Building Standards technical handbook 2017: non-domestic buildings

The Building Standards technical handbooks provide guidance on achieving the standards set in the Building (Scotland) Regulations 2004 and are available in two volumes, Domestic buildings and Non-domestic buildings. This publication is available in html and also in PDF format (in 'supporting documents' ).

3.14 Ventilation

Mandatory Standard

Standard 3.14

Every building must be designed and constructed in such a way that ventilation is provided so that the air quality inside the building is not a threat to the building or the health of the occupants.

3.14.0 Introduction

Ventilation of a building is required to maintain air quality and so contribute to the health and comfort of the occupants. Without ventilation it is possible that carbon dioxide, water vapour, organic impurities, smoking, fumes and gases could reduce the air quality by humidity, dust and odours and also reduce the percentage of oxygen in the air to make the building less comfortable to work or live in.

Well designed natural ventilation has many benefits, not least financial and environmental, although it is also recognised that inside air quality can only be as good as outside air quality and in some cases filtration may be necessary. In other cases mechanical systems or systems that combine natural with mechanical (hybrid) may provide the ventilation solution for the building.

Ventilation can also have a significant affect on energy consumption and performance and so thorough assessment of natural, as against mechanical ventilation, should be made, as the decision could significantly affect the energy efficiency of the building (see Section 6, Energy).

Ventilation should not adversely affect comfort and, where necessary, designers might wish to consider security issues and protection against rain penetration prevalent in naturally ventilated buildings when windows are partially open to provide background ventilation.

Reducing air infiltration - improved insulation and ‘tighter’ construction of buildings will reduce the number of natural air changes but can increase the risk of condensation. However leaky buildings are draughty and uncomfortable. Sealing up air leaks improves comfort and saves energy whilst proper ventilation keeps the indoor air pleasant and healthy. If poor attention to detail occurs air leakage can account for a substantial part of the heating costs. Energy savings from building ‘tighter’ could make significant savings on energy bills. There is a common perception that ‘tight’ construction promotes indoor air pollution. However both ‘tight’ and 'leaky' buildings can have air quality problems. Though air leaks can dilute indoor pollutants, there is no control over how much leakage occurs, when it occurs or where it comes from. BRE GBG 67, ‘Achieving air tightness: General principles’ provides useful guidance on how to build new buildings tighter.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirement of this standard (regulation 12, schedule 6).

3.14.1 Ventilation generally

A building should have provision for ventilation by either:

  1. natural means, or

  2. mechanical means, or

  3. a combination of natural and mechanical means (mixed-mode).

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. Ventilation should have the capacity to:

  • provide outside air to maintain indoor air quality sufficient for human respiration

  • remove excess water vapour from areas where it is produced in sufficient quantities in order to reduce the likelihood of creating conditions that support the germination and growth of mould, harmful bacteria, pathogens and allergies

  • remove pollutants that are a hazard to health from areas where they are produced in significant quantities

  • rapidly dilute pollutant odours, where necessary.

Additional ventilation provision - this guidance relates to the provision of air for human respiration and is in addition to, and should be kept separate from, any air supply needed for the smoke ventilation of escape routes in the case of fire (Section 2, Fire) and for the safe operation of combustion appliances (see Standards 3.21 and 3.22).

There is no need to ventilate:

  1. a store room used only for storage that requires a controlled temperature

  2. a room with a floor area of not more than 4m2. This is not intended to include a domestic sized kitchen or utility room where ventilation should be in accordance with the recommendations in the table in clause 3.14.5.

Ventilation should be to the outside air. However clause 3.14.3 explains where trickle ventilators may be installed other than to the external air.

Calculation of volume - for ventilation purposes, a storey should be taken as the total floor area of all floors within that storey, including the floor area of any gallery or openwork floor. Where an air change rate is recommended, the volume of the space to be ventilated may be required. The volume of any space is the internal cubic capacity of the space. Any volume more than 3m above any floor level in that space may be disregarded.

3.14.2 Natural ventilation

All buildings leak air to a greater or lesser extent. However the movement of uncontrolled infiltrating air through the fabric of a building can cause draughts and can have a significant adverse effect on the energy efficiency of the building as a whole. By improving building techniques it is possible to reduce this infiltrating air to lower levels that can improve energy efficiency (see Section 6 Energy).

Some building techniques may have little effect on air leakage and so allow the uncontrolled infiltrating air to be taken into account in the building's ventilation provision. By building with techniques designed to reduce air leakage there will need to be a reciprocal increase in the designed ventilation provision to make up for the lower levels of infiltrating air where the designer intends to use low fabric insulation rates of less than 5m3/h/m2 in the energy assessment (see Section 6 Energy). The areas of trickle ventilation shown may not suffice to maintain air quality and therefore an alternative ventilation solution should be adopted.

Natural ventilation of a room or building should be provided in accordance with the following recommendations:

  1. for a room, by the provision of a ventilator with an opening area of at least 1/30th of the floor area of the room it serves, and

    • a trickle ventilator with an opening area of at least 4000mm2, if the area of the room is not more than 10m2, or

    • a trickle ventilator with an opening area of 400mm2 for each square metre of room area, if the area of the room is more than 10m2, or

  2. for a room in a building constructed with an infiltration rate of 5 to 10m3/h/m2 at 50 Pa, by the provision of a ventilator with an opening area of at least 1/30th of the floor area of the room it serves, and

    • a trickle ventilator with an opening of at least 10000mm2 if the room is not more than 10m2, or

    • a trickle ventilator with an opening area of at least 10000mm2 plus an additional 600mm2 for each square metre of room area if the room is more than 10m2

  3. for a toilet, mechanical extract in accordance with the table to clause 3.14.5

  4. for any other building, by following the guidance in:

    • Section 3 of BS 5925: 1991 (1995), or

    • CIBSE Guide A: 1999, Design data, section A4, Air infiltration and natural ventilation, or

    • CIBSE AM10: Natural Ventilation in Non-Domestic Buildings (2005) Applications Manual AM10: 2005.

The options in sub-clause (d) provide more flexible solutions but may require complex calculations.

Wet areas - where a building is naturally ventilated, all moisture producing areas such, as bathrooms and shower rooms, should have the additional facility for removing such moisture before it can damage the building. Additional mechanical ventilation to such areas should be provided in accordance with the table to clause 3.14.5.

Opening height - where rapid ventilation is provided, such as an opening window or windows, some part of the opening should be at least 1.75m above floor level. This will reduce the problems of stratification of air.

3.14.3 Trickle ventilators

A trickle ventilator, sometimes called 'background ventilation', is a small ventilation opening, mostly provided in the head of a window frame, but not always, and is normally provided with a controllable shutter. They should be provided in naturally ventilated areas to allow fine control of air movement. A permanent ventilator is not recommended since occupants like control over their environment and uncontrollable ventilators are usually permanently sealed to prevent draughts.

The trickle ventilator should be so positioned that a part of it is at least 1.75m above floor level. This will allow at least some movement of air within the building and reduce stratification.

Although ventilation should normally be to the external air, a trickle ventilator serving a bathroom or shower room may open into an area that does not generate moisture, such as a bedroom or hallway, provided the room is fitted with a trickle ventilator in accordance with the guidance in clause 3.14.2.

A trickle ventilator should be provided in an area containing mechanical extraction to provide replacement air and ensure efficient operation when doors are closed. This will prevent moist air being pulled from other ‘wet areas’. Pulling moist air from other parts of a building will reduce the further apart the wet rooms are located. The trickle ventilator should be independent of the mechanical extract so that replacement air can be provided when the extract fan is operating. The location of the trickle ventilator and the extract fan should be located to prevent short-circuiting of the air.

3.14.4 Extensions built over existing windows

Constructing an extension over an existing window, or ventilators, will effectively result in an internal room, will restrict air movement and could significantly reduce natural ventilation to that room. Reference should be made to the guidance to Standards 3.21 and 3.22 on the ventilation of combustion appliances, as this may be relevant. There are other recommendations in Section 2: Fire, relating to escape from inner rooms.

A new ventilator and trickle ventilator should be provided to the existing room but, where this is not reasonably practicable, e.g. if virtually the entire external wall of the room is covered by the extension, the new extension should be treated as part of the existing room rather than the creation of a separate internal room. Because an extension will be relatively airtight, the opening area between the 2 parts of the room should be not less than 1/15th of the total combined area of the existing room plus the extension.

If the extension is constructed over an area that generates moisture, such as a kitchen, bathroom, shower room or utility room, mechanical extract, via a duct if necessary, should be provided direct to the outside air. Any existing system disadvantaged by the work may require to be altered to ensure supply and extracted air are still to the outside air.

3.14.5 Mechanical ventilation

A mechanical ventilation or air conditioning system should be designed, installed and commissioned to perform in a way that is not be detrimental to the health of the occupants of a building and when necessary should be easily accessible for regular maintenance.

Mechanical extract should be provided in rooms where the cubic space per occupant is not more than 3m3, and where the rooms have low ceilings and are occupied by large numbers of people.

Mechanical ventilation should be provided in accordance with the following:

  1. compliance with guidance in BS 5720: 1979, or

  2. compliance with the guidance in CIBSE Guide B: 2001, Installation and equipment data, section B2, Ventilation and air-conditioning (requirements), or

  3. for occupiable rooms, where a mechanical air supply is provided at a rate of at least 8 litres/second of fresh air per occupant, based on sedentary occupants and the absence of other requirements such as the removal of moisture, or

  4. for domestic-sized rooms where moisture is produced, such as kitchens, bathrooms and sanitary accommodation, rapid ventilation and trickle ventilation should be provided in accordance with the guidance in the following table.

    Table 3.9. Mechanical ventilation of domestic-sized kitchens, bathrooms & toilets

    Space Ventilation provision [2] Trickle ventilation >10 m3/h/m2 Trickle ventilation 5-10 m3/h/m2


    1. mechanical extraction capable of at least 30 litres/sec (intermittent) above a hob [2]; or

    2. mechanical extraction capable of at least 60 litres/sec (intermittent) if elsewhere [3]

    4000mm2 10000mm2
    Utility room or washroom mechanical extraction capable of at least 30 litres/sec (intermittent) [3] 4000mm2 10000mm2
    Bathroom or shower room (with or without a WC) mechanical extraction capable of at least 15 litres/sec (intermittent) 4000mm2 10000mm2
    Toilet mechanical extraction capable of at least 3 air changes per hour 4000mm2 10000mm2

Additional information:

  1. The trickle ventilation rates recommended relate to the infiltration rate of the building fabric which can be used in SBEM calculations (see Section 6 Energy).

  2. Where the building infiltration rate is designed to be less than 5m3/h/m2 the trickle vent rates in the above table might not be sufficient to maintain air quality and an alternative solution should be adopted.

  3. Long duct runs, flexible ducting and bends can seriously reduce fan performance and should be carefully considered during design to ensure recommended air flows are achieved.

  4. Refer to guidance to Standard 3.17 and OFTEC Technical Book 3 where an extract fan is fitted in a building containing an open-flued combustion appliance. Extract rates should be reduced.

Continuous mechanical ventilation - for smaller, domestic sized developments, a mechanical ventilation system complying with BRE Digest 398, ‘Continuous mechanical ventilation in dwellings: design, installation and operation’ may be appropriate.

Where a mechanical ventilation system gathers extracts into a common duct for discharge to an outlet, no connection to the system should be made between any exhaust fan and the outlet.

Mechanical ventilation should be to the outside air. However it may be via a duct or heat exchanger.

Care should be taken when installing mechanical extract systems where there is an open-flued combustion appliance in the same room or close by. Guidance is given in clause 3.17.8, extract fans.

Cross contamination - an inlet to, and an outlet from, a mechanical ventilation system should be installed so as to avoid contamination of the air supply to the system. The inlet to, and the outlet from, the mechanical ventilation system should be installed in accordance with the recommendations in clause 2.3.3 of BS 5720: 1979.

3.14.6 Control of legionellosis

A mechanical ventilation system should be constructed to ensure, as far as is reasonably practicable, the avoidance of contamination by legionella. The ventilation system should be constructed in accordance with the recommendations of Legionnaires’ Disease: The control of legionella bacteria in water systems - approved code of practice and guidance - HSE L8.

The guidance provided in HSE catering sheet No 10, 2000: 'Ventilation of kitchens in catering establishments' provides useful information.

There are additional recommendations in Section 2, Fire where mechanical ventilation systems pass through compartment walls, separating walls and separating floors.

3.14.7 Ventilation of sanitary accommodation

Any area containing sanitary facilities should be well ventilated, so that offensive odours do not linger. Measures should be taken to prevent odours entering other rooms. This may be achieved by, for example, providing a ventilated area between the sanitary accommodation and the other room. Alternatively it may be possible to achieve it by mechanical ventilation or, if the sanitary accommodation is well sealed from a workroom and has a door with an automatic closer, by good natural ventilation.

However no room containing sanitary facilities should communicate directly with a room for the preparation or consumption of food. This does not apply to places of lawful detention, such as integral sanitation in prison cells.

3.14.8 Ventilation of small garages

The principal reason for ventilating garages is to protect the building users from the harmful effects of toxic emissions from vehicle exhausts. Where a garage is attached to a building, designers may wish to consider making the separating construction as air tight as possible. Where there is a communicating door, a lobby arrangement could be considered.

Garages of less than 30m2 do not require the ventilation to be designed. It is expected that a degree of fortuitous ventilation is created by the imperfect fit of ‘up and over’ doors or pass doors. With such garages, it is inadvisable for designers to attempt to achieve an airtight construction.

A garage with a floor area of at least 30m2 but not more than 60m2 used for the parking of motor vehicles should have provision for natural or mechanical ventilation. Ventilation should be in accordance with the following guidance:

  1. where the garage is naturally ventilated, by providing at least 2 permanent ventilators, each with an open area of at least 1/3000th of the floor area they serve, positioned to encourage through ventilation with one of the permanent ventilators being not more than 600mm above floor level, or

  2. where the garage is mechanically ventilated, by providing a system:

    • capable of continuous operation, designed to provide at least 2 air changes per hour, and

    • independent of any other ventilation system, and

    • constructed so that two-thirds of the exhaust air is extracted from outlets not more than 600mm above floor level.

3.14.9 Ventilation of large garages

A garage with a floor area more than 60m2 for the parking of motor vehicles should have provision for natural or mechanical ventilation on every storey. Ventilation should be in accordance with the following guidance:

  1. Section 3 requirements of CIBSE Guide B2: 2001, Ventilation and air conditioning:

    • to give carbon monoxide concentrations of not more than 30 parts per million averaged over an 8 hour period, and

    • to restrict peak concentrations of carbon monoxide at areas of traffic concentrations such as ramps and exits to not more than 90 parts per million for periods not exceeding 15 minutes, or

  2. Section 4 of the Association for Petroleum and Explosive Administration’s “Code of practice for ground floor, multi-storey and underground car parks” and CIBSE Guide B, 1986, Section B2, or

  3. By providing openings in the walls on every storey of at least 1/20th of the floor area of that storey with at least half of such area in opposite walls to promote extract ventilation, if the garage is naturally ventilated, or

  4. By providing mechanical ventilation system capable of at least 6 air changes per hour and at least 10 air changes per hour where traffic concentrations occur, or

  5. Where it is a combined natural/mechanical ventilation system, by providing:

    • openings in the wall on every storey of at least 1/40th of the floor area of the storey with at least half of such area in opposite walls, and

    • a mechanical system capable of at least 3 air changes per hour.

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