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Elements of Intelligent Design - Site Selection and Planning

 

Introduction

The site chosen for the project will have major impacts on the comfort of the home's occupants and on the functioning of the building and its systems. In fact, the feasibility of using natural ventilation for cooling may depend completely on proper siting, depending on the overall site conditions. Thus, consideration of the ventilative and thermal implications of site selection and planning must be given the highest priority in the earliest stages of the design process

Sites in which the slope, elevation, orientation, vegetation and wind pattern act to increase summer and winter cooling by wind and decrease radiation effects by shading should be used. Locations near large bodies of water may be preferable if cooling breezes can be directed into the building.

To minimize heat gains from solar radiation, south, south-southeasterly and northern slopes are preferable. West and east facing slopes should be avoided due to the difficulty of providing adequate shading. The most desirable wooded sites have high tree canopies and open trunk areas, permitting air movement while providing shade. Avoid sites with dense low canopy trees which block breezes and trap humidity in dead air pockets.

 

Priorities

The first priority is to identify the most suitable location on the site to take advantage of favorable characteristics of the site and its microclimate(s), and to mitigate adverse aspects. Siting and design of the home must be related to external spaces and surroundings, as these will provide additional, alternative living areas.

In placing a building or group of buildings, provision for air movement is one of the most important considerations. Major factors to be considered in assessing local features that affect site planning are:

 

Local Winds

In addition to the general winds caused by large-scale, regional weather patterns, there are predictable 'local' winds induced by features of the terrain. The differential heating of land and water cause sea and land breezes in many coastal locations. The sea breeze tends to move inshore around midday as the land warms relative to the waterbody and the pressure differences increase, while offshore breezes develop later in the evening when the land becomes relatively cooler.

Frictional resistance of the land surface often causes the incoming air to dam up and form a small scale front which progresses inland throughout the afternoon. In locations where there is not a great temperature difference between land and water, the sea breeze layer will be shallow and the velocities weak. Tall buildings along a waterfront can completely block such a breeze.

Slope winds are caused by the radiant heating and cooling of inclined surfaces, which cause temperature differences between the air over the inclined surface and air at the same level some distance from the slope. This causes heated air to rise along hillsides in daytime and cool air to descend down slopes at night.

When slopes are arranged in a valley system, a combination of slope winds and temperature differences from valley to plain cause valley winds. these are considerably stronger than slope winds. Generally , the strongest winds are found in U-shaped valleys that have high ridges lining them and which open onto a broad plain with a considerable temperature difference between the plain and the head of the valley. Valleys oriented north or south have the strongest daytime breezes due to increased exposure to the sun.

 

Existing Buildings & Other Obstructions

New buildings can be affected by, and have an effect on, existing buildings around them and should, therefore, be sited to preserve each building's access to prevailing breezes. The most important influences on urban winds are:

  • Dimensions of obstructions: In general, the larger and taller the obstruction, the longer the wake.
  • Spacing between obstructions: A minimum clear spacing of five heights of the upwind obstruction is required to avoid effect.
  • Homogeneity or variability of building height: A highrise building in an area of low-rise development may create strong air currents at ground level that should be accounted for.
  • Orientation of streets with regard to prevailing winds: If streets are parallel to the prevailing winds, the wind will be funneled into the streets. If streets are perpendicular to the prevailing winds and buildings are continuous, the flow will depend on the street width. When siting the project, placement within grid patterns of buildings require large building-to-building spacing to maintain ventilation due to shapes of building wakes. If the buildings are staggered in a checkerboard pattern perpendicular to the wind, [FIGURE LINK TO COME] ventilation can be maintained with closer spacing and wake effects are somewhat reduced.

 

Existing Vegetation

Tall vegetation may substantially reduce windspeed at ground level. Trees are very effective at absorbing wind energy rather than deflecting it as do solid obstructions such as the terrain and buildings. Two types may be categorized: forest and windbreak. Within a forest, the velocity is a minimum near the center of the mass of the foliage in the crown; in the absence of underbrush there is a velocity increase among the tree stems.

The shape of the wind profile in a forest is contingent on the type of trees, their spacing and openings in the crown, and the distance from the edge of he stand from which ground level wind can penetrate.

Distribution, size, density, and details of planted and open areas: Planted areas can have a pronounced effect on airflow patterns and speeds. In general, grassy open areas without dense trees or bushes allow the air close to the ground to be cooled and to return to its unobstructed velocity. Sunlit open areas with manmade surfaces may heat the air above them and should be minimized on the windward sides of naturally-ventilated buildings. Trees can provide shade, but may also block wind if their understory is too dense.

The extent of the sheltered area produced by a windbreak varies with the physical dimensions and porosity of the barrier. Porous barriers cause less turbulence and can create a greater area of total shelter than solid barriers. the more solid the barrier, the shorter the distance to the point of minimum wind velocity and the greater the reduction in velocity at that point. The velocity, however, increases more rapidly downwind of the minimum point than behind a more porous barrier.

A porosity of 40% to 50% has been found to provide maximum extent of sheltered area. This reduction in leeward velocity occurs without appreciable disturbance of the airflow. Windbreaks with higher porosities do not form a turbulent wake and the airflow pattern is dependent on the velocity of the flow. Windbreaks with lower porosity exhibit a turbulent wake that provides more protection with velocities reduced to 10% of the free stream.

 

Topography

Topography has a pronounced effect on the wind at the surface. Wind flow conforms to terrain, changing its strength, steadiness, and direction as it passes over uneven ground. In general, wind acceleration on the windward side of hills and ridges is fairly predictable, but the extent of shelter in the lee is highly variable, depending on the terrain's roughness and stability of the atmosphere.

Maximum ventilation is achieved when a building is placed near the crest of hills as opposed to flat ground or, worse, in enclosed valleys or very sheltered locations. In very windy regions, locations on hills may be exposed to too much wind and may be susceptible to other problems. In general, sites at or near the top of a slope and facing south to southeast are recommended when continuous ventilation is desired.

 

Slope

A sloping site may affect the heat gain of the home if it restricts the orientation of the building and its windows.

The optimal orientation for the long faces of a building and for windows is north-south.

Sloping sites which require placement of windows to the east or west should be avoided because they are more difficult to shade.

 

Solar Shading

Topographic features and obstructions may provide shade and reduce solar gains. Buildings can be arranged to provide shade for adjacent structures and exterior spaces. The extent and timing of shading due to nearby obstructions can be determined using a sun path diagram.

Close building spacing may decrease natural daylight and adversely affect ventilation. Daylighting is not usually a problem for residential types of buildings in hot climates. Whether the ventilation is affected depends largely on the direction of the prevailing wind.

 

Reflectance

The reflectance of nearby surfaces, especially obstructions and ground surfaces near openings, can have a large effect on the interior temperatures of the building. Reflected light and local heat sources, such as nearby asphalt pavement, can substantially increase internal temperatures of naturally ventilated buildings and should be avoided, especially on the windward side.

 

Elevation/Altitude

Although not a concern in Florida, with increasing altitude, temperature and pollution decreases; precipitation (rainfall, snow, and fog), insolation, and daily temperature range increase.

 

Proximity to Water

Proximity to large bodies of water may serve to moderate temperature extremes because water stores more and radiates less solar energy than soil. Thus, near-ocean-front locations experience more moderate temperatures than further inland. On a smaller scale, ponds or sprays may be used to provide evaporative cooling when located near interior spaces when the climate is not too humid. This is not a method that is appropriate in Florida.