#1 - REDUCE LOADS / DEMAND FIRST - Shading (Heat Avoidance)
"THE EASIEST WAY TO REDUCE THE ENERGY REQUIRED TO COOL A BUILDING IS TO AVOID HEAT GAIN"
CONTROL OF SOLAR RADIATION THROUGH SHADING:
In order to control sun penetration to the interior of buildings it is important to provide exterior shading as a part of the architectural envelope design. Such shading devices can be attached to the building or can be achieved by the articulation and disposition of the building floors to create overhangs.
Sources of Solar Radiation that May Require Shading
Exterior shading is greatly preferred over interior shading as it is important to keep the solar radiation/heat from entering the building. Traditional interior blinds or drapes merely block the glare of the sun, but still allow the heat to enter the interior space.
Benefits/Detriments of Shading Locations and Types
Incidence Angle Shading:
At high angles of incidence, where the sun's rays are almost parallel to the window surface (at glazing angles), glass becomes more reflective. At incidence angles of 80o, both single and double pane glazings (clear plate glass) become about 50% reflective. This geometry occurs when profile angles are very high, window azimuth angles are high, or both. This will reduce solar radiation transmitted through the glass by an equal percentage. Some other problems like interior glare or reflected glare from the facade to the exterior may occur.
Air Mass Shading:
As a function of solar altitude, the air mass extinction becomes critical at about air mass 5.0. This relates to a solar altitude of 12 degrees above the horizon or less. At these low altitudes, the sun is more of a glare problem than heat gain. This notion works against a building which tries to collect low winter sun. Shading devices should allow for the fact that incidence and air mass shading will eliminate some periods of the year from consideration but may still require shielding from glare.
Diffuse Solar Energy:
Shading devices are usually configured to provide agains direct beam sunlight and from the brightest part of the sky on cloudy days. Some scattered (diffuse) energy will still arrive at the window. For a fully shaded window, we usually consider the incident solar energy to still equal that of a expose window on a shaded north elevation. The actual insolation received for a fully shaded window is a function of how much sky the window 'sees' and how reflective the foreground landscape is.
DIFFERENTIATED FACADE TREATMENT:
An understanding of solar geometry tells us that the exposure of each facade to the sun is different, and varies by orientation. Each orientation of the building requires a different approach to the design of shading. The north elevation (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.
Vegetative Shading
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
DURABILITY AND MAINTENANCE:
As exterior shading devices can add significant cost to the building budget, it is imperative that they be designed to be durable and properly integrated into the base envelope design. If not understood by the client as a critical means of heat avoidance, their removal as a result of "value engineering" will undermine efforts towards designing an energy effective envelope - and eventual carbon neutrality.
When designing south facing horizontal shading devices, a choice will need to be made as to whether or not these are solid or louvered. Solid shading devices may also be used (depending on their placement) as light shelves - used to reflect sunlight deep into the space to assist with daylighting. Solid shades will collect snow and ice, so are not desirable in cold climates. Louvered shades allow for air flow up the facade to assist with natural ventilation. They too will collect snow and ice, but allow for the same to drain more quickly, reducing the need to increase their structural requirements.
Louvered Shades at St. John Ambulance, Edmonton
Watch for falling ice and snow
Also ensure that the shading devices are easy to keep clean and do not make the cleaning of windows difficult. On high rise applications they can get in the way of standard window washing equipment.
Diagram Credit: Val Aiken
HELPFUL LINKS IN THE AIA 50to50 WIKI:
Building Form
Building Orientation
Passive Solar Collection Opportunities
Sun Shading
Vegetation for Sun Control
Windows and Openings
HIGHLY RECOMMENDED TEXTS FOR PASSIVE DESIGN (written by SBSE members!):
Heating, Cooling, Lighting: Sustainable Design Methods for Architects. Third Edition. Norbert Lechner.
Mechanical and Electrical Equipment for Buildings. Benjamin Stein, John Reynolds, Walter Grondzik, Alison Kwok.
Sun, Wind and Light: Architectural Design Strategies. Second Edition. G. Z. Brown and Mark DeKay.
The Green Studio Handbook: Environmental Strategies for Schematic Design. Walter Grondzik and Alison Kwok.
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Passive Heating