-glass system without horizontal or vertical mullions. The only aluminum components of the system are

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Initial Thermal Analysis and Results

As required by the specifications and based on the specified design conditions, the contractor determined that the interior dew point temperature in the winter was 37.2°F. They proceeded to perform

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Computational Fluid Dynamics Modeling

The design team needed a simulation tool capable of capturing the interaction between three modes of heat transfer, conduction, convection and radiation. Additionally, the simulation tool needed to expand the

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Revised Thermal Analysis and Results

Ultimately, the information gleaned from the CFD model provided variable values across the height of the façade that could then be used in to yield more accurate and nuanced THERM

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Conclusion

façade. By simulating conduction, convection and radiation a fuller understanding of thermal gradients across a façade in three dimensions can indicate how the actual mechanical and façade systems will interact

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Appendix

Item 1:

Subsections a. through i. of 08 88 30(1.5)(B)(1)

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