A Breath of Fresh Air
28.04.2010
“A source of fresh air is a vital requirement for all buildings. It provides oxygen or breathing, dilutes and displaces pollutants such as odour and metabolic carbon dioxide, avoids condensation by removing moisture and keeps buildings cool by removing excess heat.”[1] The above quote demonstrates the fundamental need for a fresh air supply to any building. Currently the most common means of supplying that air supply is through mechanical methods. As it stands 50% of the worlds total energy use is consumed through the servicing of our buildings[2], a figure that is not sustainable. This problem is only exacerbated by the world’s fossil fuel crisis and therefore new methods of servicing buildings need to be found. By making better use of our existing building stock through remodelling and retrofitting buildings to incorporate the first principle of sustainable design we can go some way to addressing the above.
What is Natural Ventilation
Natural ventilation makes use of natural pressure differences caused by wind and stack (buoyancy) effects. These pressure differences result in airflow and thus provide “free” ventilation. Natural ventilation is therefore dependant on three main variables wind direction, wind speed and variations in temperature. The above variables determine the magnitude of the flow rates and the pattern of fresh air entry into a building. See Figures 1.2-1.3.
Wind induced ventilation
The wind can be utilised in a number of ways to create air flow: firstly the most basic form of ventilation is cross ventilation and has been used since ancient times. As can be seen from Fig1.0, wind incident on as building causes positive pressure on the windward side (red) and an area of negative pressure on the leeward side (blue).
Fig 1.0 Plan and section showing wind pressures on a building.
This can be approximately explained with the help of Bernoullis equation P +½ þ U² = C . Where air speed drops due to the obstruction in its path the pressure P increases whereas the reverse is true where air speed is greater. In simple terms, air flows from an area of high pressure to one of low pressure and cross ventilation occurs see Fig 1.1. As a general rule this form of ventilation is only viable if the plan depth is less than 10m due to impracticalities associated with the large air velocities needed to service deeper spaces[3].
Fig 1.1 Section showing cross ventilation
Air can be drawn out of a building through the use of wind towers, they are openings at the top of a building, which create an area of negative pressure as air flows across them. Coupled with the positive pressures created by the wind incident on a façade, this sets up a pattern similar to that shown in Fig 1.2.
Fig 1.2 Section showing airflow through a wind tower ventilation system
Another method of inducing airflow is through the use of air scoops. These are omni- directional scoops that direct air into a building see Fig 1.3. They can be used in conjunction with wind towers to supply and extract air from a building.
Fig 1.3 Section showing air scoop inlet and wind tower extract
At Emrys Architects we are committed to integrating low carbon – high impact strategies to reduce the energy and lifetime costs associated with our buildings. By utilising the above strategies singularly and in combination we are making strides towards a sustainable built environment.
[1] Page 67 Low Energy Design, Richard Nicholls, 2002, Interface Publishing
[2] Source: Ecology of the Sky, Ivor Richards, 2001, Image publishing
[3] Source: Wind towers, John Wiley and Sons, 1999, Academy Editions
