About Face: Giving Existing Towers a Modern Edge With Recladding

Using new curtain walls to improve high-rise façades for better design, value, and performance

Peter J. Arsenault, FAIA, NCARB, LEED AP

Construction methods for multi-story buildings have advanced notably in just the past 100 years. Where architects were previously limited to load-bearing exterior walls of masonry, concrete, or wood, the introduction of the now ubiquitous steel frame made a dramatic difference in the way exterior walls could be envisioned, treated, and detailed. At the same time, advances in glass and glazing technology allowed for better and bigger windows whether fixed or operable. Combine this construction evolution with predominant architectural styles that favored sleek, modern, and contemporary appearances and it is easy to see why high-rise buildings have become fertile ground for creatively applying new types of façades or building skins. The system of choice for multi-story buildings has become a curtain wall, which can be manufactured in several types, take on numerous forms, offer virtually unlimited design options, and provide high levels of energy efficiency. While architects and engineers find curtain walls to be appealing, building owners also favor them because they can help to identify or brand their building significantly. This is not only true for new buildings, but for existing buildings that may have an exterior appearance that has become worn and dated, causing the value of the building or its rental spaces to decline, not to mention its energy efficiency. Office building owners in particular often seek to upgrade spaces that are not currently attractive to modern businesses and their employees into Class A office space with better human comfort and energy performance.

Perhaps the single most important material common to recladding façades with curtain wall systems is glazing products. Long seen as purely architectural statements, the current generation of all-glass buildings is increasingly being promoted as an energy-efficient, environmentally friendly solution that enhances occupant experience as well as building performance. Technological advances are responsible for this performance boost with glazing currently available that can be treated to manage daylight and glare, feature low-emissivity for thermal control, and provide user privacy or transparency without compromising light quality. In concert with the pace of these technological innovations, recent investigations into the effect of the indoor environment on people suggest that basic design choices regarding the building façade—notably natural daylighting and ventilation—can dramatically affect the performance of not only a building but also the people who use it. Hence, no longer viewed as just an assembly of materials providing environmental separation between conditioned space and the exterior environment, the curtain wall façade is now recognized as an integral component of high-performance building systems with measurable user benefits.

Design teams that understand how to transform outdated building façades by replacing them with new curtain wall systems can meet the business objectives of the owner, enhance indoor environmental quality, improve energy performance, and create notable design statements in the process.

Design Considerations: Recladding with Curtain Walls

Many high-rise commercial buildings exist in urban environments with façades that have become worn and deteriorated over time, producing an appearance that has become dated and unappealing. This could be true if the building has a façade made of traditional materials such as masonry, concrete, etc., or if it has an early version of a curtain wall system that has become obsolete. Either way, design teams and building owners will often cite a number of practical reasons to renovate existing façades with new curtain walls. The first is to take advantage of the advances in curtain wall technology that have occurred in the last 30 years or so. This is particularly true in glazed portions of curtain walls, but is also true in opaque, insulated components with improved energy efficiency and related operational cost savings. The methods of attachment and joining components and panels have also improved, allowing for better sealing and weatherproofing of the building skin.

In our current 21st century experience, a curtain wall system is defined as a complete exterior envelope that provides a non-structural, relatively lightweight, weather-tight covering on buildings. Being lightweight, it reduces both the load that must be supported and the manpower needed to erect it. Curtain walls are generally installed outside of the structural system of a building, running continuously past floor slabs and other structural elements. They are attached via tiebacks and connector plates directly to the building structure at floors, columns, and beams. As with early versions, all wind loads and dead loads imposed on the system are compartmentalized and transferred directly back to the building structure, which in turn needs to be designed to carry them.

In terms of their impact on people, new, well-designed curtain wall systems routinely achieve improved comfort for occupants. They also provide ample daylight to spaces, allowing not only human benefits, but also the opportunity to turn off electric lights, thus reducing air conditioning loads and energy costs further. In tall buildings, new glazed curtain walls offer exceptional opportunities for dramatic views of the surrounding urban landscape. All of these features, coupled with a new, up-to-date appearance, feed directly into the ability of a building owner to market rental space within the building. In particular, office buildings, mixed-use buildings, condominiums, and other high-profile properties benefit from a well-designed curtain wall upgrade by allowing them to be sold or leased as first class or Class A space. The particulars will vary from building to building, of course, but it is easy to see where an investment in the exterior building skin can produce favorable paybacks on many fronts.

Renovation Design Process

Renovation and replacement of façades on existing high-rise commercial buildings is different from the process used for new buildings. Professionals involved in this work have documented the process based on their own and their company's experience.1 They point out that replacing a building façade requires a good bit of proper planning and analysis. In new construction, the process is not encumbered by existing conditions or occupancies. By contrast, the renovation of any occupied, existing building needs to fully take into account the existing conditions, including the people who may or may not occupy the building during the renovation. Having a full understanding of those existing conditions is often critical to the success of any renovation project.

Consideration and execution of a recladding project on an existing building can be broken down into the following three phases:

1. Feasibility phase The first step is a feasibility study or assessment to determine the pros, cons, benefits, and challenges to a retrofit. It begins by articulating a preliminary set of goals and objectives as developed by the owner in concert with the design and construction team. This is followed by a basic building survey to collect the relevant information about the building structure, the existing façade, the current and intended use and layout of the building, and other relevant design factors. Concurrently, the owner will likely engage in some financial analysis and planning to determine budget parameters while preliminary construction estimates are developed. Also at this point, it is important to address and take into account the disruption to building operations and how to plan the work in light of occupants remaining in place or moving to an alternate location. The question comes down to who will be impacted the most—the occupants as work is being done, or the contractor as they adjust schedules and workflow to accommodate occupant needs. Either way, the decision needs to be made so occupants can relocate either to other spaces in the same or another building or remain where they are with some degree of temporary barrier between occupied space and the renovation work. All of this will have an impact on the proper sequencing and workflow of the renovation, and of course will affect the overall cost of the project as well. Relocating occupants to another location will likely allow the work to progress more quickly, but will create added cost for moving (out and back in) plus any difference in rental costs. Remaining in place may appear to be more cost effective, but could reduce work productivity for both contractors and occupants. The result of this feasibility phase is a collected body of information that allows the owner, designers, constructors, financers, and other stakeholders to make fully informed decisions on the best means to move ahead.

2. Design phase Information from the feasibility phase directly informs and helps direct the design phase of the façade replacement. As the design is developed, the specific details of the new curtain wall system can be moved into design options that can be tested against the parameters of the feasibility phase. That testing will likely include some ongoing review of design variables as they affect both cost and energy efficiencies of the project. From a cost standpoint, this is the time to engage in value engineering as appropriate to help ensure a positive outcome rather than wait until the design is too far developed. It is also the time to engage with façade specialists, engineers, fabricators, and contractors in the spirit of integrated design to determine specific cost drivers, design options, and other key factors. From an energy standpoint, the use of computer simulations to show the impacts of different curtain wall options on the energy performance of the building should also be done earlier rather than later in the design phase to help select the most effective solution. Once the choices are narrowed, a final structural analysis will be necessary to be sure that the change in loading conditions due to the new façade can be either borne by the existing building structure or accommodated through structural enhancements. Finally, code compliance needs to be demonstrated and a review of the relevant details related to fire safety, loading, etc. needs to be factored in.

3. Implementation phase With the design of the replacement façade fully developed, the project moves to the implementation phase which begins with securing the appropriate building permits and lining up the full cohort of construction participants. The procurement or ordering of the curtain wall system and any related components needs to be undertaken so that fabrication and assembly can then get scheduled and underway. Overall, the construction work needs to be scheduled to allow for portions of the selective demolition of the existing façade to be coordinated with shipping and delivery of the same portions of the new façade. That means that installation may occur in one building area at a time or across multiple sections of a building as determined to be most appropriate to the specific project. As work progresses, the removal and recycling of any construction and demolition debris needs to be carried out so the site remains clean and safe. Perhaps one of the most important things during this entire implementation process is quality control. Therefore, it is recommended that a façade commissioning agent be retained to assure that the end result performs correctly as designed and meets everyone's satisfaction.

Design Strategies

While the aforementioned three-phase process remains fundamentally the same across all façade renovation projects, there are five fundamental design strategies that can be considered.² Each one has varying impacts on the existing building and are discussed further as follows:

▶ Façade replacement This approach is the most complete in that it involves the complete removal of the existing façade and related building components, often down to the building structure. With the old materials cleared away, a completely new curtain wall façade system can be designed and installed. This new system will likely have little if any resemblance to the old one in terms of appearance and performance. This is usually the intent of the full replacement—to create a completely different, modernized look that also meets the current user and performance demands for the building.

▶ Recladding This approach does not require a full replacement; rather the retrofit can be limited to the replacement of selective façade or cladding materials. An assessment of the building may determine that there are some substandard or obsolete cladding components that sit next to other components that are still quite usable and serviceable. The intent is to keep and work with the ones that are in good condition and replace the poor ones with new and better-performing materials. An example of a curtain wall recladding of this type might be to replace old single-pane glazing materials with new high-performance insulated glass units (IGUs). Often portions of the curtain wall system, such as original framing and opaque wall sections, can be retained while glazing, seals, and trim can be replaced with new. Similarly, anchorages may be retained, added, or reinforced as required by the final recladding design.

▶ Over-clad system A different approach to removing anything from the existing façade involves creating a custom framing and/or panel system designed to be applied directly over the existing one. This saves on selective demolition costs and can help retain some of the positive properties of an existing system with less disruption to adjacent building construction. As with recladding, outdated glass panel materials including vision glass panels are often replaced with high-performance IGUs. Anchorages for the new over-clad system may be added or existing ones can be reinforced as required, all working within the limits and confines of the existing building structure. The new system is then trimmed out to the interior as necessary to provide a finished product.

▶ Double-skin systems This design strategy can be considered a variant of the over-clad method since it involves the addition of a new second skin to an existing façade. However it is distinguished as unique in that this second skin is separated by some distance from the first, thus creating an intentional cavity between old and new. The cavity can then be used in a variety of ways to impact the performance of the building envelope. As an added layer, the double skin can temper the exposure of the building's interior to the weather and elements. With appropriately specified new glazing over an existing skin, it can help reduce glare, adjust visible light transmittance, and control solar heat gain. Through the use of well-designed air inlets and outlets, the cavity can be used to temper fresh air used in HVAC systems. This approach may work particularly well where innovative approaches are needed to notably improve energy performance of an existing building. It may not be appropriate where the existing building is already constructed along lot lines or zoning setback lines that will not allow the added depth of the building of the double skin façade.

▶ Hybrid system When one of the four strategies above is not deemed to be the best solution by itself, then it may be appropriate to combine them. This can be particularly true where different sides or portions of a building have different needs. Different orientations (south, north, east, west) will have different sunlight conditions that may dictate different design needs. High-rise buildings often have different wind and pressure demands on higher stories compared to lower stories, indicating different curtain wall systems to be specified accordingly. A combination hybrid system can also be appropriate where the existing building has different massing conditions due to zoning setbacks, etc., suggesting different approaches to the different conditions encountered. The point is that one strategy does not need to exclude the use of others in the quest to determine the best overall solution for the building.

The determination of the best design strategy for a particular building comes about by using the feasibility study and an integrated design process that can examine each strategy for its comparative strengths and weaknesses compared to the others.

Construction Techniques

With the design fully developed and the specified system fabricated, the task of installing new curtain walls on existing buildings comes down to finding the best ways to get the construction work done. There are several techniques that are viable and commonly used. The typical approach is to use a crane and/or scaffolding with a construction elevator to move materials and personnel to the needed areas of the building façade for installation of the curtain wall system. Panels can be raised from the ground and lowered to the locations where workers are stationed. In existing buildings, it may also be possible to use building elevators to facilitate movement of people and tools if acceptable to the building owner. Alternatively, a movable scaffold can be employed which uses a motorized platform that runs up and down the building. This is similar to a window washing system used on high-rise buildings and may be the least disruptive method. It is possible that any of these construction techniques can be incorporated in multiple locations around the building to allow for concurrent activity and shorter overall construction times.

Regardless of what is happening on the outside, the inside of the building will be impacted to some degree. This is particularly true in a full replacement where the existing façade is removed and the new one installed. Interior finishes along floors and any abutting walls will certainly need to be repaired and refinished, but there may also be impacts to perimeter heating and cooling systems that operate in those existing exterior walls. The implication is not only on the materials and systems that need to be worked into the final design, but the impact on the ability of the space to continue to be used during construction since the time period may be prolonged and comfort levels lost.

Green Building Design and Curtain Wall Retrofits

It is increasingly important that green building design is simply incorporated as part of the definition of a good design. This is particularly true for building envelopes including curtain wall façades. As such, there are at least three ways that curtain wall retrofits can contribute to creating a greener, more sustainable building.

▶ Optimize energy performance Owners of older buildings often live with high energy costs which they may pass on to tenants. In a multi-story building, the exterior walls are the largest surface area with the ability to make the biggest difference on energy performance. Finding the right mix of attributes, however, is typically an iterative, not a linear process. That means there needs to be a mechanism to assess different attributes related to curtain wall U-factors, Solar Heat Gain Coefficients (SHGC), Visible Light Transmittance (VT), and Air Leakage (AL). The only reliable and time efficient way to do this during design is to use computer modeling. Too often in the past, computer modelling was left until the end of a design phase or even the completion of construction drawings. That is a huge missed opportunity. Rather, the norm is becoming to integrate computer modeling into the design process from the very outset. In the case of a curtain wall façade retrofit on an existing building, the parameters of the building can readily be put into a computer model to create a baseline condition. That baseline model could reflect the existing conditions or be adjusted to reflect the attributes of a code minimum condition. The computer software can then generate an energy usage report based on monthly and annual anticipated consumption.

Here is how to use the computer model as a design tool. Save the baseline model to a new computer file and change one design variable in the curtain wall system. It could be the amount of insulation in opaque sections, the type of glass coating used, or the ratio of glass to insulated wall (spandrel) areas. The computer can quickly re-calculate based on that one change and the design team can see immediately what the impact is, whether positive or negative and by how much. Then do the same with other variables, one at a time, to see the impact each of those make compared to the baseline. When the best-performing variables are identified, then they can be collectively copied into a computer model to further refine and identify the best performance.

Alternatively, some energy modeling programs are available that plug into popular design software such as Sketch Up® or AutoCad® or others and can provide real-time graphic indicators of energy performance as design changes are made. Annette Burgard, vice president at Sefaira Architecture, points out, “Performance doesn't happen by accident. It's built on sound decision-making throughout the design process.” By using computer software to look at different options and alternatives, designers can make informed decisions readily and quickly to optimize a curtain wall system to perform in an optimal manner related to building energy use.

▶ Materials and resources A life-cycle assessment (LCA) for a curtain wall can be well worth doing in order to identify the full environmental impact of the materials used over the life of those materials. Manufacturers who have performed LCAs on their products can share that information with design teams in the form of an environmental product declaration (EPD) in the interest of transparently revealing such things as embodied energy, recycled content, end-of-life options, and overall environmental impact of their products. The EPD can then be used to compare environmental differences between manufactured systems and products.

▶ Indoor environmental quality A new curtain wall system on an existing building can make a dramatic and positive difference on the amount of natural daylight entering a building while capitalizing on available views to the outdoors. Further, there may be cases where the renovated façade can facilitate the introduction of fresh air via natural ventilation into the building. All of this contributes to a better indoor environment for people who may spend considerable portions of their days inside the building. In addition, out of concern for assuring a healthy indoor environment, most green buildings seek to avoid introducing harmful chemicals from products installed in the building. Curtain wall systems can be made and fabricated using pre-finished, low-emitting materials that avoid or limit the use of volatile organic compounds (VOCs) such as formaldehyde.

In the end, incorporating green and energy-efficient principles into the overall design process will produce benefits for the building owner and occupants for the life of the curtain wall.

Design for Future Renovations

There is a long-term aspect to take into consideration when it comes to building façades, particularly on high-rise buildings. A technical paper presented at the Council on Tall Buildings and Urban Habitat (CTBUH) 9th World Congress in Shanghai (2013 proceedings) titled “New Skins for Skyscrapers: Anticipating Façade Retrofit”3 is focused on future retrofits. As pointed out by the authors, high-rise building structural systems easily have a 100-year service life or more. They indicate that fully 86 percent of the high-rise buildings built by or before 1920 are still standing. However, in many cases, building façades, even for newly built or retrofit curtain walls typically have a 30- to 40-year service life including things like glazing, seals, connections, etc. Hence, they contend that it is reasonable to assume that any façade designed and placed on a building today, new or retrofit, will likely need to be replaced again at least in part, if not in entirety, during the full life of the building. Insulated glass units (IGUs) in particular are known to be susceptible to a limited service life due to seal failure which can cloud or discolor the glazing.

What is the implication for design with this anticipated future obsolescence? First, it begs the need to do a life-cycle assessment (LCA) on the final system selected. Choosing to specify systems and materials with higher durability and weather-resistant properties can extend the life of the system, particularly with sensitive or critical components such as glazing seals. The LCA can also be used to identify the embodied energy and potential material re-use or recycling at the end of its service life. Second, any new or retrofit curtain wall façade designs need to account for some future repair or renovation work beyond just routine window washing or cleaning. That means that glazing, support framing, seals, and other components should be designed that are easy to change-out on the building, either on the inside or the outside. Tall buildings in dense urban settings certainly have limits on safe and reasonable access to the outside, which may make interior access a preferred option. Coordination with curtain wall specialists, fabricators, and suppliers will help identify the possible ways to facilitate future repair and replacement of components. Third, the curtain wall should be easy to inspect and maintain. It is more likely that isolated panels or sections will need attention long before the entire curtain wall will. Hence, allowing for straightforward ways to accommodate inspection (such as on window washing platforms) and conduct periodic maintenance will assist in the ability to find and resolve any maintenance or component replacement issues early before they have the opportunity to grow to bigger problems.

Achieving all of the aforementioned future retrofit planning items is not always easy, particularly if there are other, competing design priorities. However, in the interest of creating well-designed, sustainable buildings, they should be seriously considered and design teams should strive to achieve this preferred outcome of facilitating future recladding or replacement work.

Conclusion

A burgeoning number of existing high-rise commercial buildings around the world are in need of façade renovations. That need creates an opportunity to transform existing buildings from outdated, poorly performing ones to modern, efficient, and Class A green buildings. Whether recladding, replacement, over-cladding, double skin, or hybrid systems are used, the end result makes a building more valuable since it becomes better suited to the current needs of owners and users. Integrated teams of professionals that recognize this potential and who understand the options and details of using modern curtain wall systems can grow their practice by serving clients in need of such renovations. More significantly, they can leave an enduring mark on the cities where these buildings are located in terms of better performance, better buildings, and better design.

Endnotes

1. Structural Glass Façade Technology as a Building Skin Retrofit Strategy Mic Patterson, LEED AP (BD+C) and Jeffrey Vaglio, PE, LEED AP (BD+C) Enclos Advanced Technology Studio
2. Ibid – Enclos paper above.
3. “New Skins for Skyscrapers: Anticipating Façade Retrofit” Patterson, Mic; Martinez, Andrea; Vaglio, Jeffrey; & Noble, Douglas University of Southern California CTBUH 9th World Congress, Shanghai, 2013 Proceedings

Peter J. Arsenault, FAIA, NCARB, LEED AP, is an architect and green building consultant who has authored over 100 continuing education and technical publications as part of a nationwide practice. www.linkedin.com/in/pjaarch

The Ornamental Metal Institute of New York is a not-for-profit association created to advance the interests of the architectural, ornamental, and miscellaneous metal industries by helping architects, engineers, developers, and construction managers transform designs into reality. www.ominy.org