Ventilation is necessary in buildings to remove ‘stale’ air and replace it with ‘fresh’ air. This can be achieved through good design in the majority of cases. With consideration to the introduction and layout of trickle vents, utilising the natural stack effect of a property and reducing solar gain Robert Street Architects aim to develop a passive ventilation strategy. Where required this can be augmented with a mechanical system however, as noted below, this normally only becomes a requirement due to external environmental factors.

Ventilation helps to:

• – Moderate internal temperatures.
• – Reduce the accumulation of moisture, odours and other gases that can build up during occupied periods.
• – Create air movement which improves the comfort of occupants.

Very broadly, ventilation in buildings can be classified as ‘natural’ or ‘mechanical’.

• Mechanical (or ‘forced’) ventilation tends to be driven by fans.
• Natural ventilation is driven by ‘natural’ pressure differences from one part of the building to another. Natural ventilation can be wind driven, or buoyancy driven.

Whilst natural ventilation may be preferable, mechanical ventilation may be necessary where:

• – 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.

‘Mixed-mode’ ventilation uses both natural and mechanical ventilation, for example, allowing the opening of windows, but also providing a mechanical air distribution system.

How to ventilate a building must be considered on a case by case basis. With new buildings a comprehensive scheme can be developed that maximises site opportunities such as orientation, prevailing winds and site photography. When extending or renovating existing buildings the opportunities can be limited and require consideration of the existing condition. If existing single glazed windows with poor draught seals are to be replaced with highly efficient double or triple glazed widows, then the natural ventilation of the building needs to be replaced to maintain a high quality internal environment. This can be done with trickle vents however with modern automation of windows a building can be ventilated utilising the ‘stack affect’ (hot stale air rising) and purge ventilation through automated roof windows that are actuated through humidity and carbon dioxide sensors. This can be done with little expense however requires consideration at an early design stage.

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

‘Trickle ventilation’, ‘slot ventilators’ or ‘background’ ventilation can be necessary in modern buildings (which tend to be designed to be almost completely sealed from the outside to reduce heat loss or gain), so that problems such as condensation are avoided when openings are closed.

This tendency to ‘seal’ modern buildings can also adversely affect occupant comfort, as generally, occupants feel more comfortable if there is some air movement (as long as draughts are not created). This situation can be mitigated by heat recovery ventilation (HRV). This permits increased ventilation rates by recovering heat from extract air and using it to pre-heatincoming fresh air using counter-flow heat exchangers. Heat recovery is increasingly common in mechanical ventilation systems. It is also possible, although complicated with some natural ventilation systems.

Rates of ventilation in buildings can be expressed in terms of air change rates (the number of times that the volume of air in a space is changed per hour) or litres per second. The ventilation rate will be determined by the type and size of space and the way it is occupied (for example, the number of occupants, sources of heat, moisture, odour, contaminants, and so on). Ventilation in buildings is regulated by Part F of the Building Regulations.