Building Envelope Performance and Design Considerations for Mechanical Engineers

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.