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)

New Homes – using Natural ventilation with background ventilators and intermittent extract fans guidance suitable only for less air tight dwellings)

These are now simpler per room amounts;

For dwelling with multiple floors:

  • Habitable rooms and kitchens: 8,000mm2EA
  • Bathrooms: 4,000mm2EA
  • Sanitary Accommodation: No minimum

For single storey dwellings (e.g flats):

  • Habitable rooms and kitchens: 10,000mm2EA
  • Bathrooms: 4,000mm2EA
  • Sanitary Accommodation: No minimum

New Homes - using mechanical ventilation with heat recovery systems

Trickle ventilators are not required because these are balanced ventilation systems in more energy efficient house designs

New Homes using continuous extract ventilation systems

Trickle vents should provide 4,000mm2EA in each habitable room

Existing homes

  • Replacement windows should be fitted with trickle vents regardless of whether the windows being replaced had vents in them or not, if no background ventilation alternative is being installed.
  • Habitable rooms and kitchens: 8,000mm2EA, Bathrooms: 4,000mm2EA.
  • Addition of a wet room to an existing building: 5,000mm2EA.
  • Addition of a habitable room to an existing dwelling (if existing room has less than 5,000mm2EA): 12000mm2EA.
  • If the existing dwelling has continuous mechanical extract ventilation fitted then 4,000mm2EA is required in habitable rooms.

Key terms

Background ventilation is a small ventilation system designed to provide controllable whole building ventilation. (e.g. trickle vents in windows or doors)

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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