Natural Ventilation

Natural ventilation relies on moving air through a building under the natural forces caused by wind and the buoyancy effects of temperature differences. The majority of UK buildings are naturally ventilated by opening windows which provides simplistic control of the flow of air into an area. However, it is often the case that occupants find natural ventilation systems more comfortable to air-conditioning systems because they have more individual control, they understand the system and they have a system which gives immediate response.

Natural ventilation delivers low to zero energy and, hence, low running costs. However, it does not provide the same thermal consistency in the internal environment as, say, mechanical ventilation or air conditioning. For natural ventilation to be effective in cooling a building air paths need to be simple and generous as wind and buoyancy pressures are low. At certain times during the summer external temperatures can be higher than internal temperatures, yet even warm air entering the building may assist in providing improved thermal comfort for the occupants by increasing air movement around the body. In some cases it may not be appropriate to permit hot fresh air, but to let the mass of the building absorb the heat gain. Thermal mass is most effective when it is coupled with night cooling to dissipate heat absorbed by the room surfaces during the day. Natural ventilation can be used most effectively to cool the building at night by bringing in cool night time air to depress daytime peak temperatures on average a couple of degrees lower than the external. It is very important not to overcool the building to avoid the subsequent use of heating to bring the building up to temperature at the start of the occupied period.

One important issue with natural ventilation is the position of inlet vents so that outdoor noise and pollution does not enter the building.

Fig 1 – Natural Ventilation Systems

The above diagram shows natural ventilation techniques and how air flows through a building through windows and vents in relation to wind pressure and natural buoyancy. It can be seen that kitchen exhausts should be positioned well away from opening to ensure inlet air is not contaminated. Similarly, it can be seen from the illustration that contamination is reduced the further air inlets are positioned from noise and air pollution ie traffic illustrated by the bus on fig 1.

The following are three main categories of natural ventilation systems which are each discussed in further detail with respect to where the implementation of each system is appropriate:

· Openable Windows – single-sided or cross-ventilation

· Motorised vent openings and night cooling – single-sided or cross-ventilation

· Stack Ventilation and night cooling