There are many different reasons to
want to control the amount of sunlight that is admitted into a
building. In warm, sunny climates excess solar gain may result
in high cooling energy consumption; in cold and temperate
climates winter sun entering south-facing windows can positively
contribute to passive solar heating; and in nearly all climates
controlling and diffusing natural illumination will improve
Well-designed sun control and shading devices can
dramatically reduce building peak heat gain and cooling
requirements and improve the natural lighting quality of
building interiors. Depending on the amount and location of
fenestration, reductions in annual cooling energy consumption of
5% to 15% have been reported. Sun control and shading devices
can also improve user visual comfort by controlling glare and
reducing contrast ratios. This often leads to increased
productivity. Shading devices offer the opportunity of
differentiating one building facade from another. This can
provide interest and human scale to an otherwise undistinguished
The use of sun control and shading
devices is an important aspect of many energy-efficient building
design strategies. In particular, buildings that employ
daylighting often depend on well-designed sun control and
During cooling seasons, external window shading is an
excellent way to prevent unwanted solar heat gain from entering
a conditioned space. Shading can be provided by natural
landscaping or by building elements such as awnings, overhangs,
and trellises. Some shading devices can also function as
reflectors, called light shelves, which bounce natural light for
daylighting deep into building interiors.
The design of effective shading devices will depend on the
solar orientation of a particular building facade. For example,
simple fixed overhangs are very effective at shading
south-facing windows in the summer when sun angles are high.
However, the same horizontal device is ineffective at blocking
low afternoon sun from entering west-facing windows during peak
heat gain periods in the summer.
Exterior shading devices are particularly effective in
conjunction with clear glass facades. However,
glazings are now available that have very low shading
coefficients (SC). When specified, these new glass products
reduce the need for exterior shading devices.
Thus, solar control and shading
can be provided by a wide range of building components
- Landscape features such as mature trees or hedge rows;
- Exterior elements such as overhangs or vertical fins;
- Horizontal reflecting surfaces called light shelves;
- Low shading coefficient (SC) glass; and,
- Interior glare control devices such as Venetian blinds
or adjustable louvers.
Fixed exterior shading devices such as overhangs are
generally most practical for small commercial buildings. The
optimal length of an overhang depends on the size of the window
and the relative importance of heating and cooling in the
In the summer, peak sun angles occur at the solstice on June
21, but peak temperature and humidity are more likely to occur
in August. Remember that an overhang sized to fully shade a
south-facing window in August will also shade the window in
April when some solar heat may be desirable.
To properly design shading devices it is necessary to
understand the position of the sun in the sky during the cooling
season. The position of the sun is expressed in terms of
altitude and azimuth angles.
- The altitude angle is the angle of the sun above the
horizon, achieving its maximum on a given day at solar noon.
- The azimuth angle, also known as the bearing angle, is
the angle of the sun's projection onto the ground plane
relative to south.
- An easily accessed source of information on sun angles
and solar path diagrams is
Architectural Graphic Standards, 10th Edition
available from John Wiley & Sons, Inc. Publishers.
Shading devices can have a dramatic impact on building
appearance. This impact can be for the better or for the worse.
The earlier in the design process that shading devices are
considered they more likely they are to be attractive and
well-integrated in the overall architecture of a project.
ASHRAE/IES Standard 90.1 "Energy Efficient Design of New
Buildings Except Low-Rise Residential Buildings" (on which
the Federal equivalent
10 CFR 435 is based), the degree of window shading is a
major consideration. Both the projection factor (PF) for
exterior shading and the shading coefficient (SC) of glass must
be evaluated when using the Alternate Component Packages
envelope design approach.
Designing Shading Systems
Given the wide variety of
buildings and the range of climates in which they can be found,
it is difficult to make sweeping generalizations about the
design of shading devices. However, the following design
recommendations generally hold true:
- Use fixed overhangs on south-facing glass to control
direct beam solar radiation. Indirect (diffuse) radiation
should be controlled by other measures, such as low-e
- To the greatest extent possible, limit the amount of
east and west glass since it is harder to shade than south
glass. Consider the use of landscaping to shade east and
- Do not worry about shading north-facing glass in the
continental United States latitudes since it receives very
little direct solar gain. In the tropics, disregard this
rule-of-thumb since the north side of a building will
receive more direct solar gain. Also, in the tropics
consider shading the roof even if there are no skylights
since the roof is a major source of transmitted solar gain
into the building.
- Remember that shading effects daylighting; consider both
simultaneously. For example, a light shelf bounces natural
light deeply into a room through high windows while shading
- Do not expect interior shading devices such as Venetian
blinds or vertical louvers to reduce cooling loads since the
solar gain has already been admitted into the work space.
However, these interior devices do offer glare control and
can contribute to visual acuity and
comfort in the work place.
- Study sun angles. An understanding of sun angles is
critical to various aspects of design including determining
basic building orientation, selecting shading devices, and
Integrated Photovoltaic (BIPV) panels or solar
- Carefully consider the durability of shading devices.
Over time, operable shading devices can require a
considerable amount of maintenance and repair.
- When relying on landscape elements for shading, be sure
to consider the cost of landscape maintenance and upkeep on
- Shading strategies that work well at one latitude, may
be completely inappropriate for other sites at different
latitudes. Be careful when applying shading ideas from one
project to another.
Materials and Methods of Construction
In recent years, there has been a dramatic increase in the
variety of shading devices and glazing available for use in
buildings. A wide range of adjustable shading products is
commercially available from canvas awnings to solar screens,
roll-down blinds, shutters, and vertical louvers. While they
often perform well, their practicality is limited by the need
for manual or mechanical manipulation. Durability and
maintenance issues are also a concern.
Require A&E professionals to fully specify all glass. They
should be prepared to specify glass U-value, SC, and Tvis and
net window U-value for all fenestration systems. The shading
coefficient (SC) of a glazing indicates the amount of solar heat
gain that is admitted into a building relative to a
single-glazed reference glass. Thus, a lower shading coefficient
means less solar heat gain. The visible transmittance (Tvis) of
a glazing material indicates the percentage of the light
available in the visible portion of the spectrum admitted into a
building. See also WBDG
Windows and Glazing.
When designing shading devices, carefully evaluate all
operations and maintenance (O&M) and safety implications. In
some locations, hazards such as nesting birds or earthquakes may
reduce the viability of incorporating exterior shading devices
in the design. The need to maintain and clean shading devices,
particularly operable ones, must be factored into any
analysis of their use.
Among the primary types of
buildings that can benefit from the application of shading
- Barracks and other multi-family housing projects,
especially in temperate or warm climates;
administration buildings, and other structures employing
- Nearly any structure in warm, sunny climates.
RELEVANT CODES AND STANDARDS
Construction Criteria Base is the source for the full text
of all federal construction documents including Department of
Defense Unified Facilities Guide Specifications (UFGS), Unified
Facilities Criteria (UFC) and Federal Publications and
Standards. It is available on CD-ROM and DVD from the
Institute of Building Sciences (NIBS), 1090 Vermont Avenue,
NW Suite 700, Washington, DC 20005-4905; phone: (202) 289-7800;
fax: (202) 289-1092.
Building / Space Types
Applicable to all
Preservation—Update Building Systems Appropriately,
Productive—Assure Reliable Systems and Spaces,
Productive—Promote Health and Well-Being,
Productive—Provide Comfortable Environments,
Secure / Safe,
Sustainable—Optimize Energy Use
Products and Systems
Section 07 92 00:
Joint Sealants, Building Envelope Design Guide:
Delivery and Controls,
Version 2.1 Credit / WBDG Resource Page Matrix,
Antiterrorism Standards Tool
Low-E Glazing Design Guide
by Johnson, Timothy E. Boston: Butterworth Architecture,
1991.—An excellent discussion of high-performance glazings
and their specifications including shading coefficients.
Simplified Design of HVAC
Systems by Bobenhausen, William. New York: John Wiley
& Sons, Inc., 1994.—An excellent source for information on
the basics of solar geometry and sun angles.
Solar Control and Shading
Devices by Olgyay, Aladar and Olgyay, Victor.
Princeton, New Jersey: Princeton University Press, 1957.—The
classic source for information and examples of sun control
and shading devices.
Sun Protection: An
International Survey by Danz, Ernst. New York:
Frederick A. Praeger, 1967.—Out-of-print, but worth finding,
it offers many provocative photographs and illustrations of
shading devices from around the world.
Federal Energy Management Program (FEMP)
offers a series
of workshops on the use of renewable energy sources in
federal buildings. This includes a segment on shading
design. Contact FEMP for workshop dates and locations.
Design and Analysis Tools
There are a number of building analysis and design tools that
permit users to evaluate the effect of different sun control and
—Description: Comprehensive hour-by-hour
simulation; daylighting and glare calculations integrate
with hourly energy simulation. IBM or compatible Pentium is
, Lawrence Berkeley National Laboratory
Radiance 3.5 (2003)
—Description: A ray-tracing program
that accurately predicts light levels and produces photo
realistic images of architectural space in all sky
conditions. Sun Microsystems, DEC, Macintosh with (AUX),
CRAY or other UNIX machine. Availability: Environmental
Energy Technologies Division, Lawrence Berkeley National