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ABSTRACT
As the building design industry transitions from a linear process and towards a collaborative and iterative approach, it is more important for project teams to understand each team member's design criteria and considerations. High-performance buildings require mechanical systems that reduce energy consumption while maintaining indoor air quality. To achieve this criteria, mechanical engineers must understand the role building envelopes play in the overall building performance. This paper will define key design considerations of high-performance building envelope systems and address common missteps regarding building envelope performance values. Specifically, this paper will review fenestration and spandrel U-factor calculations, envelope thermal bridges and thermal comfort, mechanical system impact on surface condensation, and review envelope air leakage assumptions. Case studies of these concepts will provide practical examples demonstrating how key high-performance envelope design considerations impact occupant comfort and overall building performance. The paper summarizes fenestration and spandrel u-factor calculations and envelope thermal bridge calculations (psi-values), and how these are entered into energy models. This paper will also review surface film coefficients, how mechanical systems can impact envelope condensation risk, and discuss the principle drivers of building envelope air leakage, how it is measured, and its impact on indoor air quality and energy consumption.
INTRODUCTION
The building envelope and mechanical systems are two distinct parts of a building with dedicated consultants on the project team. Each consultant selects and designs their respective systems to meet the Owner's needs and code requirements. Both systems impact energy consumption and indoor air quality and are inextricably linked. As building envelope consultants, the authors often work closely with mechanical engineers to provide feedback on the impact mechanical systems have on building envelope systems and vice versa. Through the author's combined experience in new design, renovation, and investigation projects, several common themes emerged: fenestration and spandrel U-factor calculations, envelope thermal bridge calculations, condensation risk analysis, and envelope air leakage. In this paper, the authors explore each of these topics to provide insight to mechanical engineers on design methodologies to avoid common misinterpretations.
FENESTRATION AND SPANDREL U-FACTORS
Conventional commercial fenestration systems typically consist of thermally broken metal framing with glazing infill. Fenestration systems can be an individual panel within a punched wall opening, or it can be a panelized facade system with both transparent...