HORIZONTAL SHADING DEVICES – OVERHANGS
This diagram illustrates, the basic concept is that an overhang can be positioned to totally allow low winter sun in the entire window while completely shading the entire window from summer sun. The design calculation is performed over a certain period of mid-summer and a certain period of mid-winter, typically a month or two on either side of the two solstices. The calculation is also performed only for a certain period during the day, typically near solar noon since that is when it’s most important to increase solar gain in the winter and reduce gain in the summer (because the sun is most intense then). In fact, it is not usually possible to design a horizontal overhang that works in the early morning or late afternoon because the sun is low in the sky in both the summer and winter.
These are placed horizontally in fron tof the window, in various ways. Their, shape, type, depth and height all differs, all depending on the sun conditions. A window overhang is a (usually) horizontal surface that juts out over a window to shade it from the sun. This is desireable in order to reduce glare or solar heat gain during warm seasons.
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.
|North (equator-facing)||Fixed horizontal device|
|East or West||Vertical device/louvres (moveable)|
|South (pole-facing)||Not required|
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.
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.
To design a horizontal shading device, simply following the following steps.
- Determine cut-off date.
This is the date before which the window is to be completely shaded and after which the window will be only partially shaded.
- Determine Start and End Times.
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.
- Look up Sun Position.
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.
- Calculate HSA and VSA.
Using the formulae given above, calculate the HSA and VSA at each time.
- Calculate Required Depth and Width.
Once again, using the formulae above, calculate the depth and width of the required shade on each side of the window.
DIFFERENTIATED FACADE TREATMENT:
The northelevation (in the northern hemisphere) essentially does not require shading because except in the summer months in the early morning and late evening, no sun penetration occurs. At this time of day the sun angle is so low that horizontal projections would be useless as shading devices. It is best to limit as much as possible fenestration on the north elevation as there will be very little solar heat gain and much direct heat loss from this side. If fenestration is required for daylighting, then it is important to select a highly efficient glazing assembly to reduce energy transfer.
The south elevation (in the northern hemisphere) allows for the easiest control of solar energy. Shading devices are normally designed as horizontal projections above the windows — the length of the projection is determined as a geometric function of the height of the window and the angle of elevation of the sun at solar noon. Such shading devices can be designed to completely eliminate sun penetration in the summer and allow for complete sun penetration during the winter when such is desired for passive heat gain.
Basic Shading Strategy for a South Elevation
The east and west elevations are both difficult to shade “architecturally”. The sun angles in the morning and afternoon are low enough to preclude shading using overhangs. The morning sun is normally cooler and less offensive than the heat and glare of the late afternoon sun.
Shading Issues with East and West Facades
Shading needs to be provided in the way of landscaping and foliage. Deciduous trees are effective as they block the sun in the summer when it is not desired and allow sun penetration during the winter. Fences work to block the sun and view at all times of the year and so are not so climatically responsive. Vines on more transparent “fence like” elements are effective as they too bear leaves to shade in the summer, and keep their leaves until later in the fall when sun is again desired. Vines are often used as well on south facing elevations on trellises to achieve seasonal variation in the opaqueness of the overhangs.
The natural environment can be used to shade low rise buildings. Deciduous trees can effectively shade the facade when heat avoidance is desired, and permit solar penetration where passive solar gain is sought. Vines can be used on trellises or trellis like shading devices to the same effect. Vegetative shading also works well with the shoulder heating and cooling seasons. In the spring when heating is still often desirable, leaves are not yet present, allowing continued passive heating. In the fall, when continued warm days might suggest cooling, the leaves have not yet been shed. If natural ventilation is also desired, it is important to allow adequate wind penetration around exterior plantings or potential natural cooling will be blocked.
How Long to Shade For?
Shading devices for heat avoidance need to be designed to be effective beyond the geometry of summer solstice when the sun is highest in the sky. Depending on the local climate conditions, cooling may be a priority from the mid spring to early fall seasons. The length of the south facing shading device should be sized for this extended season.
The Shading Season
The diagram above divides the types of shading devices into fixed and movable. Movable shading devices may include awnings, hinged extensions and vegetation. If a mechanically dependent solution, the device needs to be designed for durability.
GENERAL TYPES OF SHADING DEVICES:
The basic types of exterior shading devices can be identified as HORIZONTAL, VERTICAL OR EGGCRATE. When designing shading devices for heat avoidance it will be important to also weigh the amount of solar penetration that is desired during the heating months. Where the heating degree days greatly exceed the cooling degree days (in COLD climates), be careful not to compromise the potential for solar gain in the winter months. Where the cooling degree days exceed the heating degree days (HOT climates), shading should be effective for a longer period. In some climates this may warrant the virtual elimination of south facing windows, with deference to north facing windows to promote daylighting.
Horizontal shading devices are suited to southern exposures. Roof overhangs can also easily be used to shade southern exposures on low rise buildings. This is perhaps the most economical and potentially aesthetically pleasing solution for residential applications.
Basic Typology of Horizontal Shading Devices for Southern Exposures
Where sun is hitting the facade from a south-easterly or south-westerly direction, vertical devices can effectively block the sun. Eggcrates are often used on non true south facing elevations as well.
Shading Devices for Non Southern Exposures
The general configuration of the building can also be modified to alter the orientation of windows for heat avoidance.
Various Planimetric Configurations of Non South Facing Shading Devices
For reasons of both heat avoidance and economy, it is often best to “gang” the south facing shading devices. In order to obtain shading in the late morning and early afternoon when the sun is not at its high point, the shading device should be extended either side of the window opening.
Elevation of Shading Device Configurations for South Facing Facades