Last edited 23 Sep 2016

Natural ventilation of buildings

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Ventilation is necessary in buildings to remove ‘stale’ air and replace it with ‘fresh’ air:

  • Helping to moderate internal temperatures.
  • Reducing the accumulation of moisture, odours and other gases that can build up during occupied periods.
  • Creating air movement which improves the comfort of occupants.

Ventilation in buildings can be either natural or mechanical. Mechanical ventilation tends to be driven by fans, whereas natural ventilation is driven by ‘natural’ pressure differences between one part of the building and another.

NB The term ‘assisted ventilation’ typically refers to systems where fresh air enters a building 'naturally' through windows or other openings, but is extracted by continuously running fans.

Natural ventilation is an aspect of passive building design that might also include passive heating and passive cooling. See Passive building design for more information.

Natural ventilation tends to cost less to build, operate and maintain than mechanical ventilation, and so this is generally the first option investigated during the design process. However, there may be circumstances where natural ventilation is not possible and so mechanical ventilation is necessary:

  • The building is too deep to ventilate from the perimeter.
  • Local air quality is poor, for example if a building is next to a busy road.
  • Local noise levels mean that windows cannot be opened.
  • The local urban structure is very dense and shelters the building from the wind.
  • Air cooling or air conditioning systems mean that windows cannot be opened.
  • Privacy or security requirements prevent windows being opened.
  • Internal partitions block air paths.
  • The creation of draughts adjacent to openings.

Some of these issues can be avoided or mitigated by careful siting and design of the building.

Natural ventilation is generally categorised as:

  • Wind-driven (or wind-induced) cross ventilation, where pressure differences between one side of the building and the other draw air in on the high pressure side and draw it out on the low pressure side.
  • Buoyancy-driven stack ventilation (the stack effect), where cooler air enters the building at low level, is heated by occupants, equipment, heating systems and so on, becomes less dense and so more buoyant and rises through the building to be ventilated to the outside at the top.

The effectiveness of these mechanisms is dependent on a wide number of variables, but very broadly:

  • Cross ventilation is suitable for buildings up to approximately 12 to 15m in depth (five times the floor to ceiling height, or 2.5 times the floor to ceiling height if openings can only be provided on one side). Beyond this, providing sufficient fresh air creates draughts close to openings, and additional design elements such as internal courtyards are necessary, or the inclusion of elements such as atrium that combine cross ventilation and stack effects. A disadvantage of cross ventilation is that it tends to be least effective on hot still days, when it is needed most.
  • The effectiveness of stack ventilation is influenced by; the effective area of openings, the height of the stack, the temperature difference between the bottom and the top of the stack and pressure differences outside the building. Where ventilation is needed high up in the building, this can require the addition of ventilation stacks that achieve the height necessary to create a pressure difference between the inlets and outlets. See Stack effect for more information.

Combinations of these ventilation strategies, with the additional exploitation of thermal mass can produce a wide range of natural ventilation solutions, such as trombe walls, solar chimneys and so on.

Designing natural ventilation can become extremely complex because of the interaction between cross ventilation and the stack effect as well as complex building geometries and the distribution of openings. This can require computer analysis.

Natural ventilation can also be influenced by occupant behaviour, for example, a person near to a window choosing to close it. For this reason it can be beneficial to automate natural ventilation systems, or to provide training for occupants.

In modern buildings, which tend to be designed to be completely sealed from the outside unless windows or other ventilators are open, problems, such as condensation, can occur during the winter when openings are closed. As a result, ‘trickle ventilation’, or 'background' ventilation tends to be provided to ensure there is always an adequate level of ventilation. Trickle ventilators can be self-balancing, with the size of the open area depending on the air pressure difference across it.

It is possible, although relatively complicated to include heat recovery in natural ventilation systems so that during cooler conditions, heat recovered from extract air is used to pre-heat fresh air entering the building.

Ventilation in buildings is regulated by Part F of the building regulations.

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