the implication of energy efficient building envelope details...

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14TH CANAD IAN CONFERENCE ON BU I LD ING S C I ENCE AND T E CHNOLOGY

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 high-

rises 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.

3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM Page 63

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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.

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.

PHOTO 1: FALLING ICE SIGNS A

GRIM REMINDER OF DANGER

ABOVE (STEINBERG, 2014)


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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

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).

PHOTO 2: SNOW BUILDUP ON

SILLS OR LEDGES


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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.

PHOTO 3: ICE ACCUMULATION IN

MIDDLE OF GLAZING AT AN

OBSTRUCTION

PHOTO 4: ICE ACCUMULATION ON

A GLAZING OVERHANG


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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.


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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.

3 CCBST 2014 Proceedings Book_v10 B1 37-76_Layout 1 14-10-17 3:45 PM 8

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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


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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.


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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


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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.


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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.


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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.

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.

PHOTO 5: EXTERIOR

CONDENSATION ON A

BUILDING FACADE


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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. Availablefrom http://online.wsj.com/news/articles/SB10001424052702304914204579397560926141546 [accessedMarch 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-grim-reminder-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-of-Falling-Ice-235079361.html [accessed August 18, 2014].Stangl, R., Carter, M. 2014 Should Tall Buildings in Cold Climates be Designed Specifically to StopFalling Ice? – Debating Tall. Council on Tall Buildings and Urban Habitat (CTBUH) InternationalJournal, Issue II.ASCE, 2013, Minimum Design Loads for Buildings and Other Structures (Standards ASCE/SEI 7-10)Third Printing. American Society of Civil EngineersMorrison Hershfield Ltd., 2011. ASHRAE 1365-RP Thermal Performance of Building EnvelopeConstruction 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 fromhttp://www.glasscanadamag.com/content/view/1632/132/ [accessed July 15th, 2014]


the implication of energy efficient building envelope details...

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