South Africa has significant passive design potential and it is important that architects and property developers understand the different regions’ climate characteristics in order to design for energy efficiency and comfort.
This was the message from Dr Dirk Conradie at the Green Building and Infrastructure Conference at Sustainability Week 2018. Dr Conradie is a senior researcher in the Built Environment Unit of the CSIR and currently part of a research group that focuses on predictive building performance analysis, especially by means of passive methods such as natural ventilation and solar control. He originally practised as an architect and later specialised in systems and software related to the built environment.
“Passive design helps to control comfort, heating and cooling in buildings without consuming energy or fuels,” he explains. “It is using the orientation of the building to control heat gain and heat loss, using the shape of the building to control air flow, using materials to control heat and cold, as well as maximising the use of free solar energy for heating and lighting, and free ventilation for cooling.
“In South Africa, we have a massive diurnal variation, easily 20°, so we have the benefit of high solar radiation during the day and at night temperatures drop quite low – an ideal situation to maximise the benefits of passive design. It is clearly the best way to design in South Africa, with the biggest potential for energy savings.”
Dr Conradie analysed the various climatic regions in South Africa and calculated the theoretical pure passive potential for each region. Cape Town for example has a passive potential of 84% under current climatic conditions. Inland, Pretoria has a passive potential of 76,5% and in the cooler Highveld region, it is 82,2%. While the numbers drop in areas like Upington (64,6%) . Even in the humid Durban region (55,1%), there is still scope to benefit from passive design.
“These numbers indicate the passive potential that architects can aim for in their designs without any mechanical intervention,” he says.
Understanding SA climate characteristics
Around 2010, in response to the less than optimal SANS 204, six zone map, the CSIR created a Köppen-Geiger map to quantify South African climatic conditions using temperature and precipitation data from1985 to 2005. A number of useful climate maps are freely available online on the Climate Portal of the StepSA website .
“Bioclimatic analysis is an extremely powerful tool and each climatic zone has its own set of appropriate passive bioclimatic design rules,” says Dr Conradie. “However, the Köppen Geiger map alone won’t help when you are planning passive design interventions. One also needs a cooling degree-day (CDD) and heating degree-day (HDD) climatic map to determine exactly what you are designing for from an energy point of view.”
He explains that the most accurate way to calculate degree-days is to use a weatherfile and sum hourly temperatures above and below a specific base temperature that is typically 18 °C. These heating and cooling degree hours are then adjusted to annual values. The CSIR has now developed a new generic map indicating seven sub-regions and heating and cooling zones, which is currently in the process of being taken up in the SANS 204 and SANS 10400 standards.
“We have specific figures available, but the standards writing authorities insisted that it had to be an easy-to-understand map. This is why we believe that weather files should be freely available to all professionals to enable them to do proper simulations using modern simulation software,” Dr Conradie says.
“And while this was rather difficult to apply some 40 years ago, today there is very good energy simulation software available, making it quite easy to follow bioclimatic rules.” When uploading weather files, the software suggests particular strategies for passive measures.
Glass is a weak spot
Dr Conradie points out that glass windows are typically the weakest part of a building’s armour against energy gain and loss.
“We must have respect for windows if looking to design passively. While they are an important source of natural daylight and ventilation, one has to admit solar radiation at the right time and provide appropriate solar protection. Even when using the best triple-glazed, low E glass without proper solar control, one can still be in trouble.
“This is a rather difficult subject as climate and sun angles vary, but ideally one wants a fixed solar protection solution rather than a mechanically controlled one which might fail easier. So one has to be very careful how much glass you use, how you protect it and how you combine the natural daylight solar protection and natural ventilation aspects of glass.
“In South Africa, sun shading, together with orientation and solar protection, are the low-hanging fruit of passive design and the first strategies to implement,” he states.
The basics of solar movement
Another important aspect to calculate is the angle of the sun at different times of the day and year. In summer, the sun rises south of east and sets south of west, while in winter it rises significantly north of east and sets significantly north of west – quite a significant change in the angle of the sun. Interestingly, in areas such as Makhado, north of the Tropic of Capricorn, buildings are exposed to sun angles from both the north and the south.
“The actual climatic conditions at a particular location greatly influence the solar protection that is necessary,” explains Dr Conradie.
“Currently, in central Pretoria, it is estimated when the sun is at an altitude of 62° above the horizon, one has to start shading the building. In this area, horizontal overhangs on western facades are not effective as the sun is at low elevation above the horizon. Instead, one has to opt for vertical solar protection devices or screen to exclude sun from the east or west. Eastern and western facades are azimuth dominated because of low solar angles, while northern facades are more elevation/ altitude dominated because the sun is at a higher angle.
“Screens can be a very good method of solar protection, except it reduces the benefit of the winter sun that could help to reduce the amount of energy used for heating. This is a general weakness of screens – it is not really seasonally sensitive, unless they are adjustable.”
Dr Conradie also points out that several new buildings in Sandton have solar shielding on the inside of the building to reduce solar glare, but he explains that this measure is not going to effectively stop the solar heat gains in the building. “The best place to stop solar heat gain is from the outside of the building,” he says.
Protect the roof
Heat can also come through the roof, which means that solar protection on its own won’t do the trick. “Because the solar angles in South Africa are quite high, roof insulation and cool surfaces are excellent passive design strategies,” says Dr Conradie.
“The most effective measures are to use a cool roof surface or white roof with low absorbance, and roof insulation – this can have a massive impact on heat gain into a building.”
When it comes to supplying clean air into a space, and extracting or diluting contaminants, Dr Conradie explains that one can use the shape of the building to drive natural ventilation.
“For example, when reaching a roof angle of 25° on a low-rise building, one can develop positive and negative pressure differential over the roof that will more efficiently drive airflow without mechanical intervention.”
Another interesting method used in a prison in Patensie, near Port Elizabeth, which has a round, dome-like structure, sees the buoyancy of the metabolic heat generated by the 300 inmates driving natural ventilation. This principle can easily be used in other congregate settings such as churches or concert halls.
Passive design thinking
“A good understanding of the basic principles of passive design will lead to far better ‘sun-aware’ architecture,” says Dr Conradie.
“However, when planning passive design interventions, it is important to consider the full design and not just add on so-called quick fixes. When most of the heat is coming through the roof, low E glass won’t do much to reduce the problem,” he explains.
He also cautions that the net result of combining different interventions is not always as beneficial as expected because some of the benefits could be neutralised. Using a combination of low E glass and roof insulation might only be slightly better than just using the insulation if that is where the heat is coming from, depending on the particular design.
The positioning of insulation in a wall also has a negligible effect with regards to U-value, however, it has a massive effect on active thermal capacity.
In addition, one doesn’t have to leave it all up to the building. People can adapt through the different seasons, by dressing warmer in winter than in summer.
“We have seen that different designs have similar benefits across all climatic regions, irrespective of whether heating or cooling is dominant,” adds Dr Conradie. “Solar protection is always the most appropriate passive strategy in general to improve comfort and save energy.
“I believe the time is ripe for a new profession of facade engineers who properly understand passive design principles and can design building envelopes to perform better and to be more sustainable.”
Full thanks and acknowledgement are given to Dr Dirk Conradie and the CSIR for the information contained in this article.