May 26, 2015
By Andrea Yee, B.Sc.E, and Nastassja Pearson, M.A.Sc., P.Eng.
Today, more and more architects are literally thinking outside the box. Modern buildings are taking on unique shapes and forms, and structures are reaching staggering new heights. This shift means the purpose of the building envelope is also expanding. With new materials and construction methods at their disposal, design professionals are increasingly looking to the building envelope to help supply buildings with a distinctive identity, simplify construction, and reduce costs.
Despite this trend, the industry needs to remember the building envelope’s primary purpose has not changed—it is still to provide shelter and protection from the outside elements. Therefore, it is important a building envelope’s design and construction continues to support, not compromise, performance.
In this article, three current design and construction trends for the building envelope will be discussed—use of glass, increasingly complex and unusual designs, and prefabricated cladding panels. Additionally, the potential impact each can have on building envelope performance will be examined. Finally, ways for design teams to manage the potentially negative (and often unintended) consequences of adopting these trends are explored.
Glass and the building envelope
Employing glass in buildings beyond conventional vision areas is a popular building envelope trend. In cities around the world, an increase in advanced glazing assemblies such as high-spanning curtain walls, glass canopies, and glass railings are being seen. Curved glass applications are also becoming increasingly popular, as is the use of structural glass for floors, stairs, and walls. The primary advantage of glass is it provides visible light transmission (VLT) and, if designed and used correctly, it can also be a strong and versatile material.
However, designing and installing glass without fully understanding its in-service behaviour may leave design teams dealing with unexpected consequences. The most obvious of these is glass is a brittle and fragile material, and it is much more likely to break in its everyday use than metal or concrete.
Glass can be sensitive to imperfections, such as surface and edge defects, and to solid inclusions and impurities. These compromise its strength and can lead to premature failure. Glass can also fail due to excessive buckling if it is incorrectly supported or excessively stressed. Stresses are derived from many sources, including direct impact, temperature differentials, concentrated loads at uneven or improperly designed supports, or material impurities.
The first step in minimizing the potential for glass to break is to apply good design. An example of a best design practice is to ensure the glass is separated from other hard surfaces, for example by leaving a gap between the glass edge and frame. Another is to allow sufficient accommodation for movement. Restricting the glass from moving can potentially create concentrated stresses and lead to premature failure.
Anticipating the post-breakage behaviour of glass is also critical to safeguard the public when there is a failure. Depending on the type of glass, it behaves in different ways when it breaks.
Both annealed and heat-strengthened types of glass have a tendency to crack and/or break into large shards when they fail. While this creates a hazard to people wherever accessible, it poses a risk of greater injury in an overhead application if it falls onto someone below.
Tempered glass, on the other hand, breaks into smaller pieces and is intended to cause less injury in similar applications.
Where there is an increased risk of glass falling, it is often preferred to have the glass stay within its original opening until there is an opportunity to vacate the surrounding area to allow building staff or installers to safely remove the broken glass.
Laminated glass has the highest probability of staying in place subsequent to failure. The interlayers bonding the lites together are what keep the unit intact. However, if all the laminated glass lites are fully tempered, the glass may have a tendency to sag out of its opening upon breakage. This deformation is commonly referred to as the ‘wet blanket effect.’ Sagging is less likely to occur when a stiffer interlayer material is used or at least one of the lites is annealed or heat-strengthened because those larger shards can potentially provide additional support.
Esthetics of glass
Another consideration when specifying glass for the building envelope is esthetics. While maintaining a desired appearance may not be as critical to material performance as managing breakage, it is nonetheless important to a building’s overall look and style. With multiple options available to designers through glass tints, films, coatings, and frit combinations, it is prudent to construct a physical glass mockup to better gauge the desired appearance.
Designers should also make careful and informed decisions about the materials that will be adjacent to, and on, the glass. Poorly chosen materials can cause unwanted and premature changes in the appearance of the glass, including coating corrosion, film peeling, and/or moiré effect from overlapping frit patterns.
Glass often fulfils occupants’ and designer’s desire to connect with the exterior environment. However, this intended connection is often thwarted when occupants pull blinds and curtains closed to reduce glare or prevent overheating. While there have been technological advances with low-emissivity (low-e) coatings and shading devices, their effectiveness does have a limit and cannot compete with an opaque wall with respect to thermal performance and heat control. Rather than designing with a focus on quantity of glass, designers should focus on the quality and location of the visual connection between interior and exterior. For example, there is little need for glass at floor level in a space filled with office desks.
It is also important to understand the surrounding environmental conditions. The sun’s rays reflecting off a glass façade can alter the desired appearance of the building and can also affect the surrounding environment. Highly reflective surfaces can reflect heat to neighbouring buildings and public spaces, increasing their cooling loads and making conditions less comfortable for occupants.
Designers need to ask questions
As the use of glass increases, designers need to consider a host of questions throughout the entire design and evaluation process, including:
Complex and unusual designs
Designers often seek to create a distinctive identity for a building by including feature elements such as curved walls, twisting structures, inverted slopes, overhangs, and screens. Designers are also pushing the limits of what materials are able to do and what manufacturers are able to produce.
A good deal of thought is generally given to these complicated architectural features during construction. The same cannot be said for post-construction maintenance. For example, a building clad with a large decorative screen in front of a curtain wall may have had relatively straightforward construction, with the curtain wall installed prior to the screen. Careful thought may have been given to each component to ensure it could carry the screen’s weight and any ice and snow that may build up on the screen.
However, was equal consideration given to in-service maintenance? If a curtain wall glass unit requires replacement, does the screen need to be disassembled or can a new unit be installed with the screen in place? How will the area behind the screen be accessed? Is the proposed repair plan practical and/or costly? Is there a better, simpler way to replace the glass? These are the types of questions that should be considered and answered during the project design phase, long before the building is turned over to the owner/operator.
Prefabricated cladding panels
To speed up cladding installation and lower costs, designers and contractors are using more and more prefabricated cladding panels. These all-in-one panels are fabricated in a controlled environment where quality and consistency can be closely monitored. Typically, the air, vapour, and thermal control layers are built directly into the panels.
Using prefabricated panels typically means 90 per cent of the cladding can be installed in short order. However, completing the remaining 10 per cent—which includes the joints between panels, transitions with adjacent assembles, and enclosure pockets required around anchors—can account for the majority of the labour effort or time. This is because these components—especially joints, such as those between precast concrete panels—are often small and difficult to detail since access to them is limited once the panels are installed.
However, these joints and transitions are paramount to enclosure performance as they provide air, vapour, and thermal control layer continuity. Airtightness and resistance to water penetration are critical to a well-performing wall. Therefore, if time-consuming, costly repairs are later required due to inadequate onsite joint detailing, some benefits of prefabricated panels may be lost.
Managing unintended consequences
Designers, constructors, or owners cannot be expected to independently anticipate these issues. Rather, the best course of action is to maintain open lines of communication between the entire project team so everyone involved understands the design choices. In this way, concerns can be flagged, shared, and worked through. Activities that support open communication include:
Collaborative design review
Collaborative design reviews or design charettes between project team consultants can be extremely beneficial through the planning process. Charrettes provide designers with a direct forum for quick idea sharing and feedback regarding the design of individual components and how they may affect other components or the design as a whole. What one team may see as a benefit could be a complication to another. Charettes allow designers to leverage expertise from various subject matter experts so a cohesive building envelope design can be developed.
Mockups, laboratory testing, and field testing
Similar to collaborative design reviews, using mockups and system testing is also essential to good design and execution. These are both valuable to ‘test-drive’ the design before it is fully implemented, and they provide an opportunity to catch problems earlier in the design and construction process while changes and revisions, if required, are less costly to make.
Field testing allows the design and construction team to gauge whether the fabricated product meets the original specifications. In other words, confirming building owners are actually ‘getting what they paid for’ is a good starting point to avoid unintended outcomes down the line.
Execution is as important as design. QA/QC procedures and plans are put in place to help constructors control and minimize missed steps between design and construction. These procedures can include asking for appropriate production submittals, documentation, and drawings early in the construction process, and reviewing them against the original design documents. QA/QC can also mean reviewing construction in the field and performing field testing to confirm the designs and products laid out on paper have actually been installed and executed correctly onsite.
Although QA/QC and testing are additional project costs, they should be considered valuable insurance to ensure money is not being wasted and the original design intents are being met. For example, building owners often pay a premium for window thermal upgrades such as argon fill and low-e coatings in insulated glazed (IG) units, but during field testing these authors have found the argon or low-e coating is installed incorrectly or missing altogether.
While usually unintentional, mistakes such as these can happen. Identifying and correcting the problems early can usually prevent them from escalating and becoming even larger, more costly issues in the future.
Turning over a building to the owner may symbolize the end of design construction, but the maintenance plan should not be an afterthought. Maintenance plans should be considered at the start of the design process because the design’s complexity and type of materials selected affect how a building will need to be maintained and the cost to operate it. This is also a principle outlined in CSA S478, Guideline on Durability in Buildings.
Despite their best intentions, design/construction teams occasionally develop buildings with performance issues when they do not give careful consideration to enclosure design, material choice, execution, and future maintenance. Strategies to manage these risks, including collaborative design reviews, mockups, testing, QA/QC, and upfront maintenance design can be costly. However, owners, developers, and constructors should recognize the cost of dealing with unintended consequences, and the resultant need to re-work and ‘design on the fly’ is potentially much higher. Generally, the earlier in the design process a problem can be caught, the less costly it will be down the road.
Andrea Yee, B.Sc.E., is a project manager with Halsall Associates, and has eight years of experience with building enclosure and repair/renewal projects. She can be reached via e-mail at email@example.com.
Nastassja Pearson, M.A.Sc., P.Eng, is a project manager with Halsall Associates, and has 10 years of experience with building enclosure and repair/renewal projects. She can be reached at firstname.lastname@example.org.
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