Section 4 Introduction to the provisions

Introduction to the provisions

  1. Performance
    1. The purpose of ventilation
      1. Types of ventilation
        1. Control of ventilation
          1. Performance-based guidance
            1. Equivalent area of ventilators
              1. Ventilation effectiveness
                1. Source control
                  1. Noise
                    1. Fire precautions
                      1. Modular and portable buildings
                        1. Installation of ventilation systems
                          1. Air flow rate testing and commissioning of ventilation systems
                            1. Provision of information


                              4.1 The purpose of this section is to outline briefly what ventilation in buildings is for and the philosophy behind the guidance for ventilation given in Approved Document F. More detail is given in some of the informative Appendices at the end of this Approved Document.

                              4.2 The key aim of the requirement of Part F1(1) is that a ventilation system is provided that, under normal conditions, is capable of limiting the accumulation of moisture, which could lead to mould growth, and pollutants originating within a building which would otherwise become a hazard to the health of the people in the building.

                              4.3 In general terms, the requirement may be achieved by providing a ventilation system which:

                              a. extracts, before it is generally widespread, water vapour from areas where it is produced in significant quantities (e.g. kitchens, utility rooms and bathrooms);

                              b. extracts, before they are generally widespread, pollutants which are a hazard to health from areas where they are produced in significant quantities (e.g. rooms containing processes or activities which generate harmful contaminants);

                              c. rapidly dilutes, when necessary, pollutants and water vapour produced in habitable rooms, occupiable rooms and sanitary accommodation;

                              d. makes available over long periods a minimum supply of outdoor air for occupants and disperses, where necessary, residual pollutants and water vapour. Such ventilation should minimise draughts and, where necessary, should be reasonably secure and provide protection against rain penetration;

                              e. is designed, installed and commissioned to perform in a way which is not detrimental to the health of the people in the building; and

                              f. is installed to facilitate maintenance where necessary.

                              4.4 The guidance in this Approved Document has not been formulated to deal with the products of tobacco smoking.

                              4.5 Ventilation systems in buildings result in energy being used to heat fresh air taken in from outside and, in mechanical ventilation systems, to move air into, out of and/or around the building. Energy efficiency is dealt with under Part L of Schedule 1 and Regulation 40 of the Building Regulations but consideration should be given to mitigation of ventilation energy use, where applicable, by employing heat recovery devices, efficient types of fan motor and/or energy-saving control devices in the ventilation system.

                              The purpose of ventilation

                              4.6 Ventilation is simply the removal of ‘stale’ indoor air from a building and its replacement with ‘fresh’ outside air. It is assumed within the Approved Document that the outside air is of reasonable quality.

                              4.7 Ventilation is required for one or more of the following purposes:

                               a. provision of outside air for breathing;

                              b. dilution and removal of airborne pollutants, including odours;

                              c. control of excess humidity (arising from water vapour in the indoor air);

                              d. provision of air for fuel-burning appliances (which is covered under Part J of the Building Regulations).

                              4.8 Ventilation may also provide a means to control thermal comfort but this is not controlled under the Building Regulations. Part L addresses minimising energy use due to the effects of solar gain in summer.

                              4.9 The airborne pollutants and water vapour mentioned in paragraph 4.7b and c above include those that are released from materials and products paragraph used in the construction, decoration and furnishing of a building, and as a result of the activities of the building’s occupants.

                              4.10 The pollutant(s) of most importance will vary between building types (e.g. dwelling, office, factory), building uses (e.g. industrial process, shop, commercial kitchen), and even from room to room within a building (e.g. kitchen, shower room, conference room, photocopier room). Common pollutants in a dwelling are moisture and combustion products from unflued appliances (e.g. gas, oil or solid fuel cookers) and chemical emissions from construction and consumer products. Note that the ventilation system capacity, if used appropriately, is usually sufficient to remove odours arising from normal occupant activities within a dwelling. In an office building, body odour is often the key pollutant, but there are a number of other pollutant sources including the building itself, furnishings, printers and photocopiers.

                              Types of ventilation

                              4.11 Buildings are ventilated through a combination of infiltration and purpose- provided ventilation:

                              • Infiltration is the uncontrollable air exchange between the inside and outside of a building through a wide range of air leakage paths in the building structure.
                              • Purpose-provided ventilation is the controllable air exchange between the inside and outside of a building by means of a range of natural and/or mechanical devices.

                              4.12 It is important to minimise the uncontrollable infiltration and supply sufficient purpose-provided ventilation. Air tightness measures to limit infiltration are covered in Part L of the Building Regulations and its supporting Approved Documents. Approved Document F recommends methods of achieving sufficient purpose-provided ventilation, allowing for a reasonably high level of airtightness.

                              4.13 For the purposes of Part F, a reasonably high level of airtightness means a level which is significantly tighter than the minimum target value recommended under Part L, because all new buildings are expected to better the target value to some degree. Through good design and execution, domestic and non-domestic buildings can currently achieve an air permeability down to around 2 to 4 m³/(h.m²) of envelope area at 50 Pascal (Pa) pressure difference. Some buildings constructed are tighter than this. It can be anticipated that there will be a continual trend towards more airtight buildings due to drivers for higher energy efficiency and lower carbon emissions.

                              4.14 The ventilation provisions recommended for new dwellings in this Approved Document have been specified for two standard designs of air permeability:

                              • In the default option, the guidance assumes zero air permeability and thus no infiltration. The building ventilation is reliant entirely on installed purpose-provided ventilation, thus ensuring sufficient ventilation for dwellings of all levels of air permeability. This option should be particularly suitable when intending to construct a more airtight dwelling, or where the person carrying out the building work does not have previous experience of closely matching design with as-constructed air permeability and may, as a consequence, achieve a significantly better performance than designed.
                              • In the alternative option, the guidance assumes an infiltration of 0.15 air changes per hour (ach). The recommended purpose- provided ventilation is less as infiltration contributes to the total amount of ventilation. This option is most appropriate when designing to an air permeability leakier than 5 m³/(h.m²) at 50 Pa, and it is expected from experience that the measured air permeability will be close to the design value and will not be better than 3 m³/(h.m²) at 50 Pa.

                              The ventilation strategy adopted in Approved Document F

                              Paragraphs 5.8 to 5.10 provide further information.

                              4.15 Approved Document F adopts the following strategy. (Systems which comply with the strategy are described in Sections 5 and 6.)

                              • Extract ventilation from rooms where most water vapour and/or pollutants are released, e.g. due to activities such as cooking, bathing or photocopying. This is to minimise their spread to the rest of the building. This extract may be either intermittent or continuous.
                              • Whole building/dwelling ventilation to provide fresh air to the building and to dilute and disperse residual water vapour and pollutants not dealt with by extract ventilation as well as removing water vapour and other pollutants which are released throughout the building (e.g. by building materials, furnishings, activities and the presence of occupants). Whole building/ dwelling ventilation provides nominally continuous air exchange. The ventilation rate may be reduced or ceased when the building is not occupied. It may be necessary to purge the air when the building is re-occupied.
                              • Purge ventilation throughout the building to aid removal of high concentrations of pollutants and water vapour released from occasional activities such as painting and decorating or accidental releases such as smoke from burnt food or spillage of water. Purge ventilation is intermittent, i.e. required only when such occasional activities occur. Purge ventilation provisions may also be used to improve thermal comfort, although this is not controlled under the Building Regulations.

                              4.16 This ventilation strategy can be delivered by a natural ventilation system or a mechanical ventilation system or a combination of both (i.e. ‘mixed-mode’ or ‘hybrid’ ventilation system). For mainly naturally ventilated buildings, it is common to use a combination of ventilators to achieve this strategy (e.g. for dwellings it is common to use intermittent extract fans for extract ventilation, trickle ventilators for whole dwelling ventilation and windows for purge ventilation). For mechanically ventilated or air- conditioned buildings, it is common for the same ventilators to provide both local extract and whole building / dwelling ventilation and, for buildings other than dwellings, to provide purge ventilation as well.

                              4.17 The ventilation systems and devices mentioned in this document are examples of those commonly in use at the time of writing. Other ventilation systems and devices, perhaps following a different strategy (e.g. positive input ventilation or supply air windows), may provide acceptable solutions, provided it can be demonstrated to the BCB (e.g. by use of a product or system covered by a national or European certificate issued by a European Technical Approval issuing body, provided the conditions of use are in accordance with the terms of the certificate) that they meet Requirement F1. The assessment of the suitability of a ventilation system should take account of the design air permeability of  the building, and the fact that the completed building may be ‘tighter’ than the design value because of variability in construction quality.

                              Control of ventilation

                              4.18 It is important that ventilation is controllable so that it can maintain reasonable indoor air quality and avoid waste of energy. These controls can be either manual (i.e. operated by the occupant) or automatic. Demand-controlled ventilation systems employ sensors to detect the level of occupancy, water vapour or other pollutants and adjust the ventilation rate accordingly in order to avoid over- ventilation and so reduce energy consumption.

                              4.19 Manually controlled trickle ventilators (the most common type of background ventilators) can be located over the window frames, in window frames, just above the glass or directly through the wall (see Diagram 1 in Key terms). They are positioned typically 1.7 m above floor level to avoid discomfort due to cold draughts. These ventilators often incorporate a simple flap that allows users to shut off the ventilation – depending on external weather conditions. Trickle ventilators are intended to be normally left open in occupied rooms in dwellings. A window with a night latch position is not recommended because of the difficulty of measuring the equivalent area, the greater likelihood of draughts and the potential increased security risk in some locations.

                              4.20 In dwellings, humidity-controlled devices are available to regulate the humidity of the indoor air and, hence, minimise the risk of condensation and mould growth. These are best installed as part of an extract ventilator in moisture-generating rooms (e.g. kitchen or bathroom). Humidity control is not appropriate for sanitary accommodation, where the dominant pollutant is normally odour. Trickle ventilators are available which ‘throttle down’ the ventilation flow passage(s) according to the pressure difference across the ventilator to reduce draught risks during windy weather. Manufacturers should be consulted when selecting the correct type of pressure-controlled trickle ventilator.

                              4.21 Other types of automatic control may be suitable for regulating ventilation devices (e.g. trickle ventilators, ventilation fans, dampers and air terminal devices) in dwellings. In such cases, it is important that the device controls the ventilation air supply and/or extract according to the need for ventilation in the space to remove or dilute indoor pollutants and water vapour. Trickle ventilators with automatic controls should also have manual override, so that the occupant can close the ventilator to avoid draughts and fully open the ventilator to provide maximum air flow when required. For pressure-controlled trickle ventilators that are fully open at typical conditions (e.g. 1 Pa pressure difference), only a manual close option is recommended.

                              4.22 In buildings other than dwellings, more sophisticated automatic control systems are available. These may be based on sensors located within the building, e.g. occupancy sensors (using local passive infra-red detectors) or indoor carbon dioxide concentration sensors (using electronic carbon dioxide detectors) as an indicator of occupancy level and, therefore, body odour.

                              Performance-based guidance

                              4.23 This Approved Document focuses on performance-based guidance which suggests to the designer what level of ventilation should be sufficient, rather than how it should be achieved. Therefore, the designer has the freedom to use whatever ventilation provisions suit a particular building, including the use of innovative products and solutions, if it can be demonstrated that they meet the performance standard recommended in this Approved Document.

                              4.24 The actual performance criteria for acceptable levels of moisture and pollutants are given in Appendix A. The air flow rates necessary to meet the performance criteria are given in the main guidance.

                              4.25 Simple guidance in the form of ventilator sizes for the whole dwelling is also provided to make it easier for designers to meet building regulations requirements in common situations.

                              Equivalent area of ventilators

                              4.26 Equivalent area is used in the Approved Document instead of free area for the sizing of background ventilators (including trickle ventilators) because it is a better measure of the air flow performance of a ventilator. unlike equivalent area, free area is simply the physical size of the aperture of the ventilator but may not accurately reflect the air flow performance which the ventilator will achieve. The more complicated and/or contorted the air flow passages in a ventilator, the less air will flow through it. So, two different ventilators with the same free area will not necessarily have the same air flow performance. A European Standard, BS EN 13141-1:2004 (Clause 4), includes a method of measuring the equivalent area of background ventilator openings.

                              4.27 As equivalent area cannot be verified with a ruler, it will be difficult to demonstrate to Building Control Bodies that trickle ventilators and similar products have the correct equivalent area unless it is clearly marked on the product. For this reason, it is preferable to use ventilators which have the equivalent area (in mm² at 1 Pa pressure difference), or equivalent area per metre (where the equivalent area of the product varies according to length) marked on the product in an easily visible location from the inside of the dwelling when installed. Where  it is not practical for the manufacturer to mark the ventilator because it can be used in conjunction with a range of other components, some  form of temporary marking for the installed system should be acceptable to the BCB.

                              Ventilation effectiveness

                              4.28 Ventilation effectiveness is a measure of how well a ventilation system works in terms of delivering the supply air to the occupants of a building. If the supply air is mixed fully with the room air before it is breathed by the occupants, the ventilation effectiveness is 1. If the supply air is extracted from the room before it mixes with any room air, the ventilation effectiveness is 0. If the supply air reaches the occupant without mixing with any room air, the ventilation effectiveness tends towards infinity.

                              4.29 This is important as a system with a higher ventilation effectiveness achieves acceptable pollutant levels at the occupant’s breathing zone for a lower air supply rate, and offers potentially significant energy savings. However, it has been decided not to make an allowance for any reduction of fresh air supply rates based on ventilation effectiveness in Approved Document F at this time. This is because ventilation effectiveness is dependent on the ventilation system design, its installation and the way in which occupants use the space. While it is possible to predict what the ventilation effectiveness of a system should be, there is currently insufficient knowledge of the actual ventilation effectiveness achieved in buildings to allow designers to guarantee performance and so avoid significant under-ventilation by reducing air supply rates. This is because ventilation effectiveness may be influenced by factors beyond the designer’s control such as occupant usage (e.g. seating plan and use of computers within a space and whether the space is being heated or cooled by the ventilation air). In the designs shown in this Approved Document, it has been assumed that the ventilation effectiveness is 1.0. CIBSE Guide A provides further information on ventilation effectiveness.

                              Source control

                              4.30 A complementary strategy for achieving good indoor air quality is to reduce the release of water vapour and/or air pollutants into the indoor air, i.e. source control. Source control is not considered within the main guidance of the Approved Document owing to limited knowledge about the emission of pollutants from construction and consumer products used in buildings and the lack of suitable labelling schemes for England and Wales. Some construction products such as glass, stone and ceramics are by their nature low emitters of air pollutants. Currently, some paints are labelled for their volatile organic compound (VOC) content, and some wood-based boards (class E1, BS EN 13986:2004) are available with low formaldehyde emission. This allows suitable products to be chosen when good indoor air quality is a priority, but at the present time it is not practical to make an allowance for use of these products in the ventilation requirements. Further information about control of emissions from construction products is available in BRE Digest 464.

                              4.31 Exposure to house dust mite (HDM) allergens can lead to allergic sensitisation and to exacerbation of allergic conditions. The moisture criteria needed to avoid HDM are more complex and demanding than those needed to avoid mould. The reduction of mite growth may be feasible in UK dwellings via appropriate ventilation, heating and moisture control as part of an integrated approach that involves the removal of existing mite allergens.

                              4.32 Useful information may be found in the World Health Organisation (WHO) publication, House dust mites, Crowther D and Wilkinson T (2008), which is included in Public health significance of urban pests, Bonnefoy X, Kampen H and Sweeney K, WHO Regional Office for Europe, available at:

                              4.33 Further information and suggested measures for source control are given in Review of health and safety risk drivers (BD 2518), available at:


                              4.34 The noise caused by ventilation systems is not controlled under the Building Regulations. However, such noise may be disturbing to the occupants of a building and it is recommended that measures be taken to minimise noise disturbance. For example, in noisy areas, in order to reduce noise entering the building through the ventilation system, it may be  appropriate to use sound-attenuating ventilation products depending on the noise level and any planning conditions.

                              4.35 Noise from the ventilation system may also disturb people who are outside the building, so measures to minimise externally emitted noise should also be considered.

                              4.36 Noise generated by ventilation fans (which may travel through ducts) and noise from the fan unit may disturb the occupants of the building and so discourage their use. Therefore, the designer should consider minimising noise by careful design and the specification of quieter products.

                              To ensure good acoustic conditions, the average A-weighted sound pressure level in noise sensitive rooms, such as bedrooms and living rooms, should not exceed 30 dB LAeq,T (see Note below). In less sensitive rooms, such as kitchens and bathrooms, a higher level would be acceptable, e.g. 35 dB LAeq,T . Noise from a continuously running mechanical ventilation system on its minimum low rate should not normally exceed these levels, and should preferably be lower in order to minimise the impact of the ventilation system.

                              The main issues to be addressed in minimising the noise impact of the ventilation system are the noise from the fan unit entering the ducts, and the attenuation provided by the ducts, bends and junctions and the characteristics of the roomgrill. The noise breaking out of the fan unit casing may also be significant in adjacent rooms. The characteristics of the room will also affect the noise level.

                              Methods for measuring the sound power level produced by the fan unit of decentralised extract, centralised extract, and balanced supply and extract with heat recovery systems are under development. When available, they should be read in conjunction with the appropriate parts of BS EN 13141.

                              Note: The noise index LAeq,T is used in BS 8233:1999, where T is the duration of the measurement. If the noise from the sound source is steady (e.g. fluctuating by up to 3 dB) a measuring time of 1 minute will be adequate and the LAeq,1 min level will be similar to the dB(A) level used elsewhere. If the noise from the sound source fluctuates more than this a longer measuring time (T) will be required.

                              Fire precautions

                              4.37 Where ducting passes through a fire- resisting wall/floor or fire compartment, the required measures to ensure compliance with Part B of the Building Regulations must be taken.

                              Modular and portable buildings

                              4.38 Buildings constructed from sub- assemblies that are delivered newly made or selected from stock should be treated no differently from any other new building and must comply with all the relevant requirements in Schedule 1 to the Building Regulations 2010 that were in force when they were manufactured. For guidance on energy efficiency requirements, see Approved Document L2A.

                              Installation of ventilation systems

                              4.39 It is recommended that ventilation systems are installed in new and existing dwellings in accordance with the guidance in the 2010 edition of the Domestic ventilation compliance guide, available from The guide is referenced at relevant points of this Approved Document – in Section 5, New dwellings, and in Section 7, Work on existing buildings.

                              4.40 Section 5 of the Domestic ventilation compliance guide includes an installation checklist which should be completed by the system installer. It also includes an installation inspection sheet where the equivalent area of background ventilators should be recorded.

                              4.41 Installation guidance for buildings other than dwellings can be found in the references listed in Table 6.3.

                              Air flow rate testing and commissioning of ventilation systems

                              4.42 The Regulations require:

                              • mechanical ventilation systems to be commissioned (where they can be tested and adjusted) to provide adequate ventilation and a commissioning notice to be given to the BCB;
                              • air flow rates for mechanical ventilation systems in new dwellings to be measured and a notice to be given to the BCB.

                              4.43 For dwellings, the procedures approved by the Secretary of State for measurement of air flow rates and for commissioning of mechanical ventilation systems are in Sections 2 and 3 of the Domestic ventilation compliance guide. Section 5 of the guide contains air flow measurement test and commissioning sheets which should be completed by the person responsible for commissioning.

                              4.44 For commissioning of non-domestic ventilation systems, the commissioning procedure approved by the Secretary of State is CIBSE Code M, available from

                              4.45 The commissioning of mechanical ventilation systems to provide adequate ventilation under Part F of the Building Regulations will need to be carried out with the commissioning of such systems to ensure that they use no more fuel and power than is reasonable in the circumstances under Part L of the Building Regulations. It is permissible for the notice of commissioning to be given to the BCB to cover the commissioning required under both Parts F and L. Approved Documents L1A, L1B, L2A and L2B give guidance on commissioning required by Part L.

                              4.46 In addition, guidance on commissioning ventilation ductwork is provided in the HVCA guidance documents DW/144 Specification for sheet metal ductwork: low, medium and high pressure/velocity air systems, DW/154 Specification for plastics ductwork, and DW/143 Practical guide to ductwork leakage testing, all available from

                              Provision of information

                              4.47 The Regulations require sufficient information about the ventilation system and its maintenance requirements to be given to owners so that the ventilation system can be operated to provide adequate air flow.

                              4.48 For new and existing dwellings, Section 4 of the Domestic ventilation compliance guide lists the documents that it is recommended should be given to the dwelling owner at installation handover. The inspection checklist and air flow measurement test and commissioning sheet in Section 5 should form part of the information pack. The parts of Section 5 covering air flow measurement and commissioning should also be given to the BCB.

                              4.49 For new and existing buildings other than dwellings, a way of showing compliance would be to follow the guidance in Approved Documents L2A and L2B, which reference the CIBSE TM 31 Building log book toolkit, 2006.