Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 63 THE IMPLICATION OF ENERGY EFFICIENT BUILDING ENVELOPE DETAILS FOR ICE AND SNOW FORMATION PATTERNS ON BUILDINGS N. Norris, D. André and P. Adams, M. Carter and R. Stangl ABSTRACT With advancements in building design in combination with changing weather patterns there is growing concern over the increased occurrence of hazardous ice and snow formations and their potential release from mid- and high-rise buildings. This concern is not only for the potential for building damage, but also for the risk to commuters at street level if the ice should fall during their daily commute. In cold climates, traditionally poor thermally resistant envelope assemblies readily transferred heat from the interior conditioned space to the exterior surfaces of the building envelope, especially through glazing assemblies. Glazed aluminum-framed envelopes (curtain wall/window wall) have become common for highrises and the building industry is currently moving towards using more thermally efficient versions of these assemblies in order improve overall building energy performance. While improved thermal performance for buildings is certainly a necessity, it can have unexpected consequences for ice and snow formation on building exteriors that need to be considered. Over the past 20 years, significant progress has been made in reducing heat transfer through glazing assemblies (vision and spandrels); however this reduction may be causing colder localized exterior surface temperatures which, during wet winter precipitation events (wet snow, sleet, freezing rain, etc.), contribute to more frequent hazardous ice and snow accumulation at these locations. This paper examines a case of an existing high-rise building where ice and snow formation and accumulation was observed on both the vision and spandrel portions of the curtain wall system. It is believed that the level of thermal resistance of these assemblies contributed to ice formation and accumulation that otherwise would not have occurred to the same extent under the specific weather conditions present. A 3D finite element thermal model was developed for the case building curtain wall assembly to simulate the conditions that led to the observed ice formation and accumulation, using weather data representative of the site. Changes to the thermal resistance of the glazing and framing system were evaluated to identify what effects they have on the exterior surface temperatures and subsequently to ice formation and accumulation. Additional mechanisms, such as building shape and solar radiation are also discussed. INTRODUCTION Winter storms bring wind, snow, sleet, freezing mist and freezing rain to bear on the building envelope. This exposure creates performance challenges such as ice and snow buildup, that, if not anticipated and addressed, can create a hazard to people and property below if this ice and snow falls from the building. The most often reported incidents occur from mid- and high-rise buildings in populous areas where the heights of the buildings can lead to more noticeable damage and there are more witnesses to falling ice sheets. This typically coincides with significant public events or the daily commute in urban centers when there are more people at street level. 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 63 3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 64 Ice and snow formations on tall buildings are not a new phenomenon. In 1939 the New York Times reported on falling ice dropping off tall buildings in New York City, including the Empire State Building, after a series of particularly wet snowstorms (Barbanel, 2014). Increases in heavy precipitation that have been documented over the last decade have likely increased the number of icing events (U.S. EPA 2014). The growth in high-rise construction, population densification and weather changes have all increased the potential for hazardous falling ice incidents. In cases where injury or damage occurs, the incidents are often kept confidential by building owners to avoid unwanted attention. As a result, the frequency of falling ice events may not be evident to the design industry at large. Unfortunately, this frequency is difficult to quantify beyond anecdotal evidence and injury reports (Dobnik, 2014). Nevertheless, the trend appears to be rising based on media accounts and investigations by Northern Microclimate Inc. (Carter, 2012). These wet winter precipitation events, have been reported as far south as Fort Worth, TX and Atlanta, GA, indicating that this phenomenon is not unique to cold climates (Heinz, 2013). This leads to the question of what factors influence ice formation on buildings that are within our design control. While there are many environmental impacts, such as solar exposure, wind speeds and air temperatures; ice formation can also be affected by the building design itself. Modern architectural features and industry trends, such as solar shading devices, protruding sills and mullion caps can all increase surface area where ice and snow can accumulate (Stangl, 2014). One industry trend that may be overlooked, however, is the impact of improvements to the thermal performance of the building envelope. The hypothesis is that some of these improvements, while beneficial for reducing heat flow and energy costs, have had the unintended consequence of lowering exterior surface temperatures, thereby promoting an increase in ice and snow formation at those locations that can release and fall. PHOTO 1: FALLING ICE SIGNS A GRIM REMINDER OF DANGER ABOVE (STEINBERG, 2014) With the increasing need for energy efficiency in buildings, the construction industry has been moving towards improving the resistance to heat flow through the building envelope as a way of reducing the energy consumed by space heating. One area that has made significant progress in this regard is glazing assemblies. Although still generally far less insulating than opaque wall assemblies, the use of better reflective coatings, gas filled insulating glass units (IGU) and additional panes have all reduced heat flow through glazing units compared to those produced 15-20 years ago. The case study in this paper details a sleet/freezing rain weathering event in which ice formation and ice release was observed on several buildings in a dense metropolitan area. The study focuses on one of those buildings, a newly constructed high-rise, where ice accumulation on the envelope was witnessed on multiple occasions, including at the vision glazing. It is believed that the thermal resistance of the envelope, specifically the glazing, played a direct role in the buildup of this ice and snow. A 3D finite element thermal model of a typical glazed assembly from the case building was created to simulate and evaluate the influence of the thermal resistance of the assembly on the mechanisms present in the formation of ice during the weather event. The purpose of presenting this particular case is to raise awareness within the design community of the potential for ice and snow buildup due to the influence of building envelope assemblies and to promote further investigation. It is not intended to form an argument against striving for improved energy performance in buildings. 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 64 3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 65 BACKGROUND: ICE FORMATION ON THE BUILDING ENVELOPE Currently, building standards, such ASCE 7-10 – Minimum Design Loads for Buildings and Other Structures (ASCE, 2013), refer to freezing rain and atmospheric icing with respect to their impact on the design of “Ice-Sensitive Structures” (typically suspension bridges, communication towers, power lines, etc.). However, it should be realized that structures not classified as “Ice-Sensitive”, such as high-rise or large roof buildings, can still collect varying degrees of freezing rain or atmospheric icing. The collected ice can then become hazardous to people and property below when released. Predicting the potential for hazardous ice formation on a building envelope is difficult due to the variation in form, duration and intensity of precipitation. Contributing to this complexity are additional environmental influences of wind speed, wind direction, solar exposure and air temperature, along with the elevation, size, form, shape, texture and colour of the building design. These influences will not only affect the volume of ice or snow formation, but also determine the life cycle, transformation, and release of the buildup from the building facade. Regarding the interaction of winter precipitation with the building envelope, in general heavy PHOTO 2: SNOW BUILDUP ON SILLS OR LEDGES snowfall is most problematic for roofs, canopies, and other low slope features where snowfall can easily rest. However, it is less of an issue on vertical surfaces that work with gravity, such as windows, walls and street level signposts. In order for vertical (or nearly vertical) surfaces to exhibit problematic accumulation, specific types of precipitation need to occur. This includes wind driven wet snow, sleet, freezing rain, and other forms of atmospheric icing (i.e. in-cloud or high elevation icing, freezing mist, freezing fog, and hoarfrost) that can collect directly onto vertical and steeply-sloped surfaces of high-rise buildings. These formations can either freeze on contact to surfaces that are below 0oC (32oF), or melt on contact with warmer surfaces, then drain down the façade with gravity until reaching a surface with a temperature below the freezing point, causing re-freezing and ice formation. Variations in the atmosphere during a particular weather event (i.e., a storm driven temperature inversion, supercooled wind-driven droplets, etc.) affects the form of the precipitation, which in turn influences how easily the precipitation can adhere to surfaces. The types of winter precipitation that are most problematic for ice accumulation typically occur when air temperatures are at or just below 0oC (32oF). 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 65 3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 66 PHOTO 3: ICE ACCUMULATION IN MIDDLE OF GLAZING AT AN OBSTRUCTION PHOTO 4: ICE ACCUMULATION ON A GLAZING OVERHANG Examples of wet wind driven snow, sleet, and freezing rain adhering to cold building surfaces are shown in Photos 3 and 4. Photo 3 shows how precipitation can freeze at a location where an internal structure obstructs warm interior air flow in the vicinity of the glass. Photo 4 shows a glazing panel that extends from a vision section to a soffit. In this particular case, a freezing line is clearly evident where the glass bridges from the interior heated space to the unheated soffit space. From both these photos it is also apparent that melt water produced from the adhered and melted wet snow above has run down the glazing surface and refroze, forming a thicker ice mass. This ice mass is more likely to release from the glazing in a larger, more hazardous form once the skin temperature behind the ice climbs above the freezing mark. Thus, to create the particular condition discussed, a specific alignment of warm and cold exterior building temperatures, combined with air temperatures around 0oC (32oF) that promote wet winter precipitation, need to occur simultaneously. Of these, only the building skin temperatures have some degree of control through design and operation of the building. This is analyzed further through the following case study. CASE STUDY DETAILS In the winter of 2011, a major urban center on the east coast of the United States experienced a night of snow, freezing rain and sleet with morning fog and mist. In the morning, as air temperatures warmed to just above 0oC (32oF), falling ice from bridges and some of the taller buildings were being reported within the city. The reports continued for a 3-day period as further snow/sleet precipitation occurred and air temperatures fluctuated around 0oC (32oF). The case study building, a 700ft+ office tower, was one of the buildings that experienced issues with ice formation during this period. Falling ice and snow was reported from different portions of the building facade. Remarkably, it was specifically reported that ice sheets had formed in the middle of the vertical vision glazing, which is traditionally unusual for non-sloped glazing systems. From eye-witness accounts, ice formed in the center of the glass, then released and slid vertically from the façade. Falling ice sheets were numerous enough that spectators below could hear them hitting against neighboring buildings high up in the fog. 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 66 3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 67 The case building was relatively new at the time of the snow/sleet event and was noted for its energy efficient design, including a high thermal performance curtain wall system that comprises most of the building façade. Other influencing factors such as internal air temperatures, night time set-back strategies, local microclimate influences due to elevation and wind influences, etc., are unknown, limiting the accuracy of the case study results presented. CASE STUDY THERMAL MODELLING To determine if the exterior surface temperatures and thermal performance of the envelope played a role in the formation during the snow/sleet weathering event described above, a thermal model was developed for the curtain wall system (vision and spandrel). The purpose of this analysis was twofold: 1. To calibrate and compare the model to the real observed conditions to determine if the exterior surface temperatures of the curtain wall could have played a role in ice formation; 2. Once calibrated, to use the model to see how sensitive the potential of ice formation is to adjustments in the thermal resistance of the curtain wall system. The model was created using 3D heat transfer software from Siemens called Nx. The modeling procedures and software were extensively calibrated and validated as part of the ASHRAE 1365 research project, which analyzed various building systems for thermal performance (Morrison Hershfield, 2011). The curtain wall included both vision and insulated spandrel sections, representing one full floor height, as shown in Figure 1. The curtain wall was a unitized system with the vision and spandrel glazing held in place with 4-sided structural silicone. This configuration offers better thermal performance than pressure capped systems. The IGU was double glazed with a 0.04 low-e coating on surface 2 of the outer pane, and a fritting pattern installed using a window film. The IGU had a center of glass U-value of 1.7 W/m2K (0.30 Btu/ft2hr°F). Additional components for the assembly include 4” of mineral wool, equivalent to R-16.8 (2.96 RSI) in the backpan, polyamide thermal break extrusions and a suspended floor and ceiling, instead of a knee wall (pony wall). Other vision and spandrel glazing characteristics are comparable to systems used on similar newly constructed high-rise buildings in North American cold-climate cities. Altogether, the curtain wall system is considered a “good” thermally performing curtain wall system. 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 67 3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 68 FIGURE 1: MODELED CURTAIN WALL WITH VISION AND SPANDREL SECTIONS WITH A RAISED FLOOR Weather data was collected for the case building site from local weather stations during the 3-day period in which the snow/sleet event occurred. This included exterior air temperatures, solar irradiance, wind speeds and exterior relative humidity. Using this weather data, a transient analysis was performed on the system over the 3-day period, along with the following assumptions: • Interior temperature was 21°C. It was assumed the air was well mixed. • Most material properties were considered constant and taken at 21°C, except for aluminum and air, which were temperature dependent. • Sky radiation, (night sky and cloud cover) was included using the Stefan-Boltzmann law and Swinbank formula for long wave-radiation • Exterior air film coefficients were varied to match the collected wind speed data. • Interior air film coefficients were taken from ASHRAE Handbook of Fundamentals (2013). The model did not take into account wind speed and direction, specific interior heating distribution systems or the effects of latent heat. Latent heat will be absorbed or released during freezing and melting of ice but does not result in a temperature change. Snow buildup on the glass may also insulate the surface, changing air films and effectiveness of the low-e coating, however, the transient model was simulated at one hour time steps and it was assumed that these effects would be minor in comparison to the effects of changes in the exterior air temperature. 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 68 3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 69 CASE STUDY THERMAL MODELLING RESULTS The exterior surface temperatures for the vision glass and the spandrel glass were simulated over the 3-day period. It was assumed any surface below 0oC (32oF) was considered at risk for ice buildup from sleet/freezing rain. Figure 2 shows the simulated surface temperatures for the vision and glazing sections for surfaces below 0oC (32oF) for the first 16 hours of the recorded snow/sleet event. Each image in the sequence is centered on a section of the spandrel, with vision glazing shown above and below (similar to Figure 1). FIGURE 2: SURFACE TEMPERATURE PROFILES OF THE MODELED CURTAIN WALL VISION AND SPANDREL SECTIONS OVER THE FIRST 16 HOURS OF WETTING EVENT FROM THE CASE STUDY. Comparing the colour contours to the surface temperature scale in Figure 2 it can be seen that during the bulk of the snow/sleet event in the first 16 hours, the spandrel remains consistently below 0oC (32oF). While this makes it more likely that sleet/freezing rain could build on that surface, it also gives solar radiation an opportunity to melt that buildup from the outside. With the vision glass there is only a short period of time (3 hours) where the surface temperature is below freezing. In this case, sleet/freezing rain could stick to the vision glass, but then release as the surface temperature is raised back above the freezing mark, or melt and re-freeze on the colder portions of the spandrel. It is also worth noting that Figure 2 also shows the majority of the mullion framing is above 0oC (32oF) throughout the studied period, except for small areas at the center line of the mullion. Figure 3 shows the exterior air temperatures and the exterior surface temperatures for the center of the vision glass and center of the spandrel glass, along with the vision and spandrel frame temperatures. The center of glass is the likely location of the coldest surface temperatures since they are areas farthest from the effects of thermal bridging through the edge of glass and mullions. Note that these center of glass values do not 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 69 3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 70 directly indicate the size of the areas at that temperature, however it can be inferred that the colder the center of the glass value, the larger the area on the glass that is below 0oC (32oF). FIGURE 3: MODELLED EXTERIOR CENTER OF GLASS AND FRAME SURFACE TEMPERATURES FOR VISION AND SPANDREL SECTIONS COMPARED TO WITNESSED PRECIPITATION AND FALLING ICE From Figure 3, due to the conductivity and specific heat of the materials used in the assembly (mainly aluminum) there is no advantage of heat storage in the system when air temperatures are above 0oC (32oF). The vision and spandrel center of glass temperatures follow the exterior air temperature, only transposed higher with minimal lag in response. Night sky radiation could also increases the risk of exterior frosting from condensation by making exterior surfaces colder than the surrounding air temperatures, however due to the cloud cover during the case weather event, the effects of night sky radiation was minimal. The simulated surface temperatures do not drop below the air temperature, which indicates that the ice buildup for this case is a result of precipitation, and likely not from condensation. However, this mechanism should not be discounted in general, as the conditions for exterior frosting from condensation may occur during cool days with clear night skies. Note, however, that due to cloud cover during precipitation events, it is highly unlikely ice formation from both precipitation and condensation could occur at the same time. These results were compared to the timeline of events for the case study period, which included when falling ice was observed onsite; witness interviews and media reports (also shown in timeline in Figure 3). Between approximately 3:00am to 6:00am on Tuesday morning, the meteorological reports recorded precipitation containing freezing rain and or other wet winter precipitation, during which the 3D Thermal Modelling results predicted that the vision glass surface temperature would dip below 0oC (32oF). Subsequently, falling ice sheets were reported during the early morning after 7:00am and coincided with raising air temperatures. This matched with increased vision glass surface temperatures above 0oC (32oF) shown in the 3D model. 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 70 3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 71 Finally, the 3D model results indicate that the spandrel segments of the curtain wall glazing did not rise above the freezing mark until approximately 8:00am on Wednesday, when further falling ice was witnessed and reported. Overall, this indicates that the model was generally able to capture the icing event. MODEL SENSITIVITY TO SURFACE TEMPERATURES Knowing that the model was able to simulate similar conditions as those seen in the case study, several aspects of the curtain wall system were modified in the model to examine the sensitivity of the thermal performance of the assembly on the exterior surface temperatures of the vision and spandrel glass. This analysis helps establish the strength of the connection between the envelope thermal performance and ice formation. Two aspects in the model were adjusted for analysis: the IGU performance and the frame/insulation performance. The scenarios are described in the following sections and the results are further analyzed in the Discussion portion of the paper. Figure 3 shows that, while there was no precipitation, Thursday had the largest fluctuations in air temperatures around 0oC (32oF). As such, Thursday’s exterior conditions were used as the basis of the sensitivity analysis as a worst case scenario had there been sleet/snow precipitation. IGU Performance Coatings on glazing units have steadily improved since the introduction of low-emissivity coatings and gas fills (Wilson, 2012). These coatings reduce the radiative heat transfer through the glazing depending on the emissivity and placement of the coating within the IGU, while gas fills can greatly reduce the conductive heat flow through the gap between glass panes. For the base system, the glazing had a 0.04 low-e coating on surface #2 with a 15mm airspace. Three adjustments to the base system were tested: 1) The coating emissivity was increased to 0.20; 2) The low-e coating was removed; 3) The IGU gap was filled with a 90% Argon gas mixture and a 0.04 low-e on surface #2. The surface temperatures for these scenarios are shown in Table 1. TABLE 1: EXTERIOR SURFACE TEMPERATURE RESULTS FOR VARIOUS GLAZING COATING CHANGES 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 71 3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 72 Frame/Insulation Performance With spandrel panels, designers often attempt to increase the thermal performance by increasing the amount of insulation located in the spandrel backpan. Unfortunately, due to the amount of thermal bridging from the mullions that bypass the insulation, the overall thermal resistance through the system is often not significantly impacted. For the case system, there was 4” of mineral wool insulation with additional insulation wrapped at the mullions, and large thermal breaks. For this sensitivity analysis, two adjustments were made to the curtain wall system: 1) The amount of backpan insulation was reduced to 2” of mineral wool and the mullion wrap was removed, 2) The thermal breaks were reduced in size and made of a more thermally conductive material. The surface temperatures are shown in Table 2. TABLE 2: EXTERIOR SURFACE TEMPERATURE RESULTS FOR VARIOUS INSULATION CHANGES DISCUSSION From the sensitivity analysis in Table 1, there is an argument to be made that increasing the thermal performance of the IGU’s in the vision portions of the curtain wall will decrease exterior surface temperatures and contribute to ice formation. However, from Tables 1 and 2, the center of the spandrel surface temperatures remained unaffected by the changes to the glazing, insulation levels and thermal breaks. Note that these were center of glass surface temperatures, and the decreases to the insulation and thermal breaks showed exterior surface temperature increases at the mullions, but not in the field of the spandrel. This will result in less surface area of the glass below 0oC (32oF) available for icing due to the edge effects, depending on the spacing of the mullions. There is still a potential that the ice thickness could increase at the transition between the warmer and colder areas of the glass, as shown in Photographs 3 and 4. For the vision glazing, with a less effective low-e or no low-e coatings, the exterior center of glass surface temperature is increased up to over a degree. While this may seem small, the ice formation observed in this case study occurs with specific types of winter precipitation, (ie. Wet wind driven snow, sleet, and freezing rain), which typically occur within tight temperature ranges at or just below freezing. Small increase in surface temperatures may be enough to reduce the size of ice formations, significantly lowering their potential to be hazardous. This supports the idea of why previously, before low-e coatings under 0.20, ice buildup on the envelope may not have been as prevalent. 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 72 3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 73 Using argon gas fill for windows instead of air can greatly decrease heat flow through IGU’s, and are becoming more common as prices decrease. From Table 1 it can be seen that the heat reduction from using Argon gas can also greatly reduce the exterior surface temperatures. As these gas fills for IGU’s become more standard on projects, the icing problem could potentially increase. With a large amount of new high-rises being built in cold climate cities like Toronto, Chicago, Boston, and New York, the question may be asked, why is this not happening on other buildings? As discussed in the introduction, it very well may be happening on many other buildings, however it may not be as noticed or there are factors that could play a significant mitigation role in other buildings. It is important to note that, while there has been a large amount of new construction in these cold climate cities, there is still a significant amount of older building stock that have glazing over 20 years old. It is unlikely that the glazing in these buildings have been replaced unless they have gone through a major retrofit. First Canadian Place, built in 1975 before the introduction of low-e, had only 30% of its windows replaced by 2010 (Davey, 2010). As a result, there are many tall buildings which still have substantial heat loss through the glazing, keeping the exterior surface temperatures warmer. While it is likely the IGU performance did play a role in the ice formation on the case building, there are many other factors that can also contribute to or oppose ice formation on other buildings that could be explored with further study. One major influence on ice formation on glazing may be the layout and design of the HVAC systems in use, specifically the method and location of supply of the warm air to the interior. Any system that does not provide or is prevented from providing a uniform heat and air distribution risks cold spots or delayed response to temperature fluctuations at the window exterior. Natural convection heaters (like electric baseboards) placed at the base of windows will warm the glass and cause the hot air to rise and cool as it touches the window, creating a temperature gradient along the surface. This will impact the freezing pattern on the exterior of the glass. Forced air systems distribute the air through diffusers and returns. This is often helpful in cold climates to prevent interior condensation, however can be obstructed by desks or partitions, creating cold pockets. Considering trends in architecture, many modern designs include highly sloped walls, wing walls or double facades along with features such as protruding mullion caps, fins or architectural screens not typically seen in previous construction. These design elements all increase the amount of cold surfaces on which ice and snow can form. In contrast, older designs were less angular and more likely to be constructed with stone or simple metal window sills. Older high-rise towers using concrete or other masonry can retain heat stored from sunlight, or from the building interior, which could assist in maintaining warmer surface temperatures during a sleet/freezing rain event. As a further avenue of study, it would be interesting to observe if metal cladding assemblies are also experiencing issues with ice formation as they are often higher in thermal resistance than spandrels and also have a smooth exterior exposed to ice and snow. The height of buildings and local density could also explain why some buildings may experience more icing events than others. If the building is taller than the surrounding buildings, the upper floors are exposed to greater amounts of wind driven precipitation, with higher wind speeds and colder air temperatures than that experienced at grade level. Lower buildings are more sheltered from air movement and can benefit from higher local air temperatures due to the density of the buildings and human activities at street level, otherwise known as the heat island effect. During low hanging freezing mists, these lower buildings may still be susceptible. 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 73 3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 74 A major environmental influence on ice formation is solar radiation. Depending on the cloud cover during a precipitation event, solar radiation can help keep a surface heated. Buildings that are oriented to take advantage of more sunlight during the winter may have less of an issue. With ice exposed to solar radiation, ice formation has a higher likelihood of melting from the outside, resulting in a more gradual melt and reduced potential of the ice releasing off the glass in sheets. Regarding the influence of wind, without strong winds, it is less likely precipitation will be driven onto the cold surfaces of the building. PHOTO 5: EXTERIOR CONDENSATION ON A BUILDING FACADE For this cast study, frost from condensation was not an issue. The possibility of ice formation due to condensation under some circumstances cannot be fully discounted, particularly if exterior skin temperatures might be colder with newer glazing systems, placing them more frequently below the ambient dew point temperature when there is night sky radiation. Photo 6 shows condensation and frost formation on the exterior of a triple glazed system and metal panels. Exterior frost does not typically lead directly to falling ice, however it does provide a colder surface for sleet and freezing rain to adhere to if a precipitation event follows a night of clear skies. While not a feature in this case study, there is “self-cleaning” glass available from glazing manufacturers which create a more hydrophobic or hydrophilic surface. These exterior coatings change the surface tension and can affect precipitation in different ways, which may also affect how ice forms on glass, however to the best of our knowledge this has not been investigated or studied. While mainly for specialized glazing, such as air traffic control towers or high-end penthouses, if this becomes more prevalent in the building industry for high-rises, further research may be warranted. CONCLUSIONS With so many competing influences at play on the building envelope, it is difficult to pinpoint the direct cause of ice formation for a specific event. For the case building, it was likely a combination of several factors, including the thermal resistance of the IGU, that lead to the ice formation and release. The modelling has shown in general that restricting heat flow, particularly through the IGU, can produce colder exterior surface temperatures that can contribute towards ice formation. In the end, the intent of this paper is not to state that thermal performance of the envelope of mid- and high-rise buildings is a leading cause of ice formation, but it should be considered alongside other factors in design. In terms of what can be done in design, reducing envelope thermal resistance is not desirable (no one wants to go back to not using low-e coatings or gas fills). It may have to be accepted that, under certain weather conditions, new highly glazed mid- and high-rise buildings may be at higher risk for ice formation. It is possible that the implementation or avoidance of certain design geometries, HVAC strategies, or curtain wall configurations in some geographic regions, could significantly reduce the potential for the localized melt and refreeze of wet winter precipitation on façade surfaces, such as the vision glass, that can be responsible for the most hazardous ice build-up and release. During the early design phase, options such as podiums, canopies, or alternate building geometry over critical pedestrian or accessible areas should be considered. Alternatively, it may be possible and become necessary to reduce icing potential through timely building operational strategies that would avoid night time set-backs prior to storm events or temporarily increasing internal air temperatures during wet precipitation events. 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 74 3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 75 Given the increases in severe winter weather over the last decade, future icing events are likely to be more frequent and prolonged, increasing the potential for hazard. It is important to note that this paper analyzes one occurrence of this phenomenon; however, there is a need to review additional buildings and other documented icing events to achieve a better understanding of factors that can lead to formation and release of ice and snow from the building envelope. The analysis of wind patterns, the influence of neighboring structures in close proximity to a facade, building orientation, and other complicating factors were outside the scope of this paper, but may have also played a significant role at the study building. Given the increasing magnitude of the surface areas of high-rise buildings and the hazards that are associated with falling ice, this is an issue that warrants further attention. REFERENCES Barbanel, J., 2014, Feb 21st. Falling Ice Cracks New York City’s Cool Façade. The Wall Street. Available from http://online.wsj.com/news/articles/SB10001424052702304914204579397560926141546 [accessed March 1st, 2014]. U.S. Environmental Protection Agency, 2014. Climate Change Indicators in the United States. 3rd Ed. EPA 430-R-14-004. Available from www.epa.gov/climatechange/indicators [accessed July 28th, 2014] Steinberg, N., 2014, Jan 11th. Falling Ice Signs a Grim Reminder of Danger Above. Chicago Sun-Times. Available from http://www.suntimes.com/news/steinberg/24877447-452/falling-ice-signs-a-grimreminder-of-danger-above.html#.U_JQKWP91jN [accessed August 18th, 2014] Dobnik, V., 2014, Feb 19th. Tough Winter Creates Menace: Ice from High-Rises. Associated Press. Available from http://news.yahoo.com/tough-winter-creates-menace-ice-high-rises-071130052— finance.html [accessed February 25th, 2014]. Carter, M. & Stangl, R., 2012. Increasing Problems of Falling Ice and Snow on Modern Tall Buildings. Council on Tall Buildings and Urban Habitat (CTBUH) International Journal, Issue IV. Heinz, F., 2013, Dec 9th. Viewer Videos a Stark Reminder to Beware of Falling Ice. NBC 5 –KXAS. Available from http://www.nbcdfw.com/news/local/Viewer-Videos-a-Stark-Reminder-to-Beware-ofFalling-Ice-235079361.html [accessed August 18, 2014]. Stangl, R., Carter, M. 2014 Should Tall Buildings in Cold Climates be Designed Specifically to Stop Falling Ice? – Debating Tall. Council on Tall Buildings and Urban Habitat (CTBUH) International Journal, Issue II. ASCE, 2013, Minimum Design Loads for Buildings and Other Structures (Standards ASCE/SEI 7-10) Third Printing. American Society of Civil Engineers Morrison Hershfield Ltd., 2011. ASHRAE 1365-RP Thermal Performance of Building Envelope Construction Details for Mid- and High-Rise Buildings. ASHRAE, Atlanta, GA. Wilson, A., 2012, Mar 27th. Window Performance – the Magic of Low-e Coatings. Building Green.Com. Available from http://www2.buildinggreen.com/blogs/window-performance-magic-low-e-coatings [accessed March 1st, 2014]. Davey, M., 2010, July. Icon Reborn. Glass Canada, July August 2010. Available from http://www.glasscanadamag.com/content/view/1632/132/ [accessed July 15th, 2014] 14TH CANADIAN CONFERENCE ON BUILDING SCIENCE AND TECHNOLOGY 75