on

Recommended Sun-Shading of

A Southern Elevation of Akure

Compiled by

Ayejuyoni Babatunde

ARC/05/ 5598

ARC 808 (Professional Practice Procedure II)

ASSIGNMENT

1.0 INTRODUCTION

SUBMITTED

TO

DEPARTMENT OF ARCHITECTURE, SCHOOL OF POST GRADUATE STUDIES, FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE ONDO STATE.

LECTURER: PROF. OLU.O. OGUNSOTE

September, 2011

Well planned external shading is the most effective method of reducing solar heat gain. In

addition, it offers possibilities for incorporating daylighting and passive heating. Some types

of external shading interfere with view, while other types make it possible to exploit view

that would otherwise be impossible because of solar glare. External shading is much easier to

integrate into the design of a new building than it is to retrofit. External shading has a major

effect on the appearance of the building, and it must be anchored strongly to the building

structure to resist wind loads. In retrofit, both of these factors are serious challenges.

Each face of a building requires a different shading treatment because sunlight strikes each

side from different angles. A south face is best shaded with horizontal shading. East and west

faces require shading that blocks sunlight entering at low angles. A north face can often be

left unshaded.

Shading devices can substantially reduce the cooling load of buildings. According to a

literature review (Dubois, 1997), this reduction is between 23-89% depending on the type of

shading device used, the building orientation, the climate, etc. In order to save energy,

shading devices should be integrated to a building’s facade at an early design stage.

This can be achieved using ”traditional” design tools like solar path diagrams and shading

masks or special computer programs that automatically ”generate” the optimum shading

device geometry as a function of a set of input parameters (e.g. orientation, latitude).

1.2 THE MOVEMENT OF THE SUN

The earth rotates on its north south axis in a 24 hour period and orbits the sun in a period of

one year. The rotating axis is at an angle of 23 degrees. The height at which an observer sees

the sun over the horizon (azimuth angle) depends on its location (latitude), the season

(position of the earth in its orbit) and on the time of the day (rotation of the earth). The

azimuth angle expresses the position of the sun over the horizon.

1.5 THERMAL LOADS FOR ROOFS AND FACADES

Generally, roofs are subject to much higher thermal loads than facades. East and west

facades are subject to even higher loads in summer than the south facades. South facades

however are suitable for most living spaces since the relatively flat sun falls almost

exclusively on the vertical surfaces.

The largest thermal loads on the facades are present on the south east and south west facades.

The south facades are easier to control thermally due to the steep incident sunlight.

1.6 MOVING LIGHT SOURCE

The sun is a moving light source that occupies the same position only twice per year- in the

first and second half of the year, respectively. In observing the position of the sun, it is not

only necessary to observe the solar azimuth angle, but also the direction of the incident

sunlight. Daylighting of interiors is dependent on the orientation and, in particular, latitudes

and specific weather data.

2.0 SHADING DETERMINATION TOOLS

2.1 Traditional tools

Although there exist numerous design methods based on solar path diagrams (Dourgnon,

1965; Etzion, 1992), the Olgyays’ (1992) and Mazria’s (1979) methods are probably the most

popular ones. In both the Olgyays and Mazria’s design methods the building’s overheating

period is plotted onto the solar path diagram and a shading ”mask” that avoids direct sun

during the overheating period is defined.

The main difference between the two methods is the kind of solar projection used. The

Olgyays used a projection of the sun onto a horizontal plane parallel to

the ground (Fig. 1) while Mazria used a projection onto a vertical cylinder (with the long axis

perpendicular to the ground). By “unfolding” the cylinder, a two-dimensional

diagram is obtained, where the abscissa and ordinate represent the solar azimuths and

altitudes and where the curves radiating away from the south represent the solar

time (Fig. 2). This projection is advantageous for studies of facade elements like windows

and shading devices since the sun’s projection is viewed “parallel” to the

building facade. Traditional methods have some limitations: their accuracy is limited by the

size of the charts and they yield shading devices that are larger than necessary since they are

only capable of returning a “binary” answer (Etzion, 1992). This is due to the fact that they

indicate an “unshaded” condition even when a small area of the opening is lit by direct sun

and a “shaded” condition the rest of the time.

Fig. 1 Solar path diagram used by the Olgyays (1957).

Fig. 2 Solar path diagram used by Mazria (1979).

2.2 Computer tools

Many computer design tools for shading have been developed during the past decades. These

tools are in essence similar to traditional tools but have the main advantage that the shading

device geometry is automatically ”generated” by the program.

One of the first computer design tools for shading was proposed by Shaviv (1975, 1984).

This program indicates the shape of the shading device that prevents direct

radiation from reaching the window during each month. More recently, a program combining

simulation, generation and optimisation routines was developed by Kabre (1999). This

program provides a 3D image of the optimum exterior fixed shading device, which is

determined by weighting the ”shading” versus ”heating” efficiency of

the window-shade combination. The optimum solution is determined from the results of

energy simulations and a Pareto optimisation

3.0 SHADING STRATEGIES

Control intense direct sunlight to ensure a comfortable workspace.

• This is critical for occupant visual and thermal comfort and for minimizing mechanical

cooling loads.

• Direct sun is acceptable in less demanding spaces, such as circulation zones, lobbies, eating

areas, etc.

3.1 EXTERIOR DEVICES

• Use exterior shading, either a device attached to the building skin or an

extension of the skin itself, to keep out unwanted solar heat. Exterior systems are

typically more effective than interior systems in blocking solar heat gain.

• Design the building to shade itself. If shading attachments are not aesthetically

acceptable, use the building form itself for exterior shading. Set the window back in a deeper

wall section or extend elements of the skin to visually blend with envelope structural features.

• Use a horizontal form for south windows. For example, awnings, overhangs, recessed

windows. Also somewhat useful on the east and west. Serves no function on the north.

• Use a vertical form on east and west windows. For example, vertical fins or recessed

windows. Also useful on north to block early morning and late afternoon low

sun.

• Give west and south windows shading priority. Morning sun is usually not a serious heat

gain problem. If your budget is tight, invest in west and south shading only.

• Design shading for glare relief as well. Use exterior shading to reduce glare by partially

blocking occupants’ view of the too-bright sky. Exterior surfaces also help

smooth out interior daylight distribution.

• The shade’s color modifies light and heat. Exterior shading systems should be light

colored if diffuse daylight transmittance is desired, and dark colored if maximum reduction in

light and heat gain is desired.

• Fixed versus movable shading. Use fixed devices if your budget is tight. Use movable

devices for more efficient use of daylight and to allow occupant adjustment; first cost and

maintenance costs are higher than with fixed devices. Use movable devices that are

automatically controlled via a sun sensor for the best energy savings.

3.1.1 Shading Strategies in the Window Plane

• Use exterior shades for a smooth facade. Exterior shade screens are highly effective on all

facades and permit filtered view.

• Use roller shades for a movable alternative. Open weave exterior shades are not as

effective, but acceptable.

• Don’t rely on dark glazing. Glazing treatments (reflective coatings, heavy tints, and

reflective retrofit film) can be effective at reducing heat transfer. They allow direct sun

penetration but with reduced intensity. This may not be an effective shading strategy from an

occupant’s perspective unless the transmittance is very low to control glare, e.g., 5- 10%.

Fritted glass, with a durable diffusing or patterned layer fused to the glass surface, can also

provide some degree of sun control, depending upon the coating and glass substrate

properties, but may also increase glare.

• Between glass systems. Several manufacturers offer shading systems (e.g., blinds) located

between glazing layers. Some are fixed and others are adjustable.

3.2 INTERIOR DEVICES

• Interior shading alone has limited ability to control solar gain. All interior systems are

less effective than a good exterior system because they allow the sun’s heat to enter the

building. They also depend on user behavior, which can’t be relied upon.

• If interior devices are the only shading, specify light colors in order to reflect the sun’s

heat back out. Light-colored blinds or louvers are best. Light-colored woven or translucent

shades are acceptable, but may not control glare under bright summer conditions.

• Interior shading is best used for glare control and backup shading. Supply user-

operated devices that occupants can adjust to their individual comfort needs.

3.3 DESIGN AND SELECTION ISSUES

Exterior shading requires more thought and innovation than most energy conservation

techniques because you have many choices, but not a well established doctrine for using

them. A wide variety of prefabricated shading devices have appeared on the market. For most

applications, external shading is fabricated on a custom basis by a manufacturer who

specializes in particular materials and fabrication techniques. Shading devices can be made

from metal, wood, fabric, or any opaque material.

The following factors are to be considered in recommendation of shading.

Shading Effectiveness

Simply hanging shading devices over windows may not provide much benefit, Overall

shading effectiveness depends on the performance of the device at all sun positions. For

example, a window awning on a south face may provide complete shading at noon, but poor

shading in the morning and afternoon.

Effect on View

Shading always blocks a part of the view. As a minimum, it blocks the portion of the sky

where the sun travels. On south faces, you can usually arrange window shading in a way that

preserves the view of the surrounding landscape. On east and west faces, fixed

shading may eliminate the view toward the south, or they may limit the view to a downward

angle. Movable shading on the east and west can restore views during

the portion of the day when the sun is on the other side of the building.

Daylighting Potential

External shading provides the potential of daylighting in perimeter areas, provided that the

shading never allows direct sunlight to fall inside the space. If the shading method allows

direct sunlight to enter the space even occasionally, occupants will resort to closing curtains

or blinds. Daylighting is difficult to accomplish effectively, and it requires automatic light

switching.

Appearance

External shading has a major effect on the appearance of a building. If the building is highly

stylized (e.g., neoclassical or glass cube), it may be impossible to reconcile external shading

with the original style. In such cases, the style of the building has to

change. A stylistic advantage of external shading is that you can make it have any color or

surface finish without increasing heat gain. Very little of the heat that is absorbed by the

shade is transmitted to the building interior by thermal conduction. Because of its exposed

location, the shade is cooled by the atmosphere.

Shading Masks for different shading devices

4.0 CONCLUSION & RECOMMENDATION

The sun protection effect for glass surfaces depends on several factors: the reflectivity of the

solar radiation of the applied material and its color coating The location of the shade

protection which influences the re-radiation and convection heat impacts the

specific arrangement of the applied shading and method Because of the interplay of the above

factors, it is difficult to separate the influences; however generalized conclusions about their

effect can be drawn.

It is well known that light colors reflect sun impact and dark colors absorb it. The judgment

of the eye gives an approximate measure of the relation of color to the absorption value.

Venetian blinds using off white color gives 20% more shade protection than a dark one. The

aluminum blind is 10% more protective.

The different methods of shade protection can only become categorized under certain

assumptions, such as to take certain value (as medium color or 50% light transmissions) as a

measure. With such restrictions the order of effectiveness is as follows.

1. Venetian blinds

2. Roller shade

3. Tinted glass

4. Insulating curtain

5. Outside shade screen

6. Outside metal blind coating on glass surface

7. Trees

8. Outside awning

9. Outside fixed shading device

10. Outside moveable shading device

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