While highly reflective glass efficiently blocks solar heat gain, it causes a significant impact on the neighboring environment due to exterior reflections. To calculate a building design’s potential solar reflectivity, CDC also developed computer modeling software using Computational Fluid Dynamics that shows not only the location of the reflected light, but also its intensity
In a built environment where space is at a premium and developers fight for shallower curtain-wall systems to increase leasable square footage, skyscrapers are replacing many tall buildings. These glass and metal-clad skyscrapers, combined with narrow streets and densely populated areas, can potentially create hazardous reflectivity issues for neighboring buildings and passing traffic. Most building codes do not explicitly limit solar reflectivity in the architectural design criteria; however, performing a solar reflectivity study in the initial design stages can help prevent reflectivity and exterior glare problems. Examples of recent problematic designs include the Disney Concert Hall in Los Angeles, where curved metal panels reflected intense solar radiation onto an adjacent building façade. Correcting the problem required a change in the surface finish. Another example is the Vdara Hotel in Las Vegas, which was reported in local newspapers to magnify and reflect the sun’s rays onto an area of the pool at temperatures hot enough to melt plastic drink cups. This “death ray” was due to intense solar reflections from the hotel’s concave curtain-wall geometry.
In Dallas, the 42-story Museum Tower is reflecting so much light into the neighboring Nasher Sculpture Center that it is threatening artworks in the galleries, burning the plants in the center’s garden and blinding visitors with its glare, according to the New York Times. The center’s bespoke skylights were designed to filter indirect daylight into the art galleries, and instead, are now subject to direct solar reflections from the new curved tower’s metallic coated glass facade. Sunlight is going to be reflected from the built environment no matter what we do, but the key is to limit the reflections and avoid solar reflectivity concentrations on highways and neighboring buildings. Energy performance criteria encourage the use of reflective glass in architectural design to reduce penetration of solar radiation into the building interior. However, while highly reflective glass efficiently blocks solar heat gain, it causes a significant impact on the neighboring environment due to exterior reflections. Typical clear glass has an exterior reflectance value of 9%, whereas highly reflective glass exhibits an exterior reflectance value of approximately 20% to 40%.
Currently, there are no reflectance limits in place for building facades. This is due in part to the number of variables involved. While the amount of light a building facade reflects depends mainly on the properties of the facade materials, it is also influenced by the geographic location of the project, its orientation, the climatology, etc. Glare also affects people differently, depending on the person’s age, eye pigmentation, eye sensitivity, pupil’s ability to rapidly adapt to light contrasts, and eyewear. What might cause discomfort for some might not bother others. To help building designers avoid potential reflectivity and exterior glare problems, Curtainwall Design & Consulting Inc. developed a proposed glare threshold based on the comparison of different levels of reflected light with common sources of light. The upper limit compares light reflected from buildings to direct sunlight, for example.
To calculate a building design’s potential solar reflectivity, CDC also developed computer modeling software using Computational Fluid Dynamics that shows not only the location of the reflected light, but also its intensity. Using this technology, building designers can determine whether or not the reflected glare will pose a problem. Solar reflectivity studies using CFD generate data that compiles the accumulation of reflected rays at individual locations in the surrounding environment. The algorithm provides glare data at a particular time of day; this means that the results are accurate for one particular month, on one particular day, at a given time of day. Therefore, several iterations are required in order to create representative data for the whole calendar year.
The study also takes the surrounding environment into consideration. Adjacent buildings might shade the project at various times of the day, on certain days, while roadways or neighboring facades might be sensitive to reflections bounced from the project’s reflective surfaces. The amount of heat that reflected rays produce depends on the number of reflected rays coinciding on a particular area at a given time. As a secondary step, the study can provide the temperature increase generated by the reflected rays as well. This could help avoid problems like those experienced at the Vdara Hotel, for example.
Performing a solar reflectivity study allows architects to easily rotate the building design within the modeling software to determine which orientation will best mitigate reflectivity. The CFD’s ability to then determine the potential intensity level of the reflected rays can also help prevent hazardous glare.