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Elements of Intelligent Design - Shape & Orientation

 

Introduction

(Beginning note: Please be aware that my focus in this project is on design in Hot/Humid Regions, as that is where I am located...other climatic regions have different design criteria which need to be considered accordingly)

Careful selection of building shape and orientation are primary elements in the design of naturally ventilated buildings in Warm/Hot + Wet (Humid) climates. Building orientation will determine the intensity of solar radiation falling on the walls and roof of the building, and the ventilative effectiveness of the building openings. Building shape determines the amount of exterior surface area for a given enclosed volume and the length of the interior path of the ventilation air. Together, these factors determine the relative amount of thermal transfer through the building envelope and the potential effectiveness of a design to provide cooling by natural ventilation.

 

OPTIMAL SHAPE & ORIENTATION

In Warm/Hot + Wet (Humid) climates, shape and orientation are integral factors and must be considered together in the design process. Both shape and orientation have thermal and ventilative consequences as follows:

Thermal Considerations

The optimum shape for minimizing solar gains is achieved by elongating the north and south walls, creating a prominant east-west axis. Eastern and western exposures (walls and, especially, glazing) should be minimized, since they are difficult to shade and receive longer periods of direct radiation. Southern and northern exposures are less difficult to shade, especially with roof overhangs. A variation of 15 to 20 degrees from true south has little effect on the thermal performance of small buildings. [Figure 16 to come]

 

Ventilative Considerations

For bodily cooling, the goal is to achieve the highest average room velocity in which air movement occurs in all occupied parts of the room. For an elongated building, the largest pressure differences (which drive cross-ventilation) occur when the building is perpendicular to the prevailing wind. To encourage cross-ventialtion and maximize cooling, one-room-deep plans are preferable.

 

Window Placement

When windows are in adjacent walls, the optimum ventilation occurs with the long building face perpendicular to the wind, but a shift of 20 to 30 degrees from perpendicular will not seriously impair the building's interior ventilation. This allows a range of orientations for resolving possible conflicts with the optimum solar orientation. Wind approaching at an incidence angle of 45 degrees results in interior velocities that are 15 to 20 percent lower than when the wind approaches perpendicular to the face.

When windows are in opposite walls, a 45-degree incidence angle gives the maximum average indoor air velocity and provides better distribution of indoor air movement. Wind approaching at 90 degrees is 15 to 20 percent less effective. Wind parallel to the ventilation face produces ventilation depending entirely on fluctuations in the wind and is therefore very uncertain.

 

Elevating Buildings

Buildings elevated on columns or lateral walls can have an increased ventilative potential of up to 30 percent over that of buildings on grade. Wind velocity increases with increasing height above the ground; elevating the building raises it to an area of greater free wind speeds.

Elevated buildings also allow for inlets in the floor of the structure which can permit cool, shaded air to enter the building from below. This design is common in hot, humid climates where floors are elevated to reduce structural rot.

When situated next to water, elevated buildings can allow cooler air that has passed over the body of water to enter the building from below. Elevating the building may also be worthwhile if the ground is continually damp or when the building is located in a flood plain.

 

Spacing

Buildings must be spaced to allow winds to reach the ventilation openings. In general, it is not desirable to site buildings within the wake of surrounding structures or landscaping. In most cases, dense development should be avoided. The terrain, surrounding vegetation, and other nearby structures may be used positively to "channel" or redirect breezes into the building. On sloping sites, locations near the crest of the hill on the windward side are desirable. Valley bottoms should be avoided, since they may have reduced air movement.

Street layouts can be used to channel airflow in higher density site planning. If buildings are grouped, airflow principles should be used to determine the most suitable arrangement.

 

Resolving Conflicts Between Thermal and Wind Orientations

Where optimal solar orientation and wind orientation are in opposition, solar considerations usually take precedence. However, if the building is well- insulated, has a light external color, and has effectively shaded windows, the change in internal temperature with respect to orientation may be negligible. In such cases, ventilation will have a greater effect on the internal conditions and orientation with respect to winds should take precedence.

In general, inlets for natural ventilation can more easily be designed to accommodate for less than optimal wind orientations than solar control devices. It may be possible to partially compensate for poor orientation to the wind by detailed design of the facade and windows, and by the arrangement of the building's interior plan.

 

Note: There is more to cover on this topic, which I will continue with as soon as possible...