External Shading Devices

In the tropical climates, the designer should keep the solar radiation off the opaque solid elements of the building's envelope where possible. Special care should be taken to shade the windows to reduce the incoming heat and the risk of overheating.

Shading Design

The design of shading devices can be quite complex. Computer programs exist to accurately shape shades for very specific purposes. However, in their absence, and with a little understanding of the mechanics of sun position and sun-path diagrams, manual methods can be used.
External shading devices are preferable and more effective than internal ones. This includes devices fixed to the outside of the window or attached to building envelope. Among the operable units are louvers made of wood or metal, exterior venetian blinds, shutters, awnings and fixed or movable overhangs.
As you should know from your own personal experience, the most important characteristic of solar position is its seasonal variation. At the height of summer in the southern-hemisphere the sun rises slightly south-east and sets slightly south-west. In winter it rises slightly north east and sets slightly north-west. It also rises much earlier and sets much later in summer than in winter. In the northern hemisphere, north and south are reversed.

The aim of good shading design is to utilise these characteristics to best advantage, usually complete exclusion in summer and maximum exposure in winter.

Rules of the thumb

Shading devices should be selected according to the orientation of the window. Whilst some orientations are easy to shade, others are much more difficult as the sun can shine almost straight in at times. The table below indicates the most appropriate type of shading device to use for each orientation in the southern hemisphere. These are guidelines and, of course, there are many variations to these basic types.

Orientation Effective Shading
North (equator-facing) Fixed horizontal device
East or West Vertical device/louvres (moveable)
South (pole-facing) Not required

Shadow Angles

When attempting to shade a window, the absolute azimuth and altitude of the Sun are not as important as the horizontal and vertical shadow angles relative to the window plane. These can be calculated for any time if the azimuth and altitude of the Sun are known.

Horizontal Shadow Angle (HSA)

This is the horizontal angle between the normal of the window pane or the wall surface and the current Sun azimuth. The normal to a surface is basically the direction that surface is facing its orientation. If the orientation is known,

HSA is given by:

HSA = azimuth orientation
Vertical Shadow Angle (VSA)

The vertical shadow angle is more difficult to describe. It is best explained as the angle a plane containing the bottom two points of the wall/window and the centre of the Sun, makes with the ground when measured normal to the surface. It is therefore given by:

VSA = atan(tan(altitude) / cos(HSA))

It is the VSA that determines the depth of the required shade. The diagram more adequately describes the derivation of the VSA

Shade Dimensions

These two angles, HSA and VSA, can then be used to determine the size of the shading device required for a window. If the height value refers to the vertical distance between the shade and the window sill, then the depth of the shade and its width from each side of the window can be determined using relatively simple trigonometry.

Shade Depth

The depth of the shade is given by:

depth = height / tan(VSA)

The width is given by:

width = depth * tan(HSA)

The width simply refers to the additional projection from the side of the window. Exactly which side is a matter of the time of day and which side of the window the Sun is on.

Design Requirements

The design requirements for a shading device depend entirely on a building's use and local climatic conditions. In a multi storey open plan office building, the occupancy and equipment gains are such that heating is rarely required. In this situation, to avoid unnecessary loads, shading may be designed to completely protect the windows all year-round.

In a domestic building or one that is occupied 24 hours, the release of stored heat during cold nights in winter can be important. In this case, the shading would be designed to fully protect the windows during the summer months, but to expose them as much as possible to direct sun in winter so that they have a chance to absorb heat during the day. In climates where summers are also relatively cold, the requirement may be to allow full solar access all year round.

If you look at outdoor air temperature and the intensity of solar radiation at different times of the year for Perth, Western Australia, it is clear that the transition to colder weather really begins in mid to late March. Thus, in order to take advantage of solar heating, the transition from shaded to exposed should really begin at the same time. This means that the window should remain completely shaded up until mid to late March, with maximum exposure occurring at the winter solstice in mid June.

A convenient date, by happenstance, is the 21st of March. This has the advantage of being the autumn equinox. One characteristic of the equinox is that, for a north-facing wall, the VSA is exactly the same throughout the day. This is an important piece of information as, in summer, the lowest daily VSA occurs at noon, whereas in winter noon sees the highest VSA.

Thus, if the cut-off date for a north-facing shade occurs on or before the autumn equinox, its depth will be defined by the noon VSA. If the cut off date occurs after the 21st of March, the VSA at either the start or end times will determine its depth.

Design Steps

To design a horizontal shading device, simply following the following steps.

  1. Determine cut-off date.
  2. This is the date before which the window is to be completely shaded and after which the window will be only partially shaded.
  3. Determine Start and End Times.
  4. These represent the times of day between which full shading is required. Keep in mind that the closer to sunrise and sunset these times are, the exponentially larger the required shade.
  5. Look up Sun Position.
  6. Use solar tables or a sun-path diagram to obtain the azimuth and altitude of the sun at each time on the cut-off date.
  7. Calculate HSA and VSA.
  8. Using the formulae given above, calculate the HSA and VSA at each time.
  9. Calculate Required Depth and Width.
  10. Once again, using the formulae above, calculate the depth and width of the required shade on each side of the window.
The Effects of Shading

The above method will provide a precisely shaped shade that will provide full protection over the time period selected. However, we have not yet looked at how much Sun we will get in winter, when some penetration is usually desirable. Unfortunately, a shading device will not suddenly stop working after a certain date (unless it is retractable). It will usually partially obscure the window year round, more so in summer and less so in winter.

This is where the trade-off begins the amount of which depends on the relative heating and cooling stresses in the environment. In a very hot climate you may not actually need solar gains in winter, whereas in a very cold climate solar gains even in summer may be desirable.

In order to understand the full effect of a shading device, we really need to turn again to the sun-path diagram and a percentage overshadowing graph. It may be that, whilst we want 100% shading throughout most of summer, we could probably live with only 80 to 85% shading in early Autumn in order to gain extra solar gains in winter. You will notice that the shading patterns displayed in the diagram below all display "fuzzy" edges. This is because we are dealing with a large window surface, not a single point.

sun sun sun sun sun sun sun sun sun sun