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.