E02: External Shading Devices

Requirement Summary

This measure can be claimed if external shading devices are provided on the building’s exterior.

Intention

External shading devices are provided on the building façade to protect the glazed elements (glass windows and doors) from direct solar radiation to reduce glare and to reduce radiant solar heat gain in cooling dominated climates. This method is more effective than internal shading devices such as blinds, because radiant solar gain occurs in the form of short wavelengths that can pass through glass. However, reflected wavelengths are longer and can no longer pass through glass to exit the space. This phenomenon is known as the greenhouse effect.

Approach/Methodologies

If this measure is selected, EDGE uses a default shading factor equivalent to that of a shading device that is 1/3 of the height of the window and 1/3 of the width of the window on all windows of the building. However, if shading devices are provided that are different from EDGE assumptions, then a different shading factor should be used. The shading factor varies according to the latitude and the orientation of the windows, as well as the size of the shading device, and can be calculated using the built-in calculator. Figure 7 illustrates the dimensions used to calculate the shading factor.

Figure 7. Illustration of the dimensions used to calculate the shading factor

Table 13, Table 14, and Table 15 show the relationship between the Dh and Dv (depth of horizontal and vertical shading) H (window height) and W (window width) to determine the shading factor.

This measure is assessed using an Annual Average Shading Factor, which is represented by one minus the ratio of solar radiation transmitted by a protected window (with external shading devices), compared to that transmitted by an unprotected window.

Annual Average Shading Factor (AASF) is defined by following equation:

The shading factor is expressed as a decimal value between 0 and 1. The higher the shading factor, the greater is the shading capability of the shading device.

Table 13, Table 14, and Table 15 indicate the shading factors for different orientations, latitudes, and shading device proportions. The last column of Table 15 lists the average shading factor for the combined type, which is used as the default improved case by EDGE.

The project AASF is the area-weighted average of the shading factors of all the external windows. When conducting calculations, all windows should be accounted for. If a window has a vertical and a horizontal overhang with different depths, select the more conservative (smaller factor) overhang depth for the calculation. If any windows do not have an overhang, they must still be included in the calculation and use the appropriate values for ‘No Overhang.’ The total Window Area must match the total External Window Area used in WWR calculations.

Table 13: Shading factors for horizontal shading devices at different latitudes for each orientation

*The shading factors have been derived using a solar modeling tool

Table 14: Shading factors for vertical shading devices at different latitudes for each orientation

Table 15: Shading factors for combined shading devices (both horizontal and vertical) at different latitudes for each orientation

Three basic types of solar shading are used: horizontal, vertical, and combined (egg crate).

Table 16: Typical shading devices

The effectiveness of a shading device varies depending on the location towards the equator (latitude) and the orientation of the window. Table 17 gives an early indication of the appropriate type of shading device for each orientation.

Table 17: Shading strategies for different orientations at the design stage.

Orientation	Effective Shading
Equator-facing	Fixed Horizontal Device
East	Vertical Device/Louvres (moveable)
Pole-facing	Not Required
West	Vertical Device/Louvres (moveable)

Example:

An office building in Istanbul (Turkey) has 1-meter deep horizontal shading on 3 meters high windows in all directions. What is the shading factor for these windows?

The shading factor can be calculated with the built-in calculator in the EDGE software online. If calculating the factor manually, use the following steps:

• Step 1: Determine the latitude of Istanbul (41 N) from the EDGE online tool design tab, under “Key Assumptions for the Base Case”.
• Step 2: Use the table provided for Horizontal shading (Table 13)and look for the matching latitude category which is “40o to 49o ”. As the shading is 1/3rd of the window height, “Dv=H/3” should be selected. The average shading factor is 0.30.
• Step 3: Select external shading measure in the EDGE App and input 0.30 in to the average annual shading factor (AASF) field.

Relationship to Other Measures

External shading reduces the heat gain through solar radiation, therefore a glazing type with a higher solar heat gain coefficient can be selected without a significant negative impact. As external shading cuts the solar heat before it hits the glazed element, it reduces radiative heat gain compared to a treated glass without shading, thus offering better thermal comfort conditions.

Shading reduces heat gain and, therefore, cooling loads. The extent of the savings achieved in cooling energy from shading will be impacted by the efficiency of the cooling system. With a more efficient cooling system, the magnitude of savings from shading alone will be less, even though the combined savings will be greater.

In heating mode, the heating consumption may be increased when external shading is incorporated, due to the reduction of solar heat gains in winter, if shading is not well designed. Well-designed shading cuts out the summer sun but allows in the winter sun which is at a lower altitude.

Assumptions

For the base case, EDGE assumes that no solar shading is present. For the improved case, EDGE assumes a shading factor equivalent to shading devices with a proportion of 1/3 of the height and the width of the window, fitted to all windows. The shading factor is the annual average of eight orientations as shown in the last column of Table 13, Table 14, and Table 15, which is a combination of both vertical and horizontal solar shading.

Compliance Guidance

The information required to demonstrate compliance will depend on the design solution adopted. The simplest design approach is the installation of egg crate shading devices (depth of 1/3 the height and the width) on all windows on all façades. Design teams may prefer to specify the shading device according to the orientation. Table 13, Table 14, Table 15, and Table 16 can be used as guidelines for different sizes and types of shading devices and orientation. Compliance is achieved when the design team has correctly entered the average of the shading factor of all orientations. In the case of external movable shades, the design team can select a Combined Overhang with the greatest projection (W/1 and H/1). In the instance that the building has a more complex shading design, the design team can use specialized software that uses the AASF equation given in the approach section above, to demonstrate that average shading factors have been achieved.

Design Stage	Post-Construction Stage
At the design stage, the following must be used to demonstrate compliance:  All façade elevation drawings highlighting the provision of horizontal and vertical shading devices; and  Window details clearly showing the depth of the shading device and the calculation of the proportion.	At the post-construction stage, the following must be used to demonstrate compliance:  Photographs of the shading devices on all façades; or  As-built façade drawings showing the shading devices that have been installed; or Update of shading factor calculations when changes have occurred in the design stage.

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