Siting

The location of a building in terms of its exposure to the climatic conditions can greatly affect the energy consumption. Equator facing slopes in temperate climates have greater solar insolation potential than other slopes. In Temperate and Cool climates a sheltered equator facing location is preferable to take advantage of maximum solar gain. In hot climates west slopes should be avoided where possible due to their higher solar heat gain and thus higher temperatures. A slope facing away from the equator can lead to overshadowing through the buildings as well as the slope. In hot climates a pole facing slope can provide valuable shade. 

Buildings built higher on slopes, especially in cool and temperate cliamtes, are more exposed to cooling effects of breezes especially on windward slopes thus increasing heating consumption in winter. Lower locations especially in valleys can however also be exposed to downslope winds especially at night when cooler air currents sink. In Hot and Arid climates, locations in valleys and on hill slopes can benefit from cool sir sinking along the slope at night. In Hot and Humid climates exposed locations are preferable for better ventilation conditions.

The orientation and width of the streets affect the urban ventilation conditions and the solar exposure of buildings. They are of a greater importance in densely built urban areas, such as commercial and high-density residential areas.

An optimum street layout, which provides good ventilation conditions for pedestrians in the streets and to the buildings along the street, is one with wide main avenues orientated at an oblique angle to the prevailing winds. In this way, wind can blow through the city. In climates, such as Hot – Arid, where shading is of prime importance, narrow streets ensure the shading of buildings during summer and limited solar exposure of the buildings along the streets. In hot – humid climates the street layout should be spacious to allow cooling breezes between the buildings. In temperate and cold climates, where solar insolation can play a large role in passive design, care should be taken that little over-shadowing takes place.

The height of buildings is an important factor determining the wind pattern over a built-up urban area and this, in combination with the distance between the buildings, characterises urban ventilation conditions. The airflow around and over buildings is slowed down but has higher turbulence, due to friction on them. Thus an urban wind pattern is characterised by a lower average wind speed with higher local air speeds and higher turbulence than in an open area.

Long rows of high, long buildings of the same height, which are perpendicular to the prevailing wind direction, block the wind at the first row and divert it upwards, creating poor ventilation conditions both along the streets and inside the buildings of the rows behind. The wind speed in the spaces between the buildings gradually decreases after the first row. A row of buildings of different heights with their façades at an oblique angle to the wind direction, improve ventilation conditions both on an urban scale and inside the buildings. If the buildings are of variable tallness, even a highly dense area may have better ventilation conditions than an area of lower urban density where buildings are of the same height.

Tall, narrow buildings spread around an urban area significantly increase the air speed in the streets through the deflection of urban wind. This is due to the Consideration should be given to their performance under winter conditions, as the high air turbulence could cause problems to pedestrians and surrounding buildings.

High buildings closely spaced buildings can create canyons, which can amplify the heat island effect through absorption and reflection. The high urban densities with large thermal mass and high albedo can store the solar radiation leading to increases in surface temperatures. In addition to this solar insolation is reflected and absorbed by the surroundings instead of being reflected into the atmosphere increasing the temperatures in the urban canyons. This in turn can lead to higher energy consumption for cooling. Lighter coloured surfaces and surfaces with less thermal mass can drastically reduce surface temperatures. Using “cool materials” in urban environment planning contributes to lower surface temperatures which affect thermal exchanges with the air, improve comfort in outdoor areas, and decrease the ambient temperature (Akbari et al., 1989).

Urban climate and heat island effect
Urban climates show considerable differences from that of the surrounding countryside.

Average Change of climate parameters in dense urban areas

Many factors, which determine the climatic conditions, are quite different to that of the surrounding countryside. This leads to the so called urban heat island effect. The so-called urban heat island effect is a phenomenon that occurs in cities, where the air temperature is higher in the city than in the surrounding area. The difference in temperature is generally between 1 K and 4.5 K, but larger ranges of up to 10 K have been observed (Santamouris, 2001). In regions with hot climatic spells, this leads to a further increase in thermal discomfort as well as energy consumption for cooling. This is mostly due to the sealed and darker surfaces with high thermal mass within the urban fabric with fewer trees and vegetation than in rural areas.

These surfaces have a low effective albedo and heat up during the day and store the heat. The canyon effect where the reflected solar radiation is absorbed by surrounding buildings can also aggravate this urban heat island. Airborne particles and pollution although reducing the total solar insolation also trap the heat further aggravating the problem. In addition to this, the vegetation that usually provides shading and cooling through evaporation is greatly diminished in urban areas. Rainwater runs off quickly and does not provide cooling through evaporation. In hot climates the urban heat island phenomenon can have a serious impact on the cooling load of buildings. It becomes increasingly challenging to design fully naturally ventilated buildings as temperatures rise. In temperate and cool climates on the other hand the assumed benefits for winter are small at least when compared to extra cost in the summer.

Urban areas with poor microclimatic quality due to the heat island effect use more energy for air condi-tioning in summer than areas with a higher climatic quality. LBNL estimates that about 20% (about $5 billion per year) of the U.S. national cooling demand can be avoided through a large-scale implementa-tion of heat-island mitigation measure such as cool surfaces and shade trees (Akbari, Pomerantz and Taha, 2001). In Greece it has been shown that the heat island effect increases the cooling load of buildings in Athens by 100% (Santamouris et al., 2001).

Ground and building surfaces
Ground surfaces and building surfaces can also affect the temperatures surrounding buildings. Due to the thermal absorption properties of the ground surfaces these can absorb the solar insulation and heat up higher than the surrounding air. Surfaces with high thermal mass will absorb solar insolation and transfer this heat to the surrounding air. Solid surfaces such as asphalt can reaches temperatures up to 44 °C higher than the surrounding air (Koenigshberger, 2010).

Landscaping with well designed use of vegetation can positively modify the urban climate. It is a highly effective strategy for lowering air and surface temperatures in cities, increasing the relative humidity of the air, functioning as shading devices and channelling the wind flow. Increasing the vegetation cover by 10-30%, which corresponds with one to three properly placed trees per house, may reduce the required cooling energy by as much as 50%. In addition, plants can control air pollution, filter dust and reduce the level of noise to a certain degree. Vegetation can be integrated into urban areas in the form of landscaped parks, small neighbourhood playgrounds, trees along streets and gardens around buildings.

Parks and green spaces
The influence of green spaces on the direct environment only extends a short distance into the surrounding area. It is more effective to have many small green areas than a few large parks. Integrating plants in between buildings, as in small gardens, is therefore more beneficial and reduces temperatures in the immediate surrounding of the building and therefore also inside the building. On summer days, in temperate climates, parks and planted area between 50 m to 100 m in width can be up to 3.5 K lower than surrounding built areas and temperatures over grass surfaces are also about 1 K to 2 K cooler than surrounding paved areas.

Trees and other plants
Plants offer shade to surfaces and buildings and modulate the surrounding air temperature. Trees absorb most of the solar radiation falling on their leaves, just like dark coloured surfaces, and reradiate back only portion of it. Evergreen leaves also reflect infrared light, up to 50% of near infrared. Strategic planting of trees and shrubs next to buildings can reduce summer air-conditioning costs by 15-35% and even up to 50% in specific situations (Santamouris 2001). In contrast to asphalt roads the leaves do not however store the energy as heat but use it for evaporating water. This process cools down the leaves, and consequently the air in contact with them, and releases vapour, thus increasing the air humidity. The remaining small portion of the absorbed solar radiation is used for photosynthesis. Leaves with open pores release about 50-70% of the amount of vapour that would be released from the same water area, under the same climatic conditions, effectively releasing a quantity of water about five times that of the leaves weight (Santamouris 2001). It has been estimated that a full size tree can evaporate 1460 kg of water on a sunny summer day, which is equivalent to 242 kWh of cooling capacity (McQuiston and Parker, 1982). The National Academy of Sciences of United States (NAS, 1991) reported that the planting of 100 million trees combined with the implementation of light surfacing programs could reduce electricity use by 50 billion kWh per year. This is equivalent to 2% of the annual electricity use in the US (NAS, 1991).

The position of plants around the building should be strategically chosen in order to provide shade at the most critical hours of the day. Shading of the windows is the most beneficial. Horizontal overhangs are preferable at the equator facing side of the building, whereas trees are best used on the east and west façades as the sun is at low altitudes in the morning and afternoon. Bushes and vertical trellises covered by creeping plants are also very well suited for protecting east and west façades from the low angle sun. Horizontal greened surfaces such grass can also reduce the glare and the reflected solar radiation. The most appropriate plants for good landscaping are, of course, native plants as they are accustomed to the local climate. For effective use of plants, consideration should be given to:

• Their specific biological needs such as soil type, watering needs, minimum safe temperature and exposure to the sun
• Maintenance costs
• Availability of water in the area

Green façades and roofs
The greening of façades and roofs can also play a role in improving the microclimate by providing a buffering effect. Roof and façade surfaces are often high heat absorbers due to their low albedo. Research at Trent University have shown that temperatures on a green building roofs on a typical day with an air temperature of 18.4 °C a normal roof surface temperature was 32 °C while that of a green roof was 15 °C. They also affect the microclimate of by reemitting the heat to the surroundings as well as not storing rainwater for later evaporation. Green roofs also offer some insulation. Care must however be taken with the structural planning as these can add significantly to the load.

Trees and bushes can also be positioned in such a way so as to improve ventilation inside the building by guiding the cooling breezes into the building where the main windows do not face the flow of air or even by preventing the wind from spilling around the sides of the building. A correctly planted tree can also cause wind acceleration underneath it.

Wind can be have both negative and positive effects of the thermal conditions within a building and thus on the energy consumption of the building. In colder climates they can increase the convection at the building surface as well as increase the air infiltration rate through pressure differences on the building surface both of these leading to a higher energy consumption due to higher heating loads. In warm climates they can have a cooling effect either by removing excess thermal load or by their air cooler temperatures. In all climates the effect is dependent on the wind temperature and velocity. Wind velocity varies drastically according to the proximity to the ground, the shape of the topography, vegetation and buildings.

Windbreaks can be used to impede or direct wind as needed if carefully planned. Windbreaks can either be built structures or vegetative windbreaks. Depending on the desired effect, solid, impermeable structures can be chosen which are very effective, when the buildings are positioned directly behind the windbreak (at 1 – 2 times the windbreak height) but can cause turbulence immediately downwind. Permeable structures, either built or vegetative, are slightly less effective but create a larger protected area (up to 5 times the windbreak height) with less turbulence much further downwind than solid structures.

Bushes and trees can be strategically positioned to act as windbreaks or barriers to cold winter winds. Double row vegetative windbreaks, consisting of two rows of bushes planted close to one another, are highly effective and create a large calmed area with less turbulence. The type and density of planting used, will have a significant influence on airflow patterns. A dense forest can for example reduce the wind velocity by 60% to 80 % by up to 30 m after the natural windbreak.

In Cold climates due to a decrease in wind speed, the ambient temperatures behind the windbreak can be up to 3.3 K higher than in an open field and thus reduce the heating load (Robinette, 1983) and create improved thermal comfort conditions in winter. In Hot and Arid Climates where seasonal hot winds such as the Khamsin blow, building downwind of green areas can have a positive effect as the wind is slowed, filtered as well as cooled by the “Oasis” effect due to the natural evapotranspiration of the vegetation. In Hot and Humid Climates however windbreaks should be avoided as due to the high humidity any cooling effect from the air can only be gained through air movement.

Water features can modify the microclimate of the surrounding area by reducing the ambient air temperature through evaporation and by contact of the hotter air with the cooler water surface, due to its high thermal mass. Fountains, ponds, streams and waterfalls or mist sprays can be used as cooling sources in order to lower the air temperature of outdoor spaces and therefore the ventilation air entering a building.

As water surfaces increase the air humidity, they can be problematic in very humid climates and thus circulation of the air must be enforced. In hot climates, their cooling effect should be maximised through special design features, which prevent diffusion of the air cooled by them and direct this air into inhabited places. Although water surfaces are very beneficial in dry climates, they may be limited by scarcity of water. Overheating of the water, which leads to algae growth, should be avoided through the use special features such as shading with trees, bushes, etc. The water should also not become stagnant and its circulation is recommended.

The surface to volume ratio (S/V Ratio) is the amount of surface area per unit volume of an object. The S/V ratio is calculated by dividing the surface area of the building (in m²) by the volume of the building (in m³). The lower the value the more compact the building.

The form of the building plays a significant role in the operational efficiency of the completed structure. Part of the heat transfer between the outside and internal spaces takes place through the building’s shell or surface (transmission), the other parts through ventilation and infiltration. The larger the building surface compared to the internal space (Surface to volume ratio) the greater the heat loss or gain through heat transfer.

In buildings that are actively heated or cooled, the preferred form would be a square form, where the surface-to-volume ratio would be ideal in the sense that any heat transfer between the inside and outside spaces is reduced. The least amount of energy would be ‘wasted’ through the building’s envelope (Olgyay, 1992). A square building is for example up to 17% more efficient than a similar building with the same treated floor area but with an L form.

In hot and humid climates in buildings that are not conditioned larger elongated building forms are preferred as these are more open to ventilation and are able to radiate heat better. However if buildings are to be air-conditioned at a later date a compact form is more desirable.

In hot and humid climates in buildings that are not conditioned larger elongated building forms are preferred as these are more open to ventilation and are able to radiate heat better. However if buildings are to be air-conditioned at a later date a compact form is more desirable. Other factors such as daylighting, ventilation and solar heat gains must however also be taken into account in the planning of the building shape.When considering all the above, it also becomes evident that multi-family and multi-floor buildings are usually much more energy efficient than single-family homes. The surface-to-volume ratio is optimised here, and by the sharing of walls and a reduction in roof area, less heat transfer takes place between the internal spaces and the outside.

One aspect determining the optimum building form is the orientation of the building towards the sun. Orientation plays a significant role in determining the solar heat gains, which a building receives. The largest levels of solar insolation that a building receives are that of the equator facing façade in temperate and cold climates and on the pole facing façades in the tropics. It is thus on these façades that the incidental solar radiation can be best controlled depending either to take advantage of these solar heat gains for heating or to reduce them to a minimum for cooling. On these façades the high altitude midday sun can either be admitted to the space through glazed areas for heating or easily blocked by external shading elements or overhangs that also keep the walls cool where required. Walls on the East and West sides are much more prone to overheating from the low altitude morning and evening sun, as well as heat losses in cooler climates where solar gains on these façades are lower. Reduced lengths in the East and West façades and larger South/North facades therefore result in slightly elongated shapes along the East-West axis.

The optimum building form in all climate zones, in regards to solar gain, is therefore not a square one, but one, which is always slightly elongated along its east-west axis.

Daylighting limits how deep and thus how compact a building can be. If a building has a very deep floor plan, a larger proportion of the floor area needs to be artificially lit and it will have to rely more on mechanical ventilation. It is generally accepted that a space, up to about 6 m deep from a window, will be able to take advantage of natural lighting and natural ventilation. When there are windows or openings on either side, a maximum of about 14 m width is regarded as the upper limit for the floor slab. By keeping the distance from façade to façade small, cross ventilation through the space will be possible.

Buildings can be planned so that rooms are used according to the time of day and thus take advantage of passive and cooling techniques. This can help to improve energy efficiency as the need to condition rooms can be reduced to a minimum. Rooms that are not at that time being used can then act as a buffer against the adverse thermal conditions.

In cold climates zoning of different conditioned rooms in temperate climates can act as a buffer and help to improve the energy efficiency of buildings. Colder rooms such as entrance ways and stairwells should be placed away from the equator facing facade. Rooms which are used later in the day such as the living rooms should be orientated to the south and south-west to take full advantage of the passive solar gains at midday and the late afternoon. This approach offers great potential for saving energy, as it reduces the energy loss through these facades. In hot climates rooms that are used during the hottest part of the day should be located so that the solar heat gain is kept to a minimum.

Buffering
In hot climates rooms such as entrance ways and stairwells should be placed towards the facade facing the sun at the hottest part of the year to act as a buffer. Rooms that are used for the greater part of the year can then be placed on the opposite façade. Rooms that are used later in the day can be placed on a east façade as these are cooler than those on the west façade.

The location of rooms in open buildings can also be placed to take effect of cooling breezes. Rooms with high thermal loads such as kitchens should be located so that and breezes passing through a building reach these last and no excess heat is carried into other parts of the building.

Zoning of different conditioned rooms in temperate climates can act as a buffer and help to improve the energy efficiency of buildings. In Cool and Temperate climates colder rooms such as entrance ways and stairwells should be placed away from the equator-facing facade. In Cool climates rooms which required more warmth for thermal comfort such living rooms should be placed so as to receive solar radiation. In warm climates rooms which need to be cooler should be protected from solar radiation. Rooms, such as stairs, bedrooms, and bathrooms, that are used only for short periods can be used as buffers on east and west facades. This approach offers great potential for saving energy, as it reduces the energy loss through these facades.

In all climates to reduce thermal loss to the exterior air locks should be used. These help to reduce thermal loss/gain through uncontrolled air change and also act as buffer zone. Care should be taken that the doors to an airlock be kept closed and that no two doors be open at any time.