The Engineer: Avoiding pitfalls in glass esthetics
By David Heska
Q&A on esthetics
By David Heska
Glass dominates the built environment and its quality and consistency can be an esthetic benefit or detriment. Building codes are stepping up the requirements for energy efficiency and for honest accounting of enclosure performance.
This energy reckoning for assemblies often translates into designers squeezing every ounce of performance from glazed assemblies. In this article I’ve chosen to interview my colleague, Scott Armstrong, asking him a few questions on the visual characteristics of glass.
DH: In the past year you have been involved in numerous glazing projects. What are some of the experiences you’d like to share with our readers?
SA: Achieving a quality glazing product requires rigorous control through the entire fabrication and manufacturing cycle. An IGU must meet strict requirements for elements such as thickness, stress, bow, warp, roller wave, coating quality, seal consistency and sightline. Most manufacturers have extensive in-line quality assurance and quality control procedures to catch defects before they arrive at the fabrication plant or on site. Yet, we are regularly engaged to navigate late-stage discussions with owners and designers on quality and esthetics. Buildings with large glazed areas often rely on highly reflective low-E coatings to control solar heat gain and manage cooling energy intensity. Highly reflective coatings can exacerbate issues pertaining to esthetic quality since the exterior reflected image is pronounced. The reflected image in urban or clustered suburban environments often contains strong vertical and horizontal lines from adjacent buildings. Coupled with the design trend toward sheer glass façades with narrow, flush joinery, distorted images become even more pronounced.
DH: What’s contributing to these esthetic issues?
SA: Heat treatment, especially tempering, can be a significant contributor to reflected image distortion since it can induce bow, warp, edge lift or roller wave to the finished product. Designers should consider whether the same durability and strength requirements can be achieved with heat strengthening rather than tempering to help mitigate potential distortion issues. Heat soaking can help reduce the risk of spontaneous breakage from nickel sulfide inclusions in tempered glass but using heat strengthened glass instead of tempered or heat-soaked tempered glass eliminates the risk altogether.
Another trend is the use of thicker glass lites (eight- or 10-millimeter versus standard six-millimeter) for the outer lite of an IGU. Glass flatness improves with thickness and better resists changes in flatness during heat strengthening or tempering. When coupling a thicker outer lite with a thinner inner lite, more of the cavity pressure will be distributed to the inner lite, reducing total distortion of the outer lite and exterior reflected image distortion.
DH: What can designers and owners learn from common complaints in the industry?
SA: Specifications should provide clear direction on which standards apply and what type of documentation is required to demonstrate compliance. Designers and owners should set reasonable expectations for optical flatness and should understand how some design decisions influence perceived quality of the finished product. In-line QA/QC procedures are the most reliable means for identifying non-conforming units and we advocate for third-party reviews of both production lines and QA/QC documentation. Setting expectations for quality early in the project provides the necessary benchmark against which future product can be evaluated.
David Heska, P.Eng, is a director with WSP’s building sciences team in southwestern Ontario. He oversees the operation of the Hamilton, Kitchener and Windsor offices. David has been involved on window simulation projects as well as the design and replacement of windows and residential buldings. He can be reached at David.Heska@wsp.com