Understanding Approved Document F – Ventilation

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Ventilation is the process of supplying outdoor air into a building, distributing it within the building and removing stale air from the building. In residential buildings, ventilation is crucial for maintaining healthy indoor air quality and enhancing comfort. Proper ventilation systems ensure the continuous exchange of stale indoor air with fresh outdoor air, effectively reducing indoor pollutants, controlling humidity levels, and preventing the buildup of moisture that can lead to mould growth. By implementing ventilation practices, homes can be transformed into safer and more comfortable living environments that enhance overall well-being.

ADF Volume-1 describes three ventilation strategies (types) for residential buildings:

  • Extract ventilation
  • Whole dwelling vetilation
  • Purge ventilation

Each of these types of ventilation are required for differing lengths of time and more or less often.

In according with the ADF Volume 1, ventilation in dwelling can be achieved through the following systems:

  • Natural ventilation: It refers to a ventilation using background ventilators and intermittent extract ventilation.
  • Mechanical ventilation
    • Continuous mechanical ventilation (MEV)
    • Intermittent mechanical ventilation
    • Mechanical ventilation with heat recovery (MVHR)
  • Combination of both.

Extract Ventilation

Extract ventilation means the removal of air directly from an internal space or spaces to the outside. Extract ventilation can be achieved by natural means or by mechanical means (e.g. by an extract fan or a central system).

Extract ventilation to the outside should be provided in all of the following spaces.

  1. Kitchens.
  2. Utility rooms.
  3. Bathrooms.
  4. Sanitary accommodation.

Extract ventilation can be intermittent or continuous. (Ventilation fans may be either intermittent operation or continuous operation.)

Minimum extract ventilation rates in litres per second (l/s) for intermittent operation extract systems are given in Table 1.1.

Minimum extract ventilation rates for continuous operation extract systems are given in Table 1.2.

Whole dwelling ventilation

Whole dwelling ventilation is a continuous ventilation method to provide fresh air to the rooms or spaces at a relatively low rate and to dilute and remove water vapour and pollutants not removed by extract ventilation.

In the whole dwelling ventilation, the fresh air should be delivered through one of the following means:

  1. Continuous supply fans.
  2. Background ventilators (trickle vent ventilation)

The minimum whole dwelling ventilation rate for the supply of air to the habitable rooms in a dwelling should meet both of the following conditions.

  1. A minimum rate of 0.3 litres per second per m2 of internal floor area (this includes all floors, e.g. for a two-storey building, add the ground-floor and first-floor areas).
  2. A minimum rate determined by the number of bedrooms, as specified in Table 1.3.

Purge ventilation

Manually controlled ventilation of rooms or spaces at a relatively high rate to rapidly dilute pollutants and/or disperse water vapour. Purge ventilation may be provided by natural means (e.g. an openable window) or mechanical means (e.g. a fan).

A system for purge ventilation should be provided in each habitable room.

Purge ventilation should be capable of extracting at least four air changes per hour per room directly to the outside. (NOTE: In order to demonstrate compliance with Part O of the Building Regulations it is likely that

higher purge ventilation rates than those given in paragraph 1.29 will be required.)

Purge ventilation should be delivered through one of the following means:

  1. Openings (e.g. windows or doors).
  2. A mechanical extract ventilation system.

Where purge ventilation is delivered through openings in a habitable room, the minimum opening areas in Table 1.4 should be achieved.

The values in Table 1.4 are based on BS 5925, which assumes all of the following:

  1. Single-sided ventilation.
  2. An urban environment.
  3. A wind speed of 2.1 metres per second (m/s).
  4. A temperature difference of 3°C between the air inside and outside the building.

Hinged or pivot windows with an opening angle of less than 15 degrees are not suitable for purge ventilation.

Performance testing

The air flow resistance of all components should be considered when specifying ventilation systems. The complete assembly, as installed, should be designed to meet the performance standards of this approved document. The performance of the separate components should be measured according to the relevant parts of BS EN 13141 and other relevant standards. The relevant test standards for common equipment types are given in Table 1.5.

System-specific guidance

Natural ventilation

(with background ventilators and intermittent extract fans)
(guidance suitable only for less airtight dwellings)

Intermittent extract

Intermittent extract fans should be fitted in all wet rooms. For kitchens, utility rooms, bathrooms and sanitary accommodation, the extract rates in Table 1.1 can be met using an intermittent extract fan.

If a wet room has no external walls, the intermittent extract fan should extract at four air changes per hour to meet the purge ventilation standards in paragraphs 1.26 to 1.31.

Any automatic controls (e.g. humidity control) for intermittent extract should have a manual override to allow the occupant to turn the extract ventilation on or off.

In a room with no openable window, an intermittent extract fan should be provided with controls which continue to operate the fan for at least 15 minutes after the room is vacated.

Background ventilators

All rooms with external walls should have background ventilators. If a habitable room has no external walls, paragraphs 1.42 to 1.44 should be followed.

If the dwelling has more than one exposed façade, the area of background ventilators on each façade should be similar, to allow cross-ventilation. 

If an exposed façade is close to an area of sustained and loud noise (e.g. a main road), then a noise attenuating background ventilator should be fitted.

The minimum total area of background ventilators in each room should follow the guidance in Table 1.7.

The total number of ventilators installed in the dwelling’s habitable rooms and kitchens should be at least the following:

  1. Four ventilators if the dwelling has one bedroom.
  2. Five ventilators if the dwelling has more than one bedroom.

If the dwelling has a kitchen and living room which are not separate rooms, at least three ventilators of the same area as for other habitable rooms in Table 1.7 should be provided in the open-plan space.

Replacing windows

Existing windows with background ventilators

If the existing windows have background ventilators, the replacement windows should include background ventilators. The new background ventilators should comply with both of the following conditions:

  1. Not be smaller than the background ventilators in the original window.
  2. Be controllable either automatically or by the occupant.

If the size of the background ventilators in the existing window is not known, the ventilator sizes in paragraph 3.15 may be applied.

Existing windows without background ventilators

Replacing the windows is likely to increase the airtightness of the dwelling. If ventilation is not provided via a mechanical ventilation with heat recovery system, then increasing the airtightness of the building may reduce beneficial ventilation in the building. In these circumstances, it is necessary to ensure that the ventilation provision in the dwelling is no worse than it was before the work was carried out. This may be demonstrated in any of the following ways:

  1. Incorporating background ventilators in the replacement windows equivalent to the following: 
    • Habitable rooms – minimum 8000mm2 equivalent area.
    • Kitchen – minimum 8000mm2 equivalent area.
    • Bathroom (with or without a toilet) – minimum 4000mm2 equivalent area.
  2. If the dwelling will have continuous mechanical extract ventilation, installing background ventilators in any replacement windows which are not in wet rooms, with a minimum equivalent area of 4000mm2 in each habitable room.
  3. Other ventilation provisions, if it can be demonstrated to a building control body that they comply with the requirements of paragraph 3.2.

Key terms in Approved Document F

Air permeability: The measure of airtightness of the building fabric. It is defined as the air leakage rate per hour per m2 of envelope area at the test reference pressure differential of 50Pa or 4Pa.

Airtightness: The resistance of the building envelope to infiltration when ventilators are closed. The greater the airtightness at a given pressure difference across the envelope, the lower the infiltration.

Background ventilator: A small ventilation opening designed to provide controllable whole dwelling ventilation.

Continuous mechanical extract ventilation: Mechanically driven ventilation that continuously extracts indoor air and discharges it to the outside.

Dwelling: A self-contained unit designed to accommodate a single household.

Extract ventilation: The removal of air directly from an internal space or spaces to the outside. Extract ventilation may be by natural means or by mechanical means (e.g. by an extract fan or a central system).

Free area: The geometric open area of a ventilator.

Habitable room: A room used for dwelling purposes but which is not solely a kitchen, utility room, bathroom, cellar or sanitary accommodation.

Natural ventilation:  Ventilation provided by thermal, wind or diffusion effects through doors, windows or other intentional openings without the use of mechanically driven equipment. For the purposes of this approved document, natural ventilation refers to a ventilation strategy using background ventilators and intermittent extract ventilation.

Purge ventilation: Manually controlled ventilation of rooms or spaces at a relatively high rate to rapidly dilute pollutants and/or disperse water vapour. Purge ventilation may be provided by natural means (e.g. an openable window) or mechanical means (e.g. a fan).

Mechanical ventilation with heat recovery: A mechanically driven ventilation system that both continuously supplies outdoor air to the inside of the dwelling and continuously extracts indoor air and discharges it to the outside. For the purposes of this approved document, the guidance for mechanical ventilation with heat recovery applies to centralised or decentralised supply and extract systems, with or without heat recovery.

Ventilation: The supply and removal of air (by natural and/or mechanical means) to and from a space or spaces in a building. It normally comprises a combination of purpose-provided ventilation and infiltration.

Whole dwelling ventilation (general ventilation): Nominally continuous ventilation of rooms or spaces at a relatively low rate to dilute and remove
pollutants and water vapour not removed by extract ventilation, purge ventilation or infiltration, as well as to supply outdoor air into the dwelling.

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