According to Guy Thompson at The Concrete Centre, architects and their clients are increasingly adopting a ‘back to basics’ or passive sustainability approach that provides a building with stripped-down exposed elements and structural honesty. For this they are increasingly turning to exposed concrete surfaces which need no additional finishes and offer high levels of fire resistance, sound insulation and flood resilience.
Utilising the passive sustainability potential of buildings involves taking advantage of their thermal mass as part of an integrated approach that includes a balance between orientation, glazing, ventilation and shading, as well as the provision of a high standard of insulation and air tightness.
Thermal mass is achieved by using heavyweight construction. In practice, this has been found to be best supplied by concrete – either blocks or panels, precast or in-situ. It provides a highly effective means of avoiding or reducing the risk of overheating during the summer months. Buildings with a high level of thermal mass are characterised by their inherent ability to soak up and release heat at different times of the day. In the summer, heat is absorbed on hot days, helping to stabilise the internal temperature and prevent overheating problems. Cool night air can then be used to ventilate the building and purge the accumulated heat from the building fabric. In the winter, heat gains from the sun are absorbed into the thermal mass and radiated in the evenings reducing the load on the heating system.
Passive design for the summer requires consideration of ventilation, shading and thermal mass. Effective control of natural ventilation requires well designed and user-friendly windows to take maximum advantage of potential air flow. Cross-flow ventilation, where fresh air travels across the room or home from window to window on different façades, is most effective. Stack ventilation may also be possible if air entering at low level can travel unimpeded to the upper floor and then leave via high level windows. Stack ventilation can be very beneficial on still nights when there is no wind to aid air movement.
Traditionally, shading has not been a major feature of UK buildings. However, this is likely to change as shading begins to play an increasing role in mitigating the effects of climate change by reducing overheating. Shading options include internal shading such as Venetian or mid-pane blinds or external shading using roller shutter blinds or overhangs.
Passive design for winter performance requires consideration of orientation, window sizes and shading from adjacent buildings or vegetation. Orientation is the most critical factor in determining the amount of sun that a building receives. A south orientation is required to maximise the benefits of passive solar design (PSD). In the middle of winter, the sun reaches a maximum altitude above the horizon of about 17° in southern England. During the height of summer it reaches 64° when the shading provided by a simple overhang will block the sun during the hottest part of the day. During the winter, this same overhang will allow sunlight to pass directly in and heat the building. 
Windows play a significant role in solar gain. From a thermal design perspective there are no set rules on window size. The basic requirement is for relatively large south facing windows and small north facing windows. The size of south facing glazing will need to take account of a range of factors including the insulation performance of the glass, the level of thermal mass and the general design requirements of the home.
Windows that are too large will be counter productive as their heat loss during winter nights can outweigh their ability to maximise solar gains during the day. They are also likely to increase the risk of overheating during the summer. As a general rule, windows should be at least 15% of a room’s floor area to provide adequate daylight and no more than 50% of the façade if excessive heat gain/loss is to be avoided.
It is interesting to note that many of these passive sustainability features are not new but are now being rediscovered and incorporated into designs. The use of shading and window size to moderate heat gains are evident in the building designs of the ancients, as is their use of the thermal efficiency of heavyweight construction. The sash windows of the Georgians and Victorians, when opened equally at the top and bottom, separates the in-flow of cooler air and flushes out the warmer air from the top. Get these basics right and you have a building that provides durable construction with long-term sustainability at low cost to both the environmental and the budget.
