Fixed overhang

The fixed overhang is a simple external shading system to provide protection from direct sunlight to reduce overheating in a space. As the device is fixed to the outside of the window just above window’s head, it reduces solar gains into the space without any occupant control. However, it does not offer other benefits like movable shading to adjust for changes in receipt receiving solar radiation intensity or security and privacy as well as glare control like other shading devices (Olbina, 2005). Even though, the strategy is still effective whilst blocking high angle summer sun during the hottest time of the day and allow low angle winter sun to pass directly into the dwelling. Fixed overhang design is the matter of compromising because this shading will not stop performing after a certain date (unless it’s a moveable overhang) thus it will partially shade the window all year around. A wide overhang (heavily shading) which offers a fully shade window during summer time will block too much sunlight when solar heat gain is desired. Or a narrow overhang might be not effective in reducing solar gain when heating is not required. Regarding the building form and design, an overhang over the French patio door creating the balcony cover is more suitable than the vertical fins due to its presence causing space reduction and obstruction in the balcony. If there had not been a balcony in the initial design, then vertical fins would have been suggested for effective shading on east/west facades. The improvement when adding vertical fins lies between fixed overhang and shutter (external shading) so in the scope of illustration of different solar access control, the use of vertical fins was excluded.

Overhang design

The first step is to determine the cut-off date so as to determine the width of overhang that shades window in summer and allow direct sunlight in winter. It could be taken as the period of time during which the room experiences overheating thus full shading would be useful in reducing unwanted heat gain. As shown in Figure 6-3, if the cut-off date is based on overheating event occurs indoors then the selected cut-off date is 18^th March and 24^th September. An earlier date, 24^th August is chosen for a higher solar altitude in order to provide the useful sunlight during March and early April. Therefore, the cut-off date for overhang design is 15^th April and 24^th August. The solar time to cut off can be based on peak cooling time in south, east and west zones. It can be based on previous result of analytical verifications for peak temperature and cooling load design (See Appendix B, Section B.3.2). If using that, the cut off time are below: 8:30 for east facade, 11:30 for south facade and 15:30 for west facade.

Figure 6-2: Solar altitude and operative temperature profiles for selection of cut-off date in IES<VE> graph

The dimension of the fixed overhang can be calculated using the following formula (Ballast, 1988)

h= [D x tan(solar altitude)] / cos{solar azimuth – window azimuth) (6-1)

Illustration of the dimension is shown in Figure 6-3. For sizing overhang for east or west window, a fin must be added for adequate shading, otherwise overhang can become unreasonably deep (Ballast, 1988).

Figure 6-3: Solar position in reference with window (Olbina, 2005).

Solar positions for specific cut-off date and start and end time are determined by the use of weather data file in IES<VE> as shown in Table 6-2

Table 6-2: Solar positions for the selected cut-off date and time Orientation Time Solar altitude

(°)

Solar azimuth (°)

VSA in radian

South glazing 11:30 44.2 145 0.87

East glazing 8:30 30 112.3 0.56

East glazing 9:30 30 112.3 0.56

East glazing 10:30 37.9 127.3 0.78

West glazing 14:30 45.7 209 1.13

West glazing 15:30 40.1 227.6 0.85

The light or heavy case of overhang refers to the depth of the fixed overhang above windows in which the light case is selected for higher solar altitude and the heavy case is selected for the lower solar altitude. From the weather file data, the solar time is calculated at the mid hour range and the selected parameters of solar position for designing shading were listed in Table 6-2. On the cut-off date, the time of 10:30 was selected for designing fixed overhang glazing facing east and 14:30 was selected for west facing glazing area in the second case. While in the third case, designing a deeper

fixed overhang, the time of 9:30 and 15:30 on the cut-off date were selected for glazed areas faces due east and west.

Table 6-3 lists the dimensions of fixed overhang designed with the selected cut-off date and time. However for the east case (main building façade faces due east), the fixed overhang above the glazed areas (both windows and French patio doors) designed at 9:30 were too heavy with 1.023 meters height windows, the overhang would be 1.64 m depth, or with 1.983m height glazing door, the required depth of the overhang would be 2 meters which seems unreasonable and could cause over shading that blocks useful sunlight in winter. So the later time was selected for designing overhang on east facade (i.e. 10:30 for the third case and 11:30 for the second case). Fixed overhang above the French patio door in the third case could be found in designing fabric awning or roof of a balcony as a solution. In this specific building and its fenestrations, the extent from each side of French patio door is longer the wall facade surface containing the overhang then this is designed to extend to cover the width of the façade (See Figure 6-4(b)).

The length of the overhang (L) was calculated by L=2xW+Wwindow with W is the width of the overhang from each side of the window and Wwindow is the width of windows (Olbina, 2005)

To provide a better representation of the severity of the overheating problem, the chart in Figure 6-3 show the number of degree hours over these comfort threshold temperatures for the occupied periods. Each 1°C beyond the threshold temperature (28°C living areas and 26°C for bedroom) for an hour equates to a degree hour overheating.

Simulation scenarios

Several design scenarios:

- Case 0: Base case (shading free)

- Case 1: Fix overhang on south facade only, see Figure 6-4(a).

- Case 2: Overhang on all 3 facades: partially (light) overhang / awning on east and west facades, See Figure 6-2(b).

- Case 3: Overhang on all 3 facades: fully (heavy) overhang / awning on east and west facades (similar to Case 2 but deeper overhang)

- Case 4: Fix overhang on south facade and external shutter on east and west French facades.

Table 6-3: Dimensions of overhang for different windows on 3 main orientations Orientation Glazing elements Height x

Width of glazing areas, m

Depth of overhang , m

Width of overhang (from each side), m

Depth x Length of overhang, m x m

South Large windows 1.023 x 0.886 0.86 0.6 0.86 x 2.09 Small windows 1.023 x 0.623 0.86 0.6 0.86 x 1.82 French patio door 1.983 x 1.173 1.67 1.17 1.67 x 3.88 East Large windows² 1.023 x 0.886 0.6 0.86 0.6 x 2.61

Large windows³ 1.023 x 0.886 1.0 0.80 1.0 x 2.48 Small windows² 1.023 x 0.623 0.6 0.86 0.6 x 2.34 Small windows³ 1.023 x 0.623 1.0 0.80 1.0 x 2.22 French patio door² 1.983 x 1.173 1.17 1.67 1.17 x 3.88 French patio door³ 1.983 x 1.173 2.03 1.54 2.03 x 3.88 West Large windows² 1.023 x 0.886 0.48 0.87 0.48 x 2.62 Large windows³ 1.023 x 0.886 0,90 0.82 0.9 x 2.53 Small windows² 1.023 x 0.623 0.48 0.87 0.48 x 2.34 Small windows³ 1.023 x 0.623 0.9 0.82 0.9 x 2.23 French patio door² 1.983 x 1.173 0.94 1.69 1.17 x 3.88 French patio door³ 1.983 x 1.173 1.74 1.54 1.74 x 3.88 Overhang shading design options were illustrated in the figure below:

Figure 6-4: Fixed overhang simulation model in the south case.

The simulation outputs of heating demand and overheating analysis for fixed overhang options were presented in Figure 6-5.

( a) Fixed overhangs on south windows only (b) Fixed overhangs on 3 facades

129

Figure 6-5: Heating load and overheating degree hours in different fixed overhang design options

0

Energy consumption (kWh/m² pa)

West Heating load Overheating - Windows kept shut 0

Energy consumption (kWh/m² pa)

South Heating load Overheating - Windows kept shut

0

Energy consumption (kWh/m² pa)

North Heating load Overheating - Windows kept shut

0

Energy consumption (kWh/m² pa)

Heating load Overheating - Windows kept shut

East

130 6.2.1.2 External shutter

The blinds and shutters vary widely from country to country. They could be louvered shutters, venetian blind or curtain roller blind type and are made from different materials like plastic, wood, aluminium or curtain fabric. They are used for reasons of security, privacy, solar control and/or thermal insulation. Unlike fixed overhang, they require occupant control and restrict view when in use. For their best effective in solar control, the system is made from low heat storage materials with reflective finishing.

This could reduce the amount of energy absorbed and stored within the system, thus less heat radiates back into the room. It is advisable to allow ventilation between the external shading system and glazing windows to remove remaining heat.

A full use of shading devices reduces the illuminance in the room causes requirement for artificial lightings during the occupancy. A trade off point in energy demand is raised as whether cooling energy savings from fully shaded windows in summer time could outperform lighting energy spent to maintain room’s illuminance. Whilst considering that people tend to welcome sunlight when it is available and prefer to be able to look outside the windows when they are active (quiet and dark room is preferred when resting and sleep), it is assumed that during the occupied period in summertime, a half lowered blind will be used.

The selected external roller shutter blind for the study allows limited ventilation and day lighting though it requires sash or inward opening windows. The profile of external shutter usage for modelling is half shaded describing that the roller blind is partially (i.e.

half) lowered to let daylight enter internal space during the occupied period (i.e.

weekend daytime). The simulation results of cooling load and overheating risk analysis and performance when the external shading is applied for 4 main orientations is shown in Figure 6-6 together with analysis of internal shading devices.