Ventilation design is fundamental to the design of the building. It will impact on space planning, building form, the location of openings and space for distributing any ductwork and locating air handling equipment. The ventilation design must therefore be an early design decision.


Ventilation is needed to dilute pollutants and to provide summer cooling. Two types of ventilation are necessary:

  • Background ventilation provides a continuous but controllable rate of ventilation to reduce levels of moisture and pollution;
  • Rapid and higher levels of ventilation are required to for summer cooling and to get rid of higher levels of pollution such as in kitchens, bathrooms and utility spaces.

Controllable openings should be provided in order to meet the ventilation loads of the space.  So that ventilation does not cause excessive heat loss. Background ventilation can be achieved through trickle ventilators, while more rapid ventilation needs larger openings.

It is often thought that natural ventilation is more energy efficient than mechanical ventilation. However, for high density occupied spaces, the level of natural ventilation to maintain good air quality may prove difficult to achieve, say through opening windows. Also, during the heating season delivering large quantities of outside cold air may give rise to thermal discomfort, even though internal heat gains from people, etc., may be high. It may prove more efficient to provide outside ventilation air via some mechanical ventilation heat recovery system that can deliver the large amounts of fresh air at a comfortable temperature.  Although, in warmer weather, natural ventilation may be appropriate. But still large amounts of fresh air are required. A natural ventilation system must therefore be designed to be able to deliver these large fresh air rates, in response to variations in inside outside temperature differences and wind effects.

A hybrid ventilation system is a combination of natural ventilation and mechanical. It may be spatial or seasonal.

Natural ventilation provides sufficient fresh air for the occupants, and removes odours and unwanted heat, without the use of mechanical systems or air conditioning. These mechanical systems require large amounts of electrical energy and generate significant CO2 emissions; consequently well designed natural ventilation can provide savings and reduce emissions.

Natural ventilation can be either buoyancy driven or wind driven. In its simplest form, natural ventilation can be achieved through opening windows. Arranging openings at high and low level helps improve the buoyancy effect and generates a circuit. Cross ventilation with openings at either side of a room can increase the depth serviced. More complex systems with computer control and roof mounted ventilator systems need careful design.

Spatial Hybrid:  For example, some larger spaces may prove difficult to naturally ventilate as some areas may be some distance from the perimeter of the space where openings are located. It may prove beneficial to naturally ventilate the perimeter and mechanically ventilate the deeper plan zone.

Seasonal Hybrid: For example, it may be appropriate to naturally ventilate a space in summer when the external air temperature is warm, and mechanically ventilate (with heat recovery) in winter, when external air temperatures are low.

Some natural ventilation solutions may require open plan spatial arrangements to enable the air flow paths between inlets openings and extract. Such arrangements may incur acoustic problems due to a lack of spatial separation.

What should I do?

Obtain professional assistance from mechanical services engineers.

WLGA - CLILC Llywodraeth Cynulliad Cymru / Welsh Assembly Government