Building Regulation Regarding Trickle Vents


Ventilation is the process of supplying outdoor air to an enclosed space and removing stale air from the space. It can manage the indoor air quality by both diluting the indoor air with less contaminated outdoor air and removing the indoor contaminants with the exhaust air.

The Building Regulations consider three ventilation types as follows :

  • Whole Building or background ventilation (also known as trickle ventilation) to provide fresh air to dilute and disperse low levels of water vapour and other pollutants, usually by the provision of background ventilators or mechanical supply ventilation. Trickle ventilators are ideal for meeting this requirement
  • Local extract or extraction ventilation in rooms where most water vapour or concentrated pollutants are released, usually by mechanical means such as extract fans.
  • Purge or rapid ventilation is achieved usually by opening windows. The requirements for Purge ventilation are set out in Appendix B of Part F.

There are four approaches that can be taken to providing ventilation, referred to in the Approved Document F:

  • System 1 – Background ventilators and intermittent extract fans
  • System 2 – Passive stack ventilation (PSV)
  • System 3 – Continuous mechanical extract (MEV)
  • System 4 – Continuous mechanical supply and extract with heat recovery (MVHR)

System 1 – Intermittent extract fans with background ventilators

This is the most common approach to meeting Building Regulations Approved Document Part F in the UK. System 1 is suitable for use in houses and many flats or apartments with multi façades. This is the simplest system available, both to install, and for occupants to control. Fans though, must be regularly maintained, in order for best efficiency, and to reduce any potential noise issues.

The System 1 comprises of background ventilators, usually trickle ventilators fitted to windows and extract fans fitted in moisture producing areas or “wet rooms” such as kitchens and bathrooms.

Teh background ventilators (usually trickle vents) are fitted to windows to provide the whole building ventilation, and provide air to the extract fans, which will aid to remove odours and humidity.

Extract fans are located in bathrooms, kitchen, and utility rooms. These fans can operate in conjunction with light switch, remote switch, pull cord, humidistat, presence detector, or timer. They must comply with ventilation airflow rates set out in Part F, dependant on number of bedrooms and occupancy levels, ensuring that adequate air quality is provided indoors for occupants.

The minimum level of background ventilation for all habitable rooms, kitchens, utility rooms and bathrooms is determined by a number of factors:

  • Dwelling floor area
  • Number of bedrooms
  • Number of occupants
  • Number of storeys
  • Number of façades

Minimum background ventilation in System1 should be provided as follows as defined in Building Regulations Part F, Section 7:

  • Habitable rooms: 5000 mm2 equivalent area
  • Kitchens, utility rooms and bathrooms: 2500mm2 equivalent area
  • New rooms (in the case of an extension): 8000mm2 equivalent area

Trickle vent requirement for replacement windows and doors in existing dwellings

The Building Regulations require that adequate ventilation is provided to existing dwellings and you are reqired to replace windows like for like. So you must not worsen the existing room’s ventilation provisions. If the original windows have already background trickle ventilation, then the replacement windows should also be fitted with trickle vents to at least the same equivalent area as the ones replaced.

When assessing a property for replacement windows the following should be considered regarding the use of trickle ventilators and other forms of ventilation within replacement windows:

  • Trickle ventilators are not mandatory unless the existing windows have them.
  • Replacement ventilators must be no smaller in geometric open area than the existing ventilators. If the geometric area is not known, habitable rooms should have trickle ventilator of minimum 5000 mm2 EA and wet rooms should have minimum 2500 mm2 EA.
  • If the window being removed did not have trickle vents fitted, it is recommended to provide the background ventilation in the replacement window, due to the health benefits.

Trickle vent requirement for windows and doors in new dwellings and extensions

The required ventilation levels for a new house or dwelling are dependant upon a number of factors. 



Key terms

Background ventilator is a small ventilation opening designed to provide controllable whole building ventilation.

Equivalent area is a measure of the aerodynamic performance of a ventilator. It is the area of a sharp-edged circular orifice which air would be pass through at the same volume flow rate, under and identical applied pressure difference, as the opening under consideration.

Free area is the geometric open area of a ventilator terminal.

Intermittent extract fan is a mechanical ventilator that does not run all the time, usually only running when there is a particular need to remove pollutants or water vapour (e.g. during cooking or bathing). Intermittent operation may be under either manual control or automatic control.

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