Energy efficiency and green living are fast becoming buzzwords, thanks to recent conclusive evidence that our activities as humans are contributing directly towards an increase in CO2 emissions and, as a result, global warming. Problems regarding electricity supply from Escom have further highlighted the need for energy efficiency – from a sustainability point of view we may one day be thankful for the awakening created by this problem. We are advised, and correctly so, to consume less, completely switch off appliances when not in use, install geyser blankets, lower the thermostat, and minimise the use of electric heaters. Despite all of this, the vast majority of new houses being built are still designed the same way they were when we were not yet aware of the urgent need for energy efficiency.
Hence the need for passive solar design – a design strategy that aims to provide maximum thermal comfort while utilising the minimal amount of conventional energy. This is achieved by taking seasonal sun angles and local climatic conditions into account, designing the orientation, shading and ventilation of the house accordingly, and selecting the building materials and their use by considering their thermal properties.
A change in seasonal sun angles is the result of the earth’s tilt remaining constant as it rotates around the sun. In December, the southern hemisphere is more exposed to the sun as the south pole is tilted towards the sun, hence our summer. In June, the northern hemisphere is tilted towards the sun resulting in our winter. The further away one is from the equator, the more the difference is between the summer and winter sun angles. At a latitude of 34 degrees (the Southern Cape) for example, the midday in mid-winter sun angle is 32,6 degrees, and the midday in mid-summer sun angle is 79,6 degrees.
Local climatic conditions in South Africa vary greatly from area to area, with a temperate Mediterranean climate along the Western Cape coast, a sub-tropical climate along the east coast, and an interior which includes cold, temperate and hot climates. Besides the obvious aim of keeping our homes warm during cold winters and cool in summer, particularly in the hot interior and sub-tropical areas, it is the fluctuation of day/night temperatures, called diurnal fluctuation, that is an important consideration in passive solar design. Generally the further inland, away from the temperature-moderating effect of the ocean, the greater the diurnal fluctuation.
The optimal orientation for achieving thermal comfort is to have the main living areas facing north and the house ideally elongated along an east-west axis for maximum penetration of north light into the house. Where facing the living areas to the north is not ideal due to other reasons, for example on a typical sea-view coastal property in the Southern Cape, with views to the south, allowance must be made for penetration of north light from the back of the house. Orientation and shading work hand in hand, as one wants low sun angle winter sunshine into the house for warmth, and the shading of high angle summer sunshine. This is achieved by eaves overhangs and shading devices designed in accordance with sun angles. The period during which we would typically want minimal sun penetration into the house in South Africa, called the shading period, would be from around late October to late February, and this equates a sun angle of 67 degrees and above.
In cold conditions, most of our heat loss occurs through the glazing, which should ideally be used minimally to the south. In the case of a southern sea view as in the above example, double glazing should be considered. Double glazing at this stage is rarely used in South Africa, and due to the small market is still relatively high priced. As the cost of energy rises and awareness of sustainability increases, this may start becoming more viable and more popular, with a resulting decrease in relative cost.
Optimal ventilation, for summer cooling, and to provide a flow of fresh air, requires consideration of the design and location of window and door openings for effective cross ventilation. Convective ventilation, based on the principle that warm air rises, is achieved with high level windows or roof space vents. A typical problem in a vaulted roof space where this is not allowed for is that warm air which moves upwards by convection is then trapped below the ceiling, and radiates warmth downwards, causing overheating.
Materials, with regard to their appropriateness according to thermal properties, can be evaluated in terms of their insulation and thermal mass retention properties. As a general rule, lightweight materials, such as timber, have high insulation values, and high mass materials, such as concrete and masonry, have lower insulation values and high thermal mass retention properties. Lightweight construction methods, such as timber frame, have the additional advantage of providing a hollow wall cavity which can be filled with insulation material appropriate to the local climate.
In the selection of a material, or a combination of them, according to climatic zone, the higher the diurnal fluctuation the more important it is to incorporate materials of high thermal mass in the construction to take advantage of this. For example, a timber frame house built on a conventional masonry foundation and concrete slab in a hot semi-arid region, with correct orientation and shading, would allow the sun to penetrate the building during a warm winter’s day warming up the concrete slab. At night, the concrete slab, as it has high mass retention, would still be warm, and would slowly give off its heat during the cold night. The highly insulated timber frame walls play their role by keeping this heat in. During summer, the windows, as they are shaded, allow minimal direct sunlight into the house. The concrete slab, with its high mass, having cooled the night before, in addition acts a heat sink, absorbing heat. Thermal mass could also be achieved by building a large stone or masonry fireplace or the incorporation of any other elements of high mass.
In moderate or sub-tropical climate zones, with a typically lower diurnal fluctuation, thermal mass is of less importance and lightweight timber frame construction, or solid timber construction, is ideally suited. In moderate climate zones some high thermal mass materials are recommended, in combination with highly insulated walls to keep the building warm in winter.
In the sub-tropical climate zone, where the idea is to keep the building cool, high levels of insulation and correct shading, particularly to the north and west, are important and thermal mass is not required. Ventilation and the capturing of the breeze for cooling are most important, and raising the house on poles often assists with this.
In very cold climates, timber buildings are ideal, as when having to heat the building by conventional means, such as a fireplace or electrical or gas heaters, the low mass walls don’t absorb any of the heat but provide excellent insulation. In a masonry house, by comparison, the high mass walls absorb heat from the heat source and provide little insulation.
It is often claimed that timber homes are as a rule cooler in summer and warmer in winter than their conventional masonry counterparts. This is true as they are a lot better insulated. When cold, they will respond a lot quicker to a conventional heat source such as a fireplace or heater. In order to achieve optimal passive solar design however, along with correct design in terms of orientation, shading and ventilation, they are often best used in combination with some materials of high thermal mass such as concrete, masonry or stonework.
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