by Katie Daniel | May 25, 2018 9:54 am
By Jeff Ker
Recent instances of extremes of weather have prompted conversations about issues that can no longer be ignored. Building façades, as the “front line” facing punishing weather systems, are a key focus of consideration when addressing designs for the future. Many current cladding choices have a long lifespan, causing the architectural community to consider whether façade designs will withstand the trajectory of more extreme weather on the way.
This author, along with John Kubassek of Engineered Assemblies, presented his perspective on façades for the future at a recent conference. The presentation has been adapted for print here, with the central focus of bridging design with the field of construction to look at the complete envelope system and encompass all areas of consideration that can contribute to a successful installation. That means taking a step away from day-to-day business as an individual specialist to consider the industry as a whole.
According to the author, there are three fundamental goals the industry should pursue:
The automobile industry did an excellent job of showing us “tough isn’t the answer.” Years ago, cars were built heavier, with more mass, and they were designed to be tougher. When these older-model cars were involved in a collision, shockwaves went through the vehicle and, of course, gravitated to the lowest common denominator (the driver and any passengers). Not only were these cars built before fuel efficiency concerns, but they were also heavy to move and produced excessive exhaust fumes.
Fast-forward to today. Cars are a lot lighter with less mass, much more respectful of fuel efficiency, and have crumple zones and airbags. The suggestion is not modern cars are superior in all respects to what came before, but there has been substantial improvement, and it is due to smart engineering. They are now safer, employing intelligent systems rather than sheer muscle. It is a case of brains over brawn. In this way, façades have followed the same path.
Building envelope systems were first structured solely to withstand the forces of nature. Factors like embodied energy and sustainability were not considered. To do better for the environment, building professionals now use lighter systems with less mass and clever engineering. These new façades absorb more energy and can transfer it accordingly, instead of standing up like an unyielding monolith. They will flex and give, as seen in seismic zones. So, similar to cars, façades have evolved to contain less mass. Smart engineering and science have also led to greater façade strength.
Looking to the future
As mentioned, façades are critical because they are the first line of defense from extreme weather. With the real threats related to climate change, façades are a vital component for designers and builders to address. What is going to make them fruitful for the future and successful now? The author suggests these considerations:
Additionally, it is worth noting façades play an essential role in protecting buildings of significance; if they fail, the entire building is compromised.
Façade systems are generally produced using durable materials with lower embodied energy. Crafting environmentally responsible products and policies is not new to this industry, but the concept of lower embodied energy is less known.
Embodied energy has two components: initial and recurring. Initial embodied energy is the energy needed to fabricate a product and place it where it belongs. On the other hand, for products that do not last, recurring energy is required to maintain or replace them. This is a crucial concept relating to sustainable materials on the subject of environmental conservation.
Several façade materials could be considered to have low embodied energy. In the case of cement, for example, this applies to high-density fibre cement. Other types of materials that can fall into this category include phenolics, ceramics, porcelains, and some metals.
Fibre cement has been around since the early 1900s. It can be compressed to a high density and is composed of a few raw materials: Portland cement, water, natural fibres, and sand. Most high-density fibre cement panels are unique, displaying the raw, untreated texture of the base material. Life expectancy is at least 50 years. To diminish the environmental impact of manufacturing practices, some manufacturers of fibre cement have worked to remove trucks from the distribution chain in favour of using local waterways. They can also reuse waste and water, and doing so does not seem to sacrifice the product’s esthetic quality or ability to address various types of architecture. In particular, fibre cement seems to suit edgy and contemporary designs.
Phenolics are another type of material well suited to engineered exterior façades. Robust and resilient, these rigid homogenous panels are manufactured using tough thermosetting resins reinforced with cellulose fibre for added strength and durability. Some panels have a decorative surface on both sides. Interestingly, some manufacturers of phenolic panels now integrate real wood veneer. This is not a high-pressure laminate skin, but a real wood veneer skin, which means each panel is visually different. This type of product can enjoy the benefits of a natural, organic esthetic, and at the same time—in keeping with the consideration of embodied energy—it requires little recurring energy for maintenance.
Reuse and recycling of waste is an important aspect of manufacturing these systems that cannot be left out, bearing in mind the flexibility of the product to address various types of architecture. Designers and specifiers should also be looking for companies with sustainable forest management at heart. Specifically, International Organization for Standardization (ISO) 14021, Environmental labels and declarations—Self-declared environmental claims, and ISO 14001, Environmental management systems, can apply to forest management systems.
Rear-ventilated rainscreen (RVRS) systems embrace continuous insulation (ci). Moisture is never welcome in the building envelope, nor in most aspects of the structure. An ideal system substructure is thermally broken, is a rear-ventilated rainscreen, and is covered with sustainable materials. A good ventilated façade can be at least 35 mm (1 3/8 in.) deep with the potential of adding an outboard insulated system in a thermally-broken substructure giving good effective R-values and protecting everything behind the weather membrane.
RVRS systems are subject to the European Deutsches Institut für Normung (DIN) standard 18516, Cladding for external walls, ventilated at rear. This standard applies to rear-ventilated cladding for façades without background, including anchoring, connection, and fixings. It specifies calculation, design, and construction principles for permanent structures.
Microclimate for balance
A building envelope plays an important role as the leading edge of a structure. Its purpose is the protection of the superstructure/core. Notably, ventilation is a vital part of the building envelope. Gravity alone will not remove moisture. The rainscreen does not propose to prevent any and all moisture from entering the façade, but gives it a front and back door from which to exit. This exit is part of the active plenum created in a true RVRS system. Air continuously flows through the system working to dry out the RVRS. This is a simplified explanation of a complex process.
The active plenum within a substructure behind a façade is referred to as a “microclimate,” since its small climate is completely independent from the inside of the building, the rest of the envelope, and the outside. Consider a hot, sunny day in July, when temperatures outside are 28 C (82 F) or greater, and one wants to maintain a much lower temperature inside for comfort. The exterior panels essentially act as shades, in layperson’s terms. Rather than the sun beating down onto the panels and conducting heat to everything attached to them, which causes heat to be pushed through and inside the building, the microclimate moves air, hot air in this case.
Moving air dissipates the heat off the front panel, serving a purpose for the envelope in terms of heat exhaustion, and also for the panel in terms of dissipating heat for stability. If the ventilation was not properly set up, there would be a buildup of condensation, which, combined with solar heat gain, would create a virtual sauna. Imagine what such excessive, moist heat behind a panel system would do.
The active plenum exists to keep things dry, and to balance temperature and integrity. Canadian engineering firm Morrison Hershfield found having 25 mm of air space in the active plenum could produce up to 0.7 additional R-value. (For more information, see Morrison Hershfield, Thermal Performance of EAi Thermal Clips, March 22/2012, report#5123226.00.) Attaching an insulation value to the plenum is important, as it supports the idea the system strength as a whole can be more valuable than the individual components.
Some façade systems are holistic; like a high performance RVRS, everything works in a particular relationship with the other components to provide higher overall performance. In this author’s view, this is the performance the industry must move towards, leaving the old ways behind because they do not work well enough. Looking ahead to changing building codes, it is clear standards are getting tougher and both manufacturers and designers must rise to higher levels of performance.
Beyond their obvious structural function, walls are important to the thermal performance of a building. Acknowledgement of this required the industry to change gears. Previously, the predominant school of thought said roofs were important. Buildings have become a focus for energy consumption—in fact, a building can lose up to 70 per cent of its energy through the façade, slab edges, and fenestration. (For more information, visit www.e-education.psu.edu/egee102/node/2070, or www.oaa.on.ca/oaamedia/documents/Thermal%20Bridging%20And%20Whole%20Building%20Energy%20Performance.pdf.) Hence, walls are important.
When insulation is installed into an RVRS or an outboard continuous insulated system, designers must consider heat bypasses. The façade must be set in a substructure and that substructure could conduct energy; thus, it could make the insulation less effective. The solution to this problem is thermally-broken substructures.
Design guides exist for thermally-broken substructures and how to specify the substructure for a high-performance façade system.
Consider the example of mineral wool in the context of a thermally-broken substructure and effective R-value. Mineral wool is noncombustible and does not have the highest performance in terms of thermal value but for this example assume that 127 mm (5 in.) of mineral wool will produce R21. What happens to that R21 value when we put it into an envelope? Does the performance value change because the envelope design introduces other conductive elements, such as substructures? Is the energy from the inside going to race out through a pathway? Will it be lost? Yes, in fact, it will.
An R-value table created by Morrison Hershfield looked at the idea of an outboard insulated façade. If the starting point is R21, the end result will be R11 to R15 when employing the “old ways.” R11 is only 55 per cent of R21; 45 per cent of the insulation value has been thrown away. We could produce a value of R15 with cross-girtting, but that is still only 70 per cent.
It is possible to achieve more with a thermally-broken substructure. If the builder employs this technique, the system should achieve around R19 to R21—a lot closer to the insulation value of the insulating material. (For more detail, see Engineered Assemblies’ SYSTEM2 design guide/TcLip.) All the components of the building envelope, put together in a system, contribute to the thermal performance. Various documents and data suggest every single component of the envelope is either neutral, takes away efficiency or provides efficiency. (For more information, see Linear and Point Transmittance Evaluation for Engineered Assemblies T-100 Cladding Attachment System, Oct 6/2016.)
Vertical expansion and contraction
Extreme weather conditions affect the façade system. Expansion and contraction are more pronounced with lightweight, low-mass façade systems, especially when subjected to frequent, extreme weather changes. Heat waves, more transitions over the freeze/thaw line, and higher winds mean components are going to be moving.
Certain design guides can offer prescriptive ideas on how to create a façade system that can work within this dynamic environment.
A building face is a large area. Consider the example of a face measuring 30 x 30 m (100 x 100 ft). One cannot just cover that area with a big steel cage (i.e. a substructure). It would be like a big steel net trying to hold everything in place. There is too much movement in a building from expansion and contraction of the materials, as well as movement from the control joints. Ideally, one should stop and start the substructure at every floor slab. None of the vertical hat bars creating the 25-mm plenum should go over a floor slab, because it is point of deflection in a building. Horizontal z girts, in this example, should be about 3 m (10 ft) long. The gaps between the vertical hat bars and the horizontal z girts forming the grid for this sample RVRS system should be >/=3 mm (11.8 mils).
The “quadrants” created with the sample grid above will operate independently, allowing them to move and shift since all the pieces are going to expand and contract. Wind loads are higher on the corners of a building than they are in the centre of a facade, and therefore, the outside corners of this sample building will experience more natural forces. Due to the different forces acting on different parts of the building, it is not advisable to connect one system completely together. In a high-performing system, quadrants should be employed, and within quadrants, the system should address control joints.
To avoid failures in façade systems, one has to engineer a system accounting for adjustability and flexibility and responding well to the inevitable realities of nature—wind loads, freeze/thaw, expansion, contraction, and perhaps seismic movements. In a large building, one must certainly allow for shifting and movement in the superstructure. This cannot be seen with the human eye. However, the engineering design should respect this type of movement and so should the substructure.
Engineering for panel dynamics incorporates expansion and contraction allowances within the panel relationship to the substructure, and allows the substructure to respond to the forces of nature and to the superstructure. Some of this engineering can be realized in addressing movement through fixed and floating points of some components. One way to think of a high-performing façade system is to divide the whole system into three zones. The panel is zone one, the substructure is zone two, and the superstructure, zone three. All three zones have agendas in terms of expansion, contraction, and movement. They must respect each other. To avoid failures and not negatively affect the panel in terms of its integrity, future façades must be engineered a little bit differently as they are on the front lines facing the forces of wind, solar heat gain, freeze-thaw cycles, and the higher frequency of extreme weather due to climate change.
Uniting design with construction
More complex weather patterns create conditions where architects need technical support to select the best façade material for each project. Beyond esthetics, the architectural community has responsibility to build for post-disaster scenarios and to ensure structures are still standing, in good condition, 50 years from now.
In this author’s view, it is important to unite the house of design, defined as engineering and architecture, with construction, to ensure projects are built the way they are designed.
There can be synergy among design, construction, extreme weather, and environmental sustainability. Façades need to deliver strong thermal performance, embody less energy, and confront the effects of climate change head-on. With this in mind, the industry is on the way to building better façades for the future.
Jeff Ker has more than eight years of technical sales experience in the Ontario architectural and design industry. His experience includes two years in the West Coast market and more than six years in the Eastern Canadian market, representing a variety of rear-ventilated rainscreen (RVRS) systems. Ker has a solid background in technical sales, project management, and liaison with the construction community. He can be reached via e-mail at firstname.lastname@example.org.
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