Indoor and Built Environment http://ibe.sagepub.com/

Overall Environmental Modelling of Newly Designed Curtain Wall Façade Configurations Gon Kim, Hong Soo Lim, Laura Schaefer and Jeong Tai Kim Indoor and Built Environment published online 7 December 2012 DOI: 10.1177/1420326X12470281 The online version of this article can be found at: http://ibe.sagepub.com/content/early/2012/12/04/1420326X12470281

Published by: http://www.sagepublications.com

On behalf of:

International Society of the Built Environment

Additional services and information for Indoor and Built Environment can be found at: Email Alerts: http://ibe.sagepub.com/cgi/alerts Subscriptions: http://ibe.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav

>> OnlineFirst Version of Record - Dec 7, 2012 What is This?

Downloaded from ibe.sagepub.com by guest on February 1, 2013

Original Paper

Indoor and Built Environment

Indoor Built Environ 2012;000;000:1–12

Accepted: November 7, 2012

Overall Environmental Modelling of Newly Designed Curtain Wall Fac¸ade Configurations Gon Kima Hong Soo Limb Laura Schaeferc Jeong Tai Kima a Department of Architectural Engineering, Kyung Hee University, Yongin, Gyeonggi-do, Republic of Korea b Department of Architectural Engineering, Kangwon National University, Chuncheon, Gangwon-do, Republic of Korea c Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, USA

Key Words Double-skin E Fac¸ade design E Renovation E Sustainable building E Building energy

Abstract There has been a rapidly growing interest among contemporary architects in recent years in the innovative use of curtain walls as a building enclosure. A curtain wall is a glass and metal skin that wraps the entire building in a continuous, transparent layer. This modern design provides a relatively desirable environment for the occupants. The glass fac¸ade allows light and diffused views into the surroundings during the day and can become a glowing and aesthetically pleasing aspect of the skyline at night. The key visual features of curtain walls are the glazing appearance and sightlines, but the window selection plays a key role in determining the overall building’s environmental performance. The complex layering of glass should be strategic as well as artistic, balancing light and shade to ß The Author(s), 2012. Reprints and permissions: http://www.sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1420326X12470281 Accessible online at http://ibe.sagepub.com Figures 1–3 and 5–13 appears in colour online

optimize light and minimize heat gain. Recently, a series of curtain wall system have been developed with an emphasis on the technical issues, such as structural stability, energy efficiency, sustainable strategies, daylighting, and materials. This design case project explores in depth the overall environmental performance of the leading-edge curtain wall assemblies using computer programs such as Virtual Environment and Radiance, which allow estimation of total U-values and helps predict daylighting performance.

Introduction A curtain wall system is a specialized type of cladding with intensively-designed glazing typically used in the construction of modern buildings. However, this special type of building envelope requires an overall understanding of the design criteria as they relate to both the Jeong Tai Kim, Department of Architectural Engineering, Kyung Hee University, Yongin, Gyeonggi-do 446-701, Republic of Korea, Tel. þ82-31-201-2539, Fax þ82-31206-2109, E-Mail [email protected]

Downloaded from ibe.sagepub.com by guest on February 1, 2013

structural stability and the interaction with and filtering of the environment. A curtain wall system with glazing materials (which offer a wide range of transparency) affects every aspect of the environmental interaction of the building such as daylight apertures, the thermal resistivity of the materials, the shading devices, and glare control. Allowing too much daylight into the interior through very large openings such as a curtain wall will expose occupants to excessive brightness differences that will result in poor visibility and visual discomfort [1]. In addition, architectural and optical solutions both to attenuate the harmfulness of sunlight and to improve the visual satisfaction should be issued and their performance is reviewed [2]. Outdoor shades in curtain walls are commonly used to intercept the sunlight before reaching the building interior, which also reduces heat gains in the form of direct beam radiation, particularly in summer. The target illuminance value can be set for specific tasks in buildings, and the difference between the standard and actual illuminance levels can then be converted to either an excessive or needed energy [3]. Solar radiation and the outdoor temperature are the key factors that influence the optical and thermal conditions in curtain wall design via thermal gains/losses and incoming light. It is important to address the role of shading devices on daylight availability, visual comfort, total energy use, peak electric demand, and energy cost savings in a building, preventing unwanted solar gains in summer [4]. There are multiple studies, both computational and experimental, that have examined these effects, as discussed below. Richman and Pressnail [5] developed a numerical model of a solar dynamic buffer zone (SDBZ) within a curtain wall system. It was demonstrated that this system was effective in reducing building heating energy consumption. The SDBZ curtain wall system was shown to be an innovative tool to collect solar energy that would otherwise be lost to the exterior, while using existing components within the curtain wall. Lim and Kim [6,7] developed four different geometries of shading devices to improve both indoor natural illuminance and open views for the visual environment based on computer simulation analysis. Offiong and Ukpoho [8] evaluated two external shading treatments for controlling the thermal conditions in a building: an external overhang and internal vertical side fins. Also Tzempelikos and Athientis [9] focused on the interrelated impacts of the glazing area, shading device properties, and shading control on cooling and lighting needs. Venetian blinds were studied, and the above properties were

2

quantified as a function of rotation angle, taking into account slat thickness. Dubois [10] used the Radiance program to simulate the impact of seven different shading devices on daylight quality at different times of day. Tsangrassoulis et al. [11] used a genetic algorithm to develop design tools for slattype blinds, based on the relative light intensity distribution under a uniform light source for good sun-shading properties. Loutzenhiser et al. [12] noted that building energy simulation programs can be valuable tools in the design phase of a new building, since they can be used to assess potential daylight control savings by performing parametric studies of varying window sizes and materials and shading devices to optimize the energy performance of a building. Kim [13] presented an alternative to a high-performance glass wall through a transparent composite fac¸ade system. A composite core acts as a shading device while airspace between the polymer skins provides insulation. Kim characterized the environmental performance of the system both through the energy consumption and the life cycle CO2 emissions [13]. Yun et al. [14] extended the understanding of window use patterns and control behaviour which is of importance in creating comfortable, energy-efficient, and healthy indoor environment. Yin et al. [15] used eQuest to simulate the energy savings from applying solar window films in a commercial building in Shanghai, China, with large curtain wall areas. It was found that the position of the installed window film and the configuration of the original glazing system were the two most significant factors. Using the film reduced the shading coefficient and solar heat gain coefficient by 44% and 22% for application to the outside and inside, respectively. Finally, Shih and Huang [16] used computer-aided visualization to determine the influence of reflected sunbeams from glass curtain wall buildings. The boundary of the reflection area was used as a performance index for evaluating the level of glare. Tilting walls or rotating plans were simulated to evaluate how the tilted angles or orientation of the fac¸ade would affect reflected glare. In recent years, low energy and environmentallyfriendly architecture has been an area of increasing interest in South Korea. There are various standards and laws for encouraging green technologies in residential buildings, but for office buildings, there are only set point temperature standards to save heating and cooling loads. It should be noted that, generally, a curtain wall makes a building shape thicker and narrows the perimeter space, attenuating daylighting efficiency.

Indoor Built Environ 2012;000:1–12

Downloaded from ibe.sagepub.com by guest on February 1, 2013

Kim et al.

This paper evaluates the overall environmental performance of newly-designed types of curtain wall systems for the Seoul, Korea. It reviews the design criteria for curtain walls and gives their performance in terms of illuminance, uniformity, cooling load, and energy. For the prototype geometry of a unit floor in a commercial building, the primary variables of interest for the environmental control performance are the glass structure and venetian blinds. A daylighting analysis program, Radiance, is used to evaluate the visual environment, and a building energy analysis program, Virtual Environment, simulates the thermal environment of an office building with the curtain wall system. After the various configurations have been analyzed, potential future directions for development of configurations to facilitate wider acceptance of built-in shading devices are suggested.

Design of a Curtain Wall System Environmental Considerations The environmental performance of a glazed envelope is one of the most important factors in the determination of energy consumption in modern buildings. It is known that overall curtain wall thermal performance is a function of the glazing infill panel, the frame, the construction behind the opaque areas, and the perimeter details [17]. Rapone and Saro [18] used a PSO algorithm to conduct an automated search aimed at finding the optimal values of the curtain wall envelope features and found that the features varied in importance based on different orientations of the fac¸ade. In a curtain wall, the complex layering of glass should be strategic as well as artistic, balancing light and shade to optimize light and minimize heat gain. Specifically, a fenestration design with thermal performance requirements must be integrated with the building’s heating and cooling systems. The primary variables that affect the thermal performance are the solar energy transmittance through the glazing, the reflectance of the glazing, the width of the air space, the material properties, and the location and design of the spacers [19]. The function of the shading elements is to control the transmission of direct sunlight, and, thus, to reduce the cooling load (or prevent overheating) while still maintaining enough daylight in the space. The choice of glazing is therefore a key factor in determining the environmental performance of a building. Almost all curtain walls used in commercial buildings incorporate insulating glazing, both for the improved energy efficiency and the level of visual comfort [19].

Environmental Modeling of Curtain Wall Fac¸ade

Furthermore, when the glass is covered with a reflective or low-emissivity coating, both the aesthetics and the thermal performance (by reflecting visible light and infrared radiation) of the glass are improved. In addition to the individual glazed panel performance, a well-designed structure of glazed layers promises better environmental resistance in the design of curtain walls. Usually, a curtain wall consists of two or more panes of glass with a continuous spacer. The air space further reduces heat gain or loss, which gives the glass wall unit a superior thermal performance. Most commercial windows, curtain walls, and skylights contain multilayered glass units. The condensation resistance of a curtain wall is also a concern for the environmental comfort of building occupants. The thermal performance of glazing materials can affect indoor air quality, allowing water leakage or condensation. Glazing strategies for eliminating condensation include insulating glazing and providing supplemental heat to the glazing to increase surface temperatures. Many researchers have tried to predict the surface temperature of windows or curtain walls precisely. In a recent work by No et al. [20], the correlation between a computer simulation and a mock-up test (using a large-scale thermal chamber for the curtain walls) was studied. A method was proposed to adjust the simulation result to match the test result by changing the convective film coefficients of the frame and glazing to find the optimal convective film coefficients for the chamber [20]. Finally, in spite of their many positive aspects, UV rays remain very harmful. A larger glass area for windows can result in a harmful level of UV light, which poses a threat not only to occupants’ visual quality of life but also to their health. Methods to provide UV protection include providing laminated glazing, certain applied films, or curtains and shades. Laminated glass can filter out more than 95% of the UV radiation [21,22]. Design of the Curtain Wall Layering In designing a curtain wall, the most important consideration is the complex layering and perimeter choices of different materials such as glass, aluminium, sealant, and breaks. To reduce heat loss and prevent condensation in cold seasons or to minimize heat gain in hot weather, reduction of the overall U-value of the wall is usually a good long-term investment. The perimeter of the glass typically has a higher U-value than the centre due to the higher rate of heat transmission through the spacer. This means that heat loss and condensation problems will almost always occur near the perimeter of the glazing.

Indoor Built Environ 2012;000:1–12

Downloaded from ibe.sagepub.com by guest on February 1, 2013

3

Similarly, the curtain wall framing will also have a different U-value and encounter the same issues [19]. Aluminium has a very high thermal conductivity; considerably higher than glazing materials. This translates into high heat loss through aluminium curtain wall mullions. One approach to correct this problem is to replace the metal with a design that uses materials that are better insulating. The most common approach is the addition of thermal breaks of low conductivity materials, traditionally Neoprene rubber, polyurethane, and, more recently, polyester-reinforced nylon, or polyamide for improved thermal performance. The curtain wall layer frequently includes gaskets. The gaskets provide both thermal breaks and acoustic isolation. Deeper thermal breaks (greater than 6.25 mm) can improve the thermal performance and condensation resistance of the system. Some curtain wall systems address the problem of condensation by adding gutters to collect and drain the condensate from spandrel areas to the exterior. Proper placement of insulation at the curtain wall perimeter can reduce the need for these specialized condensation treatments (as well as reducing energy loss). Another approach to reducing heat loss and sound transmission has been to replace the metal spacer with one that is less conductive and/or to change the cross-sectional shape of the spacer. As new highly insulating multiplelayer windows are developed, an improved edge spacer becomes an ever more important element [19]. In addition to condensation and heat transfer considerations, there are also indoor environmental air quality issues to be considered. Fixed curtain wall glazing, meaning there is no access to the exterior of the building except through doors, can cause a variety of indoor environmental problems. It must be stressed that nearly any type of operable windows or vents can be glazed into the curtain wall system as well, to provide the required ventilation. The ventilation performance is primarily dependent on the width of the cavity space, the fac¸ade height, and the configuration of the air inlet and outlet. Finally, the structural impact of the curtain wall must be analyzed. Because of the relatively light weight of the materials used, the curtain wall fac¸ade does not carry any dead load weight from the building. Gravity forces affecting structural design are generally small in comparison with those imposed by wind action. The horizontal wind loads on the curtain wall are transferred to the main building structure through connections at the floors or columns of the building. Additionally, while the thermal breaks decrease the thermal conductivity of the curtain

4

wall, they also interrupt the aluminium mullion, so that the overall moment of inertia of the mullion is reduced. Based on all of the above considerations, the basic configuration of curtain wall has been developed consisting of two panes of pair-glass, equipped with built-in venetian blinds to formulate a differentiated glazing function, as shown in Figure 1. The curtain wall configuration, in terms of daylighting, is specialized with the venetian blind between glass panes, delivering the daylight into the deep interior space by adjusting the slat angle. The control strategies for excessive direct sunlight should be emphasized in the development of the thermal capability of the curtain wall configuration. In case of the developed curtain wall system, only the wind load for the fac¸ade was considered. It was assumed that the wind speed was 30 m s1 and the roughness was B, which means somewhat severe ground surface conditions around the target building. As a result, the calculated deflection was 97% of the allowable figure and 75% for the moment. A total of three different sizes of the glass, which are 1,500 mm  4,000 mm, 1,500 mm  2,800 mm, and 1,500 mm  1,200 mm have been applied to the curtain wall.

Daylighting Properties of the Curtain Wall Design of the Daylighting Analysis Daylighting modelling of the building interior was completed using Radiance, a computer software package developed by the Lighting Systems Research group at Lawrence Berkeley Laboratory. Radiance can help establish a reasonable estimate for the intensity and distribution of daylight with various slat adjustments within the curtain wall system. Radiance is a research tool for accurately calculating and predicting the visible radiation in a space based on the ray tracing technique. The program uses three-dimensional geometric models as input, to generate spectral radiance values in the form of photo-realistic images. However, Radiance has many additional functions beyond just serving as a photo-realistic renderer [23]. By using accurate inputs into the program, as shown in Figure 2, such as manufacturers’ photometric data for specific lighting fixtures, designers are able to confidently evaluate their designs by both visualized images and photometric data. For the purpose of predicting the performance of the curtain wall, a prototype plan for a commercial building has been established. As shown in Figure 3, the unit floor

Indoor Built Environ 2012;000:1–12

Downloaded from ibe.sagepub.com by guest on February 1, 2013

Kim et al.

Fig. 1. Developed model of a curtain wall system.

Fig. 2. Example model of Radiance.

consists of a total of four different zones, and daylighting performance is individually analyzed for each zone, since there are entirely different potentials for each configuration. The floor plan is a rectangular shape with glazed curtain walls, of which more than 80% is in the exterior area. Regardless of the orientation of the curtain walls, a series of horizontal shading device has been placed. The result is given in terms of the Daylight Factor and illuminance level with different slat adjustments. The Daylight Factor is a convenient index to represent the performance characteristics and to predict the potential light level of a daylighting design. It can be also used to evaluate the performance of daylighting design alternatives, since the value of the daylight factor is altered by any

change in architectural parameters, such as window dimensions or height above the working plane, ceiling height, surface reflectance, ground reflection, and obstructions. Based on the literature review in the previous chapter, Table 1 presents the outline of the experimental design with key assumptions and simulated ranges of the research variables. Five significant aspects of the simulation must be considered: solar transmittance of glazing, solar reflectance of the interior, orientation of openings, time, and sky condition. The optical properties of the glazed materials impact the characteristics of the solar transmittance, which determines the ratio of sunlight influx and the dispersion of daylight in the space. The transmittance of the curtain wall was set as 80%, which matches real-world

Environmental Modeling of Curtain Wall Fac¸ade

Indoor Built Environ 2012;000:1–12

Downloaded from ibe.sagepub.com by guest on February 1, 2013

5

Fig. 3. Model space module in an office building.

Table 1. Input data for Radiance simulation Building type Surface reflectance

Glazing transmittance Orientation Time Sky condition

Office building in Seoul Wall 50% Floor 40% Ceiling 80% Blind 80% 50% West and South March 21, June 21, December 21 (noon) CIE (International Commission Illumination) Clear and overcast sky

observations. To avoid over estimating, the interior reflectance was assumed as the lower than normal. In defining the sky condition, the position of the sun in the sky can be described by its altitude angle and its azimuth angle, and depends on the orientation of the openings and time. In order to realistically evaluate the equipped blinds system between the glass panes, two standard sky conditions, diffuse overcast and clear sky, were simulated by Radiance. This enables the determination of which types of slat configuration should be primarily considered as a representative solution for daylighting and energy performance. As shown in Figure 4, three different configurations of the blind slats have been applied to control direct sun light. Daylighting Performance of the Curtain Wall Systems When glass is used as the curtain wall, a great advantage is that natural light can penetrate deeper within the building. Figure 5 shows the seasonal values of the Daylight Factor at different zones under an overcast

6

sky condition. The red dotted line is the recommended minimum level of Daylight Factor by both LEED and BRE. Except for zone 1, which is much higher, the minimum level of Daylight Factor for all zones in the spring and fall is higher than the criteria by at least 2%. In the spring, most of the direct sunlight cannot penetrate into the space, whereas diffuse daylight primarily contributes to the illumination. Illuminance with the shading devices installed was acquired under CIE clear sky. As shown in Figure 6, the curtain wall equipped shading devices block direct sunlight in the spring, summer and winter, which leads to illuminance levels ranging from 1,000 lux to 1,500 lux. However, the curtain wall is not able to block the direct sun as effectively in the winter due to the lower position of the sun. Although the shading device can reduce the heating load, the resulting non-uniform illuminance distribution causes problems such as glare or visual discomfort. To determine the optimal configuration of the blinds to control direct sun in the curtain wall, three different types of slat, as shown in Figure 4, were simulated. Figure 7 shows the seasonal Daylight Factor with the different slat configurations. The figure also shows the relation between the illuminance level and the distance from the window, indicating that the type B slats could best direct the reflected component of the sunlight to the rear of the space. As shown in Figure 8, the analysis of hourly illuminance for spring shows that the shading device delivered direct sunlight into the indoor space when the solar angle became lower in the early morning and late afternoon. This means that horizontal shading devices cannot effectively control sunlight except during the summer time.

Indoor Built Environ 2012;000:1–12

Downloaded from ibe.sagepub.com by guest on February 1, 2013

Kim et al.

(a)

(b)

(c)

Fig. 4. Different configurations of the blind slats. (a) Type A, (b) type B, (c) type C.

Fig. 5. Seasonal values of daylight factor at different zones.

Fig. 6. Seasonal illuminance at different zones.

In terms of visual comfort, venetian blinds can reduce glare and provide a view to the outdoors, by adjusting slat angles. Figure 9 shows the tendency of illuminance with different slat angles in the summer and winter. The result shows that slat angles allow daylight to be transmitted with venetian blinds.

By adjusting slat angles, daylighting performance is changed in the rear space. Illuminance in the rear space exceeds 135 lux with the slat angles from 808 to 808 owing to direct sunlight. As a result, in terms of light level and uniformity, a blind slat angle of 208 might be the most appropriate.

Environmental Modeling of Curtain Wall Fac¸ade

Indoor Built Environ 2012;000:1–12

Downloaded from ibe.sagepub.com by guest on February 1, 2013

7

Fig. 7. Seasonal daylight factor with different slat configurations.

Fig. 8. Comparison of illuminance in March.

Thermal Properties of the Curtain Wall Design of the Thermal Analysis Thermal modelling of the building using energy analysis software can help establish a reasonable estimate of the thermal performance of the glass and frame. IES_VE (Virtual Environment), the building energy simulation program, integrated with various 3rd party applications, was used to carry out the thermal performance of the curtain wall configuration. As shown in Figure 10, the basic model configuration of the building with the curtain wall has been created in ModelIT, and the annual impact of the glazed wall on the amount of energy consumption has been analyzed. Apache-sim calculated the heating and cooling load in the process of the energy analysis. In order to reduce the data noise and for a more effective simulation, three layers have been equipped to mediate the negative impact of sol-air temperature with direct solar radiation.

8

The most important factors in the thermal performance of a building are the set temperature and the amount of ventilation for the interior. The set temperatures for the heating and cooling seasons recommended by the Korean government, and the standard value of 0.7 ACH have been applied for the building energy simulation. The complex layering of the glass in the curtain wall could be strategically varied to optimize the thermal performance. A double-glazed layer and a triple-glazed structure of the curtain wall were considered for which provided the better energy performance in terms of the heating and cooling load. Table 2 shows the boundary conditions for the simulation. The weather data provided by the Korea Meteorology Administration was used. Table 3 depicts the construction materials of the elements of the office building for the energy performance simulation. The key factor in the design of a functional curtain wall is to control direct sunlight, and, thus, to reduce the

Indoor Built Environ 2012;000:1–12

Downloaded from ibe.sagepub.com by guest on February 1, 2013

Kim et al.

Fig. 9. Illuminance with slat angle in different seasons.

Table 2. Virtual environment boundary conditions Boundary conditions

Values

Design temperature

Cooling set-point 268C Heating set-point 208C 30–70% 9 am–6 pm 0.7 ACH Seoul, Korea (latitude: 378, longitude: 1278) Korea Meteorology Administration

Relative humidity Use schedule Air change rate Location Weather data

Thermal Performance of Curtain Wall Systems To determine the effect of the curtain wall systems on the overall thermal performance, as noted above, two different configurations of the glazed layer (triple glazed and a double glazed) have been applied. Figures 12 and 13 illustrate the monthly heating and cooling load for each

case. Unsurprisingly, the triple glazing is better at lessening the heating and cooling to some degree. Because of its unique thermal properties, the various layers of glazing in an insulating unit show different thermal performance. In the curtain wall configuration, the amount of both the heating and cooling load might be dependent on the glazing layers (in particular, a couple of panes or triple panes of glass). Generally, in residential buildings, the focus is on reducing the heating load, while the cooling load is regarded as more important burden in office buildings. In case of the model office building in this study, the overhang system plays a role in attenuating the cooling load, and the layer of triple glazing makes it possible to reduce heating load simultaneously. As shown in Figures 12 and 13, the triple glazing allows not only a

Environmental Modeling of Curtain Wall Fac¸ade

Indoor Built Environ 2012;000:1–12

Fig. 10. Model building of V.E. simulation.

cooling load while still maintaining thermal comfort. As shown in Figure 11, the period of overheating in Seoul, South Korea (Latitude 37.68), is not bisymmetrical based on the summer.

Downloaded from ibe.sagepub.com by guest on February 1, 2013

9

Table 3. Office building material combination for V.E. simulation Element

Wall Internal partition Glass Slab Ceiling Curtain wall frame

Constructions

U-value (W m2K)

Dense concrete þ insulation þ brick Concrete (200 mm) Refer to the blueprint of office building Sandy soil þ polystyrene þ concrete Sandy soil þ polystyrene þ concrete Alloy aluminium

0.397 2.4823 Diverse values 0.4121 0.9565 5.6470

Fig. 11. Period of overheating in the site (Seoul, South Korea).

Fig. 12. Monthly heating and cooling load with double glazing.

cooling load savings of up to 8% but a heating load reduction of 7.6%.

Conclusion There is a deep-rooted preference for a full view from an interior space, which can be satisfied using a large area

10

of glass. However, the introduction of a curtain wall has critical concerns that must be addressed in terms of the health and sustainability of the space. Glazed curtain walls lead to challenges in balancing the desire for more natural daylight versus addressing the heat gain. There are also issues relating to having too much uncontrolled daylight entering the space, not only because of glare but also energy demand.

Indoor Built Environ 2012;000:1–12

Downloaded from ibe.sagepub.com by guest on February 1, 2013

Kim et al.

Fig. 13. Monthly heating and cooling load with triple glazing.

To test the effects of various curtain wall designs and strategies, a simulation was developed for a typical office space. Using a daylight analysis with control strategies, it was found that the minimum level of Daylight Factor for all of the interior space in the spring and autumn was at least 2% higher than the required levels. In the spring, most of the direct sunlight could not penetrate deep into the space; instead, diffuse daylight was the primary contributor to the illumination. The curtain wall was equipped with shading devices to block direct sun in the spring, summer, and autumn. These shading devices allowed for illuminance ranging from 1,000 lux to 1,500 lux for most of the interior spaces for most of the year. However, the curtain wall is not able effectively to block direct sun in the winter due to the lower position of the sun. Although the shading device can still reduce the heating load, the non-uniform illuminance distribution causes problems such as glare or visual discomfort.

The challenge in the design of an environmentallyfriendly curtain wall is to strive for the highest visible light transmittance and the lowest solar heat gain coefficient while controlling glare. In terms of visual comfort, venetian blinds reduce glare and provide a view to the outdoors, by adjusting slat angles. For a desirable and uniform light level, the most appropriate blind slat angle was found to be 208. In addition to the influence of the slats, the role of the glazed layer was also found to be an issue of interest. Not surprisingly, it was demonstrated that triple glazing promises better performance, lessening both the heating and cooling loads to some degree.

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012-0000609)

References 1 Kim W, Kim JT: The scope of the glare light source of the window with non-uniform luminance distribution: Indoor Built Environ 2011;20(1):54–64. 2 Kim G, Kim JT: Healthy-daylighting design for the living environment in apartments in Korea: Build Environ 2010;45:287–294. 3 Kim G, Lim H, Lim T, Kim JT: Comparative advantage of an exterior shading device in thermal performance for residential buildings: Energy Build 2012;46:105–111. 4 Ziva K, Mitja K, Mateja T, Ales K: Fuzzy control system for thermal and visual comfort in building: Renew Energy 2008;33:694–702. 5 Richman R, Pressnail KD: Quantifying and predicting performance of the solar dynamic buffer zone (SDBZ) curtain wall through

6

7

8

9

experimentation and numerical modeling: Energy Build 2010;42:522–533. Lim H, Kim G: Predicted performance of shading devices for healthy visual environment: Indoor Built Environ 2010;19:486–496. Kim JT, Kim G: Advanced external shading device to maximize visual and view performance: Indoor Built Environ 2010;19(1):65–72. Offiong A, Ukpoho AU: External window shading treatment effects on internal environmental temperature of buildings: Renew Energy 2004;29:2153–2165. Tzempelikos A, Athientis AK: The impact of shading design and control on building cooling and lighting demand: Solar Energy 2007;81:369–382.

Environmental Modeling of Curtain Wall Fac¸ade

10 Dubois M: Shading devices and daylight quality: an evaluation based on simple performance indicators: Light Res Technol 2003;35:61. 11 Tsangrassoulis A, Bourdakis V, Geros V, Santamouris M: A genetic algorithm solution to the design of slat-type shading system: Renew Energy 2006;31:2321– 2328. 12 Loutzenhiser PG, Maxwell GM, Manz H: An empirical validation of the daylighting algorithms and associated interactions in building energy simulation programs using various shading devices and windows: Energy 2007;32:1855–1870. 13 Kim K: A comparative life cycle assessment of a transparent composite fac¸ade system and a

Indoor Built Environ 2012;000:1–12

Downloaded from ibe.sagepub.com by guest on February 1, 2013

11

glass curtain wall system: Energy Build 2011;43:3436–3445. 14 Yun GY, Kim H, Kim JT: Thermal and nonthermal stimuli for the use of windows in offices: Indoor Built Environ 2012;21(1):109– 121. 15 Yin R, Xu P, Shen P: Case study: energy savings from solar window film in two commercial buildings in Shanghai: Energy Build 2012;45:132–140. 16 Shih N, Huang Y: Case study: an analysis and simulation of curtain wall reflection glare: Build Environ 2001;36:619–626.

12

17 Vigener N, Brown MA: Building Envelope Design Guide – Curtain Walls. Simpson Gumpertz & Heger, Waltham, MA, USA, 2011. 18 Rapone G, Saro O: Optimisation of curtain wall fac¸ades for office buildings by means of PSO algorithm: Energy Build 2012;45:189–196. 19 Vigener N, Brown MA: Building Envelope Design Guide – Glazing. Simpson Gumpertz & Heger, Waltham, MA, USA, 2009. 20 No S, Kim K, Jung J: Simulation and mock-up tests of the thermal performance of curtain walls: Energy Build 2008;40:1135–1144.

Indoor Built Environ 2012;000:1–12

Downloaded from ibe.sagepub.com by guest on February 1, 2013

21 Kim G, Lim H: Spectral characteristics of UV light existed in indoor visual environment: Indoor Built Environ 2010;19:586–591. 22 Kim G, Kim JT: UV-ray filtering capability of transparent glazing materials for built environments: Indoor Built Environ 2010;19:94–100. 23 Ward G: RADIANCE User’s Manual, Lawrence Berkeley Laboratory, Berkeley, California, 1997: Available at: http://radsite.lbl.gov/radiance/refer/manpages.pdf (accessed June 2012).

Kim et al.