See Sod Roofs and Feng-shui and Biotecture below. (See also Biotecture I: Living Houses and Pleaching, and the comments on Biotecture and Turf Roofs in Non Timber Building Materials)
It is surprisig that in our search for energy-conserving cooling devices we have largely overlooked nature's own near-perfect solution. Everyone knows how pleasant it is to sit in the shade of a tree in summer but few of us think of the situation as being 'energy efficient'. Yet one mature tree potentially provides nearly as much cooling as five 3 kilowatt air conditioners (a domestic air conditioner uses generally about 2 kW). A survey conducted in an American city showed that public street trees saved approximately $US 800 per day in costs compared to their equivalent in machine air conditioning.
The concept of landscaping for energy conservation is not new. In early agricultural developments, primitive humans saw that they could alter the character of the land and climate around them. The use of soil cultivation techniques and judicious planting arrangements allowed them to settle areas even now considered inhospitable and unsuited for habitation. These early farmers created an 'oasis' for their settlements.
The early white settlers in America, South Africa and Australia employed a similar approach. Landscaping around the early farms in the Sydney region demonstrates not only the use of wide verandahs to shade walls, but also the extensive use of plants to keep these buildings cool in summer and warm in winter. Take, for instance, the use of windbreaks to shelter buildings from cold winter winds; large shade trees to the north, east and west which filtered the direct sunlight onto buildings; and large vine-covered pergolas which created cool outdoor spaces for summer living.
Our favourable climate means a great deal of time is actually spent outside the house: around the pool, sitting and relaxing, or barbecuing. But how many gardens are well suited to enjoying these pleasures?
All human-made structures alter the microclimate of an area. Perhaps the most important change is to the surface properties of the city and suburban areas. We might well lament the replacement of plants with large areas of concrete, asphalt and brick. These hard, rock-like surfaces conduct heat more rapidly and retain more heat than grass and tree-covered fields. Streets, walls and roofs form a maze of reflecting and absorbing surfaces for the capture of heat. The entire surface area of our urban environment is absorbing energy and releasing heat. Air in these places is heated simply by contact with the surfaces. Insulated building elements, while reducing the rate of heat flow into the buildings, raise the surface temperature on the outside which further heats the surrounding air.
In the urban setting, rainfall is not absorbed by these hard surfaces it is quickly removed by gutters, drains and an elaborate stormwater system. Robbing the soil of water means that yet another cooling mechanism is lost.
Essentially, vegetation can cool a building and the area around it by reversing these changes. Vegetation also modifies the rate at which energy is exchanged. Plants can intercept and dissipate heat before it reaches your house.
Trees can intercept most of the solar radiation arriving at the top of the leaf canopy, or they can allow some sunlight to filter through. Obviously it is cooler beneath a tree that has relatively dense foliage than one with sparser leaves. The leaf area index (LAI) or leaf area per unit of ground area, determines how much solar radiation a plant will intercept. As the LAI increases, a smaller proportion of the solar radiation reaches the surfaces below. With the exception of plants with a single layer of leaves (such as light vine cover), the LAI is greater than 1; generally it ranges from 3 to 6. On this basis, plants have at least 36 times as much surface area for energy interception as a canvas awining or metal sunshade.
Plants usually intercept around 70 % of the incoming solar radiation, although some plants intercept as much as 90%. Landscaping can, therefore, cool surfaces below and around them by reducing the amount of energy which passes through. The obvious advantage with deciduous trees and vines is that in summer they can dramatically reduce the amount of heat entering your house, while in winter they will allow the sun to pass through and heat the house.
As plants are also living organisms, they interact with their environment. As the air temperature rises, the leaves lose water or transpire, thereby remaining at a relatively constant temperature. Building materials on the other hand lose no water. So, while sunshades or sunscreens, and heat-absorbing and reflecting glass decrease some forms of heat entering the home, they also raise the air temperature around them. Eventually, this increased air temperature will find its way into the house. As a simple experiment, on the next hot day, touch the aluminium awning on a neighbour's house, then touch the leaves of a nearby tree.
In a similar way, any concreted areas around your house will heat up in summer and pass the heat to the interior. You won't have this problem if the surrounding area is grassed, nor will you burn your feet while taking a stroll around your 'estate'.
A large tree is able to reduce the surface temperature of an iron roof by nearly 30°C, while the suface temperature of a concrete carpark may be 30° warmer at 2 pm and 10° warmer at 2 am than an adjacent grassed area.
In areas with large temperature variations between day and night, the building heats up during the day through incident solar radiation and convective heat exchange with the environment. The longwave loss during the day is slight. At night however, the longwave loss to the sky and environment increases, thereby cooling the building.
Settlers in the arid region of Western Australia utilised both the shading and cooling effects of transpiration in the design of their buildings. Building an exterior structure over their sheet iron buildings, they trained plants, frequently imported from the east coast of Australia, to cover the entire structure. Because of the lack of sufficient soil depth, slight moundings around the building were common. Trees were also planted to protect the site from the hot westerly winds blowing off the desert.
The effect of the plant canopy is such as to reduce the diurnal temperature fluctuations. By day, the canopy intercepts solar radiation and dissipates the absorbed energy via transpiration and convective heat loss. By night, the canopy acts as a blanket, reducing the longwave loss.
CHOICE OF SPECIES
For vegetation to be successful, plant material should be chosen according to a number of requirements which may be summarised as:
1. Deciduous vs. evergreen
2. Complete vs. incomplete canopies
3. Leaf distribution vertically hanging leaves through to horizontal leaves
4. Transpiration capabilities high transpiring plants vs. low transpiring plants.
The first three points can be covered through inspection of the mature plant material, while a rule of thumb can be used for the fourth. Plant materials with a low transpiration rate are generally considered to be drought-tolerant and are usually characterised by some modification of the leaf structure eg, the leaves may be small, leathery, have fine hairs, have a waxy quality, etc.
CONVECTION AND TRANSPIRATION HOW PLANTS MODIFY TEMPERATURE
Convective transfer of heat can be either positive or negative, depending on whether the air is warmer or cooler than the plant. Vogel (1968) has shown that leaf shape is also important in the convective exchange process, lobed leaves being more efficient in dissipating energy than entire leaves. The same author has shown a similar relationship between the size of the leaf and the rate of convective cooling, small leaves (Pinus and Casuarina spp.) being generally more efficient than broad, large leaves (Populus and Platanus spp.).
Convection acts across a thin layer of atmosphere around the leaf known as the boundary layer. The rate at which energy is transferred across the boundary layer depends on its thickness and on the difference in temperature between the surface and the atmosphere.
In still air, a canopy under the influence of solar radiation will absorb energy and, in so doing, cause the air mass in the boundary layer to heat and expand. As this warm air rises, cooler air replaces it along the canopy surface, and the plant and canopy are able to stabilise the air temperature about them. At night, or whenever the canopy is cooler than the air, the situation is reversed. Convection now forces the warmer air to give up energy to the canopy. Again, this process tends to stabilise the canopy temperature.
Dissipation of Heat through Transpiration
Transpiration converts water in the leaves of the canopy from liquid into gas; the water vapour then passes from the leaf into the surrounding atmosphere. The process consumes energy and the transpiring canopy grows cooler. The effectiveness of transpiration in energy transfer can be judged by the fact that a transpiration rate of only five ten thousandths of a gram of water per square centimetre gives rise to an energy loss of approximately three tenths of a calorie. This is enough to lower the temperature of a transpirng leaf by as much as 15° C (Gates, 1969).
|'Whoever wants to avoid an energy war in the future should make a pact with this reality, should themselves try to change their own dwelling, their own house into a small bioversity, for the benefit of their children who otherwise will be stultified by mickey-mouse technology' Rudolph Doernach|
DESIGN WITH VEGETATION FOR ENERGY CONSERVATION
Landscape integrated design can take numerous forms, but for this report, their usage with buildings is discussed in four areas:
a. Windbreaks; Shelter belts
b. Plants near buildings
c. The use of Vines
d. Sod or Turf Roof and Earth-sheltered Designs.
WINDBREAKS AND SHELTER BELTS
Good building area; 1015 times the height of trees
Shelterbelts Hot Climates
Shelterbelts Cold Climates
Shelterbelts should be positioned such that they deflect cold winds around and over the building. Trees used for this should be evergreen.
Care should be taken when using shelter belts on sloping sites where they may also act as a dam to the movement of cold air. In such situations, the shelter belt should be staggered to allow cold air to drain away from the building.
PLANTS NEAR BUILDINGS
The effect of plants on altering the radiation and energy balance can be accurately articulated. Design with plants therefore need not be a fuzzy process with unpredictable results. Plant species and arrangements can be chosen for specific requirements to achieve a particular level of control. For example, one might consider the design of planting for a west wall where the main problem is the penetration of solar radiation at low altitudes. Consider also that there are undesirable hot winds associated with this orientation. The planting for such a situation may take the following form:
The use of plants for the interception of low elevation solar radiation and the dissipation of advected 1 heat energy.
At the outer zone, the use of fine-leafed plants with a multi-layer canopy would be appropriate for initial interception of solar radiation, but more importantly for the dissipation of energy through convective transfer. Within this zone, the use of a tree with a monolayer canopy, branching close to the ground, with predominantly an erectophile leaf distribution, or a vine with a regular leaf dispersion would be most adequate. The canopy of the tree or vine, because of its leaf distributions and dispersions, would be expected also to transpire at a higher rate (Horn, 1971), therefore being more efficient in dissipating intercepted energy.
This drawing shows how deciduous and evergreen planting can be used around a building to admit or screen solar radiation from season to season.
Generally, the zone of planting between the north-eastern and north-western sectors (corresponding to the winter sunrise and sunset), should be deciduous, to allow sun penetration in winter while excluding hot summer sun. The zones corresponding to the summer sunrise and sunset (south-east and south-west) should be planted with evergreen species to screen the building from the early and late hours of summer sunshine.
SOD OR TURF ROOF
Depth of Soil:
1. Depth of a sod roof can vary between 150600 mm; optimum depth is 200300 mm
2. The shallower the soil, the more water will be required for th healthy maintenance of the plants.
1. Use local soils, unless they have a very high clay content
2. Mix compost/organic matter into the soil (at least 20%) to improve water holding potential of soil
3. Sandy clay loams with organic matter mixed through are optimum
SLOPE OF ROOF
1. 527° (over 27° the soil will tend to slide off the roof)
1. Turf directly laid on roof and then well watered
2. If sowing seed, use lower pitch (say, no greater than 16°) and use a mixture of seeds which includes a soil-binding species, eg subterranean clover
1. Local drought-resistatn field grasses
2. Avoid local 'domestic' turf grasses, since they require too much water
3. If shrubs are to be included, plant them nearer the base of the roof since there will be more moisture
1. If roof water is to be collected for drinking, do not use any bitumen or bitumenised paints or materials
2. Butyl rubber is ideal but costly; another membrane available is Sarlon Polyfabric
Vines growing on or near a wall can reduce heat-gain of a building by as much as 70%, as well as reducing heat loss by 30%. It can therefore be considered as an alternative to insulation
NOTE: If vines are to be grown directly on a wall, choose both plant and materials carefully; eg, do not use vines directly on earth walls (use a supporting structure away from such walls). Provide a support structure consistent with plant's climbing mechanism:
|PLANT TYPE||SUPPORT STRUCTURE||EXAMPLE SPECIES|
|Tendril||mesh||Grape (vitis spp.)|
|Twiner||timber struts||Honeysuckle (lonicera spp.)|
|Scrambler||solid support, eg timber||Bougainvillea spp.|
(Vines should be planted at small intervals to promote vertical growth; walls and roofs not facing north should use evergreen vines.)
With Feng-shui 2, or Chinese Geomancy, one finds an extraordinarily well documented and well articulated system of design in Nature. More specifically, it is a valuable system of structuring the landscape to make it more amenable to human settlement. Although Feng-shui still gets written off as mere superstition, others find it to be a clear articulation of cosmological understanding. It contains also a large number of practical lessons relating to siting, orientation, the use of planting and water not only for individual buildings, but also for entire cities. The tradition of the useful implementation of planting noted by Marco Polo in his travels continues even today.
A relationship between changeable environmental conditions and a particular site is noted in many Feng-shui manuals. Obstacles, whether mountains or vegetation, were seen to alter local site conditions; eg, mountains or trees in the distance could restrict winter solar radiation, while their absence in the east and west were seen as a disadvantage for summer. Recognising that wind patterns also changed with different months or seasons, shelterbelts and windbreaks to deflect or redirect winds were also suggested. With the use of special compasses and other tools, the Geomancer was able to assess the viability of any site for development as well as proposing schemata for alterations to the site climate if necessary.
1. Advection: The transfer of heat by horizontal movement of air; horizontal convection. Macquarie Dictionary
2. Feng-shui: translates literally as 'wind and water', or 'the action of the wind on the water'.