Abstract
A shading device, as an integrated component of a window and facade, protects space from direct sun, overheating, and glare; and provides increased daylight levels, desired privacy, or a view to the outside. This paper presents a classification of shading devices based on their assembly, their material, their position relative to the facade, and the control strategy used for the shading device position adjustment. The paper also introduces the decision-making framework (DMF) that can help in the selection of the most appropriate shading device for a specific building. The DMF is a tool for the analysis of the shading device performance; it is meant to be used by architects, engineers, window manufacturers, and shading device manufacturers.
INTRODUCTION
The proper design of a building with a window as a building component has the goal of providing comfort for the occupants, as well as energy efficiency for the building, by reducing the heating and cooling loads of the building. Direct sun radiation—one of the most significant cooling loads—is admitted through the windows. Therefore, direct sun radiation through the window should be prevented by the appropriate application of shading devices.[1]
The window is part of any conventional facade system, including both single-skin and double-skin facades. Windows as multifunctional systems provide thermal, visual, and acoustic comfort, and affect air quality. The thermal requirements for windows are to protect the building from heat loss in winter and heat gain in summer, and to collect energy in winter. The visual requirements for windows are to provide the occupants with a view, daylighting, privacy, and protection from glare.
The shading device is an integrated component of the window. Proper application of the shading device is especially important in curtain wall systems. Large glass areas can create a greenhouse effect, contribute to overheating, and increase cooling loads. Glass can also cause visual problems with direct and reflected glare.[2] Therefore, the application of shading devices in windows and large glass facades is necessary for controlling sunlight penetration through the glass.
Shading devices regulate heat by maximizing the reception of welcome heat in winter, and excluding excessive heat penetration in summer.[3] The shading device protects the space from direct sun and overheating in summer, reducing the cooling loads of the building by 23%-89%.[2] As a result, the appropriate use of a shading device in a window contributes to energy savings. When designing a window and a shading device for the window, the goal is to achieve a low total energy transmittance while maintaining high light transmission and good transparency.1-4-1
Advanced shading devices also provide daylight for the interior space. “The maximization of daylight is recognized as one of the key-goals in low-energy design.”[5] A shading device as a daylighting system can redirect daylight to spaces where daylight is needed; for example, to spaces at a large distance from the window wall. Use of daylight decreases the use of artificial lighting, decreasing the following:
• Use of electricity
• Internal heat gain from lighting
• Cooling loads
This leads to energy savings for the building. The application of daylighting can decrease energy cost by 30%.
The shading device can also provide the following benefits:
• Protection from glare
• View to the outside
• Privacy
• Collection of sun energy in the double-skin facade
• Thermal insulation during winter nights
“Venetian blinds, draperies and roller shades inside single-pane, clear glass windows, reduce heat losses by
25%-40% and metallic coated shades may further reduce losses by 45%-58%.”[2] However, use of the shading devices in the window can obstruct the view to the outside and limit the amount of daylight that penetrates into the interior space.
The second section of this article presents a classification of shading devices based on their assembly and material, their position relative to the facade, and the control strategy used for the shading device’s position adjustment. The third section explains the decisionmaking framework (DMF) that can help in selection of the most appropriate shading device for a specific building.
CLASSIFICATION OF SHADING DEVICES
Various shading device systems available on the market can be classified based on the shading device’s assembly, its material, its position relative to the facade, and the control strategy used for the shading device adjustment.
Shading Device Assemblies and Materials
Various assemblies and materials are used for the shading device systems:
• Architectural solutions—Shading devices are an integral part of the building (e.g., overhangs, fins, brise-soleils, window setback, and light shelves).
• Window treatments—Shading devices are industrially manufactured systems (e.g., awnings, louvers, blinds, roller blinds, solar films, shades, sun screens, drapes, and shutters).[1]
The description of some of the shading assemblies follows:
• Overhangs and fins: fixed architectural shading elements, usually in the form of the balconies or projected spaces. Horizontal overhangs are effective devices for the south orientation, while the vertical fins work better for east-west orientation.[3]
• Brise-soleils: fixed architectural shading elements that consist of horizontal or vertical brise-soleil louvers. They are effective for east-west orientations.
• Awnings: consist of a frame that supports a horizontal or sloped surface on the exterior of windows. Awnings can be made of fabric, plastic, and aluminum. They can be fixed or moveable.[1]
• Louvers and blinds: consist of multiple horizontal or vertical slats. Horizontal devices are the most efficient for south orientation. Vertical devices give the best protection for east-west orientation. Slats can be either flat or curved, fixed or moveable. Louvers are exterior devices made of galvanized steel, anodized or painted aluminum, plastics, or glass (Fig. 1). Blinds are interior or between-glass devices made of painted aluminum, perforated metal, wood, glass, or plastic (Fig. 2).[1'6]
Advanced shading devices not only meet thermal performance requirements but also improve daylight levels in the space. The examples of advanced devices are as follows:
• Light shelves: flat or curved elements that reflect light either outside (exterior light shelf) or inside (interior light shelf). They divide the window into two areas; the upper area provides daylight, while the lower area provides shading.[7]
Fig. 1 Louvers: an example of the automatically controlled exterior shading device, made of glass.
Fig. 2 Venetian Blinds: typical example of the manually controlled interior shading devices. The slats are made of perforated aluminum.
• Mini light shelves: for example, Okasolar units have a concave and convex shape and are made of a highly reflective light-gauge steel.[8'9] Louvers are fixed at a predetermined angle and spacing to respond to different seasonal conditions. Louvers are installed between the two panes of glass (Fig. 3).
• Prismatic and refraction elements: can be made of acrylic (Fig. 4). They can be installed in the upper part of the window to protect the space from glare and veiling reflections, while the lower part of the window provides the view.[8]
Position of Shading Devices Relative to the Facade
Based on their position relative to the facade, shading devices can be classified into three major groups:
• Exterior devices
• Interior devices
• Between-glass devices
An exterior shading device is installed in front of the facade (Figs. 1 and 4). Examples include overhangs, fins, awnings, louvers, brise-soleils, fabric blinds or screens, and roller blinds.[9] Exterior devices provide better solar protection in summer than interior devices, because exterior devices block sun radiation before it enters the glass panel and interior space. Shading effectiveness increases 35% by using an exterior shade instead of an interior one. As a result, building cooling loads are reduced. The exterior shading device’s maintenance is more complicated and expensive than maintenance of the interior shading device. Exterior devices are more expensive because of the structural and durability requirements.
Fig. 3 Mini light shelves: the Okasolar unit is an example of the fixed, between-glass shading device, made of a highly reflective light-gauge steel.
Interior shading devices are installed in the building’s interior space. Examples include Venetian blinds (Fig. 2), traditional roller shades, drapes, and blackout screens.[7] Interior shading captures sun energy that can be used in winter for space heating. Maintenance of interior devices is easier and less expensive than for exterior devices. Efficient solar protection in summer is difficult to achieve with an interior device, since sunlight enters the interior space and overheats the space between the blinds and the interior glass layer.
Fig. 4 Prismatic shading elements: an example of a moveable exterior shading device made of acrylic.
A between-glass shading device can be installed in an air cavity in one of two ways:
• Between two panes of glass in a double insulating glass unit (DGU) (Fig. 3).
• Between two facade layers in a double-skin facade.
The between-glass devices are less exposed to dust and dirt, so there is less need for cleaning. If the blinds are moveable and fully automatically controlled, maintenance can be complicated and more expensive, and a more complex window structure can be required. In summer, the between-glass shading device in the DGU usually reflects all sun energy in order to protect the interior from overheating. Any energy absorbed by the shading device contributes to the heating of the glass panes and of the air in the cavity between glass panes. This creates a problem in the DGU. However, in a double-skin facade, this warm air can be exhausted at the top of the facade, so that the facade and interior space can be protected from overheating in the summer.
Control Strategy for the Shading Device Position Adjustment
Shading devices can be either fixed (Fig. 3) or moveable. “Fixed systems are usually designed for solar shading, and operable systems can be used to control thermal gains, protect against glare, and redirect daylight.”[6] The use of fixed blind systems requires higher energy consumption than moveable blind systems.[10] Moveable systems follow the dynamic exterior thermal and luminous conditions.1-1-1 Position of the moveable shading device can be adjusted manually (Fig. 2) or automatically (Figs. 1 and 4), depending on the sun position, the sun radiation intensity, and the requirements for interior temperature and light levels. The moveable shading device can have three basic positions: open, partially open/partially closed, and completely closed.
Manually operated systems are generally low energy-efficient, because occupants may or may not operate them “optimally.”[6] Occupants very often close the blinds completely to protect the space from overheating and glare, but at the same time the amount of daylight in the space is reduced; therefore, both the use of electric lighting and, thus, the cooling loads are increased. “If the blinds are open when a large amount of solar radiation enters, excessive energy is consumed for air-conditioning… When the blinds are closed on days without solar radiation, the advantage of the view from the window is lost.”[11] The occupants will adjust the blinds to protect the space from direct sunlight, but will rarely adjust the blinds again when the direct sunlight is gone, and daylighting can be admitted.[10]
The automated shading device systems optimize energy use and control interior conditions without relying on occupants.[7] Automated systems can achieve savings in both cooling loads and lighting energy.[12] Automated blinds have better thermal and daylighting performance than both fixed blinds and manually controlled blinds. Automated systems “close automatically when the interior becomes too glary or too hot, and re-open later to admit useful light.”[10] The use of automated Venetian blinds decreases the energy cost by 30% during the winter and by 50% during the summer. However, automatic systems can produce discomfort in occupants who dislike the feeling of not having personal control over the system.[6] Automated devices are often high-maintenance, and therefore expensive, solutions.[8]
SHADING DEVICE SELECTION
Several criteria should be considered when selecting the most appropriate shading device among the devices available on the market. To make the proper choice of the shading device, the required or desired performance for the shading device and the variables that affect that performance need to be determined. Fig. 5 shows the structure of the DMF for the shading device selection. The user of this DMF can be an architect, engineer, windows manufacturer, or shading device manufacturer. The DMF is an analysis tool that can help its user to select the most appropriate shading device for a building.
The structure of the DMF includes the following:
• Independent, dependent, and shading device variables that influence shading device performance.
• Performance parameters (thermal, visual, acoustic, aesthetic, cost, and control) that are used as criteria for the shading devices’ evaluation and selection.
• Relationships and interactions among the variables and performance parameters.
Independent Variables
Independent variables such as climate, location, site, and building type are given to the user of the DMF.
The United States has four major climate zones: hot dry, hot humid, cold dry, and cold humid. For each of these climates, characteristics need to be determined. Climate directly affects the type of heat transfer, Heating, Ventilation, and Air Conditioning (HVAC) conditions, the facade type, shading device variables, the shading device’s thermal and visual performance, the operational cost of the shading device, and the control strategy for the devices’ adjustment.
Fig. 5 The decision-making framework (DMF) for the shading device selection: the figure shows the structure of the DMF including variables (independent, dependent, and shading device variables), performance parameters (thermal, visual, acoustic, aesthetic, cost, and control), and the relationships and interactions among the variables and performance parameters.
The building location is defined by latitude and longitude. Based on the building location, the climate of the region and the microclimate of the particular building’s site can be determined. Location indirectly affects heat transfer, HVAC conditions, the facade type, the shading device’s thermal and visual performance, and the control strategy.
The building site is given to the designer, who has two choices: to select the position of the building on the site, or to accept the predefined position of the building, as in the case of the dense, urban setting. The site has a strong relationship with the climate and location. The site has direct influence on the facade type, the shading device variables, the shading device’s thermal and visual performance, the operational cost, and the control strategy for the blinds’ adjustment.
The building type defines a function of the building; for example, residential, commercial, education, or health.
The building type directly influences heat transfer, HVAC conditions, facade type, the position of the shading device, the shading device variables, and the performance of the shading device.
Dependent Variables
The user of the DMF defines dependent variables, such as heat transfer, HVAC conditions, the facade type, and the position of the shading device relative to the facade.
The user of the DMF defines dominating heat transfer conditions in the building, such as heat gain or heat loss. Heat transfer depends on the climate, location, and building type. Heat transfer affects the HVAC conditions, the facade type, the shading device variables, and the values of the performance parameters.
By selecting the HVAC conditions, the user of the DMF decides whether or not there is a need for heating, cooling, or air conditioning—or a combination of any of these systems. The selection of the HVAC conditions is made based on the heat transfer in the building, the climate, and the building type. The HVAC conditions have an impact on the shading device variables, the shading device’s thermal performance, and the control strategy for the devices’ adjustment.
The selection of the facade type is affected by the climate, site, and building type. The facade type has an effect on the shading devices’ variables and on the shading device’s thermal, visual, aesthetic, and cost performance. For example, if the double-skin facade is chosen, the shading device can be installed between the two facade layers and function as a solar collector, thus improving the facade’s thermal performance.
The position of the shading device is dependent on the climate, site, building type, heat transfer, HVAC conditions, and facade type. The position of the shading device strongly influences the shading device variables. For example, an exterior device should be made of different material than an interior device because exterior devices must be weather resistant. The position of the shading device affects the performance of the shading device, especially the maintenance cost (e.g., it is more expensive to clean an exterior device than an interior one).
Shading Device Variables
This DMF includes the following shading device variables: the shading device’s geometry, width, and thickness; applied materials and coatings; and in the case of Venetian blinds, distance between the blinds, the blinds’ direction, and the blinds’ tilt angle. Shading device variables are independent in the process of the shading device’s selection because the shading device variables are already defined by the manufacturer and given to the designer of the building. These predefined variables are used to analyze the performance of the shading device. Shading device variables, together with the independent and dependent variables, directly affect the performance of the shading device.
Performance Parameters for the Shading Devices
Performance parameters considered in this DMF are the thermal, visual, acoustic and aesthetic performance; the cost of the shading devices; and the control strategy for the shading devices’ position adjustments. The performance parameters’ values depend on the independent, dependent, and shading device variables. There are also interactions among the performance parameters in this DMF.
The thermal performance of the shading device includes protection from overheating in summer, protection from heat loss in winter, and collection of sun energy. Thermal performance strongly depends on the climate, site, building type, heat transfer, facade type, position of the shading device, and shading device variables. In a hot climate, the office building’s shading device will be required to provide protection from overheating. The shading device’s geometry, materials, and position will be chosen to achieve the required protection from overheating. The level of protection from heat loss during winter nights or in cold climates and the collection of sun energy are also measures of the shading device’s thermal performance. The device can absorb solar energy instead of reflecting it, and also collect this energy for application in the building’s mechanical systems. There is a strong relationship between the shading device’s thermal performance, its visual performance, and its control strategy. The shading device can be designed to provide maximum overheating protection but also to allow the sufficient daylight level in the space. To achieve this goal in the case of Venetian blinds, control systems should provide the blinds’ optimum tilt angle.
The visual performance of the shading device includes providing the following desired effects:
• Illuminance
• Luminance
• Protection from glare
• Privacy
• Darkening of the space
• Visual contact to the outside space
Climate and site affect illuminance, luminance, and protection from glare. The building type strongly affects the requirements for providing privacy, darkening of the space, and direct visual contact to the outside space. For example, providing privacy and darkening of the space is often desirable in residential buildings, but not necessarily in office buildings. The visual performance of shading devices depends on the facade type, the position of the device, and the devices’ geometry, dimensions, material, direction, and tilt angle. Also, there is the interaction among the thermal, visual, and aesthetic performance, and the control strategy. When selecting the shading device, the user of the DMF needs to understand that good visual performance can be achieved only with a thoughtful control strategy of the shading device’s adjustment. The user of the DMF also must consider the impact of such a shading device on the appearance of the facade, i.e., on the aesthetic performance.
Acoustic performance parameters of the shading device include sound transmission and vibration of the blinds. The acoustic performance is significantly affected by the following:
• The building location and site: a higher level of noise occurs in urban areas; therefore, the shading device has to be designed to reduce this noise.
• The building type: different levels of acoustic comfort are required in different types of buildings, and a specific shading device can help in meeting the acoustics requirements.
• Facade type and position of the shading device: the exterior shading device can vibrate because of wind, resulting in increased noise level. For that reason, structure of the exterior device should be designed properly to avoid the problem of vibration.
• The thermal performance: the shading devices that protect space from heat loss during cold nights can also protect the space from noise, because devices can be made of materials with good thermal and acoustical properties.
• Control strategy: completely closed blinds provide the lowest sound transmission, but at the same time they block the daylight. The control strategy needs to balance the sound transmission, penetration of the daylight, and protection from heat loss or gain.
• The shading device variables: the shading device’s material, shape, dimensions, and position need to be selected to achieve the best possible protection from noise.
The shading device has a significant impact on both the exterior and interior appearance of the facade. To achieve an aesthetically pleasing look for the shading device and facade, the device’s transparency or translucency and the percent of the window area obstructed by the device need to be considered. The shading device’s aesthetic performance is affected by the climate, site, and building type. The appearance of the surrounding buildings also affects the appearance of the analyzed building, and, consequently, the appearance of the shading device. Different aesthetic requirements are imposed for the shading devices installed on different building types. The shading devices can have a different look if they are installed on an office building than they would on a hospital or industrial building. The aesthetic performance of the shading device is influenced by the facade type; the shading device’s position; its thermal, visual, and acoustic performance; the cost of the shading device, and the applied control systems. The shading device’s transparency or translucency and the percent of the window area obstructed by the shading device depend on the requirements for the protection from overheating, the desired light level in the interior space, and the choice of the shading device’s variables.
Total cost of the shading device consists of initial, operational and maintenance cost. Cost analysis should also include calculation of the ratio between total cost and energy savings achieved due to use of the shading devices. The initial cost of the shading device includes the cost of material, cost of manufacture of the system, and cost of installation of the shading device on the facade. The initial cost includes also the cost of the moving mechanisms and control systems if the device is moveable. The operational cost includes the operating cost of the shading device itself and the cost of the heating, cooling, and lighting of the space, which is a result of the application of the shading device on the building. The cost of maintenance includes cleaning and repair costs. The cost of the device is influenced by the climate; location or site; building type; thermal, visual, acoustic, and aesthetic performance parameters; and control systems.
The control strategy for the shading devices’ adjustment in this DMF considers two options: shading devices without need for control (fixed) and shading devices controlled manually or automatically. The control strategy for the shading devices’ adjustment depends strongly on the following:
• Climate, particularly sun radiation, sun angle, and sky conditions
• The building type, because different building types require different values of the performance parameters for the shading device; and, therefore, different positions and tilt angles of the device
• The facade type
• Heat transfer
• HVAC conditions
• The shading device variables, particularly geometry of the device, dimensions, and materials
• The required thermal, visual, and acoustic performance, and the cost of the shading device
Making the Decision by Using the DMF
The process of making a decision about selection of the shading device by using this DMF includes the following steps:
• Identifying input for the DMF—The user of the DMF prepares the input for
— The independent, dependent, and shading device variables
— The required values of the performance parameters that can be taken from active standards, codes, and recommendations
• Testing the shading devices—Separate testing is performed for each type of shading device to analyze thermal, visual, acoustic, aesthetic, and cost performance; and the effect of the applied control strategies. Depending on the nature of the performance parameter, the actual values of the performance parameters can be obtained by experimental testing, computer simulations, and mathematical calculations.
• Obtaining output results of testing—The actual values of thermal, visual, acoustic, and aesthetic performance parameters; the cost of the shading device; and the effect of the control strategy are collected for each type of shading device. The results are organized in an understandable and useful format and prepared for analysis.
• Making the decision about the selection of the most appropriate shading device for the particular building—Output results of testing are compared to the required values of performance parameters for the shading devices. If the shading device’s actual performance meets the requirements of the standards, then the particular device can be considered for further analysis and application on the building. Then the alternative shading devices are compared to each other. The actual values of the performance parameters for each shading device are compared, and the device with the best overall performance is selected for use on the specific building.
CONCLUSION
Shading devices are integrated elements of windows and building facades. They increase the energy efficiency of buildings and improve comfort for the building occupants. Shading devices provide the following benefits:
• Protection from direct sun
• Protection from overheating
• Protection from glare
• Increased daylight levels
• Privacy
• View to the outside space
Different types of shading devices that exist in the market can be classified based on their materials, their assemblies, their position relative to the facade, and the control strategy for the shading device position adjustment. The DMF presented in this paper helps the user, whether an architect, engineer, window manufacturer, or shading device manufacturer, to select the most appropriate shading system among several available systems.
