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THE IMPLICATION OF ENERGY EFFICIENT BUILDING ENVELOPE DETAILS FOR ICE AND SNOW FORMATION PATTERNS ON BUILDINGS

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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
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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. 63 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 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. 64 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. PHOTO 2: SNOW BUILDUP ON SILLS OR LEDGES 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 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 0 o C (32 o F), 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 0 o C (32 o F). 65 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 0 o C (32 o F) 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 0 o C (32 o F), 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 0 o C (32 o F). 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. 66 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/m 2 K (0.30 Btu/ft 2 hr 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. 67 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. 68 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 0 o C (32 o F) 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 0 o C (32 o F) 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 0 o C (32 o F). 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 0 o C (32 o F) 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 69 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 0 o C (32 o F). 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 0 o C (32 o F). 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 res
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