Building standards technical handbook 2020: domestic

The building standards technical handbooks provide guidance on achieving the standards set in the Building (Scotland) Regulations 2004. This handbook applies to a building warrant submitted on or after 1 March 2021 and to building work which does not require a warrant commenced from that date.

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 dwelling 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.

So that contaminants do not exceed acceptable levels and thereby endanger the health of the occupants, it is important that dwellings are adequately ventilated. Research has shown that occupants of dwellings are, for the most part, unaware of the standard of air quality within their homes. The lack of recognition of poor air quality has frequently resulted in occupants not being aware of the need to open ventilators or windows, particularly in bedrooms.

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.

Occupants should have the opportunity to dry washing other than by a tumble dryer which uses a considerable amount of energy. Drying of washing internally can generate large quantities of moisture that should be removed before it damages the building.

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 dwelling 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).

Small rooms - there is no need to ventilate 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 clause 3.14.3.

Ventilation should be to the outside air. However clauses 3.14.6 and 3.14.8 explain where ventilators and 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 Ventilation awareness in dwellings

Carbon dioxide (CO2) is present in the external air we breathe at concentration levels of around 400 parts per million and is not harmful to health at low concentration levels. However, as people release CO2 into the air when they exhale, increased levels of CO2 in occupied buildings can occur. This is generally accepted as being a reasonable indication that ventilation action is necessary.

CO2 monitoring equipment should be provided in the apartment expected to be the main or principal bedroom in a dwelling where infiltrating air rates are less than 15m3/hr/m2 @ 50 Pa. This should raise occupant awareness of CO2 levels (and therefore other pollutants) present in their homes and of the need for them to take proactive measures to increase the ventilation. Guidance on the operation of the monitoring equipment, including options for improving ventilation when indicated as necessary by the monitor, should be provided to the occupant. For more detailed information on the provision of guidance to occupants, reference may be made to “Domestic Ventilation” Scottish Government 2017

The installed monitoring equipment for CO2 should be mains operated and may take the form of a self-contained monitor/detector or a separate monitor and detector head. The monitor should have an easily understood visual indicator and be capable of logging data to allow the occupant to gain information on CO2 levels for at least the preceding 24 hour period. If the detector/monitor has an audible alarm this should be capable of being permanently deactivated.

CO2 monitoring equipment should be capable of recording and displaying readings within a range of at least 0 – 5,000 parts per million. The equipment should also be capable of logging data at no more than 15 minute intervals, over a 24 hour period.

Where carbon dioxide monitors/detectors are within the scope of either or both:

  • European Directive 2006/95/EC – Low Voltage Directive, and/or

  • European Directive 1999/5/EC – Radio and Telecommunication Terminal Equipment Directive

they should be constructed to fully comply with all applicable safety aspects of the Directive(s).

A carbon dioxide detector head requires a flow of air over it to operate correctly, therefore, it should not be located in an area that is likely to restrict the free movement of air. Unless otherwise indicated by the manufacturer, a carbon dioxide detector head should not be sited:

  • if ceiling mounted, within 300mm of any wall

  • if wall mounted, within 150mm of the ceiling or a junction with another wall

  • where it can be obstructed (for example by curtains, blinds or furniture)

  • next to a door or window, or

  • next to an air vent or similar ventilation opening.

Unless otherwise indicated by the manufacturer, a carbon dioxide monitor, with or without an integral detector, should be mounted between 1.4m and 1.6m above floor level. A carbon dioxide detector head (or monitor if integrated) should not be sited within 1m of the expected location of a bed-head.

Where a separate detector head and monitor is installed, the monitor may be located other than in the room containing the detector head, for example, the hallway. This may be desirable if more than one detector head is installed.

3.14.3 Ventilation of dwellings

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.

Recommendations for trickle ventilation in the table below are made on the basis that infiltrating air rates of 5 to 10m3/h/m2@ 50 Pa will be achieved as a matter of course in modern dwellings. However where the designer intends to use low fabric infiltration air rates of less than 5m3/h/m2@ 50 Pa in the SAP calculations (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 (see clause 3.14.11).

Table 3.5. Recommended ventilation of a dwelling

  Ventilation recommendations Trickle ventilation [1]
Apartment A ventilator with an opening area of at least 1/30th of the floor area it serves. 12,000mm2


  1. mechanical extraction capable of at least 30 l/sec (intermittent) above a hob [3], or

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

  3. a passive stack ventilation system [4].

Utility room


  1. mechanical extraction capable of at least 30 l/sec (intermittent) [3], or

  2. a passive stack ventilation system [4].

Bathroom or shower room (with or without a WC)


  1. mechanical extraction capable of at least 15 l/sec (intermittent), or

  2. a passive stack ventilation system [4].



  1. a ventilator with an opening area of at least 1/30th of the floor area it serves, or

  2. mechanical extraction capable of at least 3 air changes per hour.


Additional information:

  1. Where the trickle ventilator is ducted, the recommended areas in the table should be doubled (see clause 3.14.6).

  2. The overall provision of trickle ventilation in a dwelling may be provided at an average of 11,000mm2 per room with a minimum of 11,000 mm2 for each apartment.

  3. 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.

  4. Refer to Section 2: Fire where a passive stack ventilation system is installed in a building containing flats and maisonettes.

  5. Long duct runs, flexible ducting and bends can reduce fan performance and should be carefully considered during design.

Work on existing buildings - where infiltration rates in a dwelling exceed 10m3/h/m2 @ 50 Pa, which may often be the case in existing buildings, the size of trickle ventilation may be reduced to 8000mm2 for apartments and 4000mm2 for all other rooms. Alternatively, the overall provision of trickle ventilation in a dwelling may be provided at an average of 6000mm2 per room, with a minimum provision of 4000mm2 in each apartment.

Height of ventilator - to reduce the effects of stratification of the air in a room, some part of the opening ventilator should be at least 1.75m above floor level.

3.14.4 Ventilation of conservatories

With large areas of glazing, conservatories attract large amounts of the sun’s radiation that can create unacceptable heat build-up. Efficient ventilation therefore is very important to ensure a comfortable environment. A conservatory should have a ventilator or ventilators with an opening area of at least 1/5th of the floor area it serves. Although this is the minimum recommended area, a greater area can provide more comfortable conditions particularly in sunny weather. Notwithstanding the recommended opening height of 1.75m for ventilators, high level or roof vents are best placed to minimise the effects of heat build-up and reduce stratification.

3.14.5 Ventilation of areas designated for drying of washing

Where clothes are dried naturally indoors large quantities of moisture can be released and this will need to be removed before it can damage the building. Normally a utility room or bathroom is used and mechanical extract is the usual method of removing moisture. Where a space other than a utility room or bathroom is designated, that space should be provided with either:

  • mechanical extraction capable of at least 15 l/s intermittent operation. The fan should be connected through a humidistat set to activate when the relative humidity is between 50 and 65%, or

  • a passive stack ventilation system provided in accordance with the recommendations in clause 3.14.7.

Guidance to Standard 3.11 gives information on the space recommended for the drying of washing.

3.14.6 Trickle ventilators

A trickle ventilator, sometimes called 'background ventilation', is a small ventilation opening, normally provided with a controllable shutter. Although routinely provided in the head of a window frame this is often not the best location as the free movement of air can be restricted, for example by curtains or blinds. They should be provided in naturally ventilated areas to allow fine control of air movement. The location of trickle ventilators should be carefully considered so that they are capable of providing the intended ventilation, taking into account factors such as the size and shape of the room and availability of external walls. A permanent ventilator is not recommended since occupants like control over their environment and uncontrollable ventilators are usually permanently sealed up to prevent draughts.

Trickle vent efficiency - it is recognised that the air flow performance through trickle ventilators can vary, dependent on the design and arrangement of air routes through the ventilator. For the purpose of performance, the recommended areas in the table to clause 3.14.3 should be achieved by the use of ventilators that are sized by the equivalent area, as determined using BS EN 13141-1:2004. When determining the equivalent area, the whole ventilator installation, including the external grille or canopy, should be considered as a single unit.

Where the trickle ventilator has to be ducted, e.g. to an internal room, the equivalent area of the trickle ventilator should be increased to double that shown in the table to clause 3.14.3, to compensate for the reduced air flow caused by friction. This may over-provide ventilation in some cases but can be regulated by the fine control.

Alternatives to proprietary trickle ventilators - fitting proprietary trickle ventilators is the preferred method of fine tuning room ventilation. However in some cases it may be acceptable for background ventilation to be provided through small windows, such as top hoppers, but other issues need to be considered if this method is to be adopted:

  • a partially open window on a night latch is a possible point of forced entry to a dwelling even when the window is locked in position and because of this it is less likely to be left open at night or when the dwelling is empty, even for short periods. Small, upper floor windows in a well lit, open location that are difficult to access may be appropriate

  • it tends to be windier where flats and maisonettes are at high level and windows on night latches do not have fine adjustment to reduce draughts. They are therefore more likely to be kept closed

  • manufacturers will need to show that the opening area when on the night latch is to the recommended sizes in the table to clause 3.14.3

  • some windows might be too small to incorporate the recommended size of trickle ventilator in the frame and careful thought will need to be given to the design and location of trickle ventilators in the window itself or additional trickle ventilators through the external wall

  • trickle ventilators supply replacement air for mechanical extract and passive stack ventilation systems and routes for extracting air provided by mechanical input air systems. It is recommended that proprietary trickle ventilators are used in rooms where such systems are installed since it is more likely that they, rather than windows, will be left open.

Location of trickle ventilators - should be positioned to encourage movement of air within the dwelling and reduce stratification. To assist air movement consideration should be given to providing two or more trickle ventilators within rooms, installed at different heights.

Although ventilation should normally be to the external air, a trickle or permanent 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 area is fitted with a trickle ventilator in accordance with the guidance in clause 3.14.3. In these cases, noise transmission may need to be limited, see Section 5.

A trickle ventilator should be provided in an area fitted with 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’. The trickle ventilator should be independent of the mechanical extract so that replacement air can be provided when the extract fan is operating. Consideration should be given to the location of the ventilator and the fan so as to prevent short-circuiting of the air.

To assist air movement within dwellings with an air infiltration rate of less than 10m3/hr/m2@ 50 Pa, trickle ventilation to rooms with dMEVs could be formed by “undercutting” the room door to achieve an air space of at least 8,000mm2. This air space should be clear of any actual or notional floor coverings.

3.14.7 Passive stack ventilation systems

A passive stack ventilation system uses a duct running from a ceiling (normally in a kitchen or shower room) to a terminal on the roof to remove any moisture-laden air. It operates by a combination of natural stack effect, i.e. the movement of air due to the difference in temperature between inside and outside temperatures and the effect of wind passing over the roof of the building.

A passive stack ventilation system should be installed in full compliance with BRE Information Paper BRE IP 13/94. These systems are most suited for use in a building with a height of not more than 4 storeys (about 8m maximum length of stack) as the stack effect will diminish as the air cools.

Every passive stack system should:

  1. incorporate a ceiling mounted automatic humidity sensitive extract grille that will operate when the relative humidity is between 50 and 65%, and

  2. be insulated with at least 25mm thick material having a thermal conductivity of 0.04W/mK where it passes through a roof space or other unheated space or where it extends above the roof level. This will prevent the walls of the duct from becoming too cold thus inhibiting the stack effect and reducing the likelihood of condensation forming inside the duct.

The flue of an open-flued combustion appliance may serve as a passive stack ventilation system provided that either:

  1. the appliance is a solid fuel appliance and is the primary source of heating, cooking or hot water production, or

  2. the flue has an unobstructed area equivalent to a 125mm diameter duct and the appliance's combustion air inlet and dilution air inlet are permanently open, i.e. there is a path with no control dampers which could block the flow, or the ventilation path can be left open when the appliance is not in use, or

  3. the appliance is an oil firing appliance which is a continually burning vapourising appliance (only) such as a cooker or room heater and the room is fitted with a ventilator with a minimum free area of 10,000mm2.

Non-combustibility - a duct or casing forming a passive stack ventilation system serving a kitchen should be non-combustible. However this is not necessary where it passes through a roof space.

3.14.8 Conservatories and extensions built over existing windows

Constructing a conservatory or extension over an existing window, or ventilator, will effectively result in an internal room, restrict air movement and could significantly reduce natural ventilation to that room. Reference should be made to clause 3.16.2 relating to natural lighting, and to the guidance to Standards 3.21 and 3.22 on the ventilation of combustion appliances, as this also may be relevant. There are other recommendations in Section 2: Fire relating to escape from inner rooms.

A conservatory may be constructed over a ventilator serving a room in a dwelling provided that the ventilation of the conservatory is to the outside air and has an opening area of at least 1/30th of the total combined floor area of the internal room so formed and the conservatory. The ventilator to the internal room should have an opening area of at least 1/30th of the floor area of the room. Trickle ventilators should also be provided relevant to the overall areas created.

An extension may also be built over a ventilator but a new ventilator should be provided to the room. Where this is not practicable, e.g. where there is no external wall, the new extension should be treated as part of the existing room rather than the creation of a separate internal room because the extension will be more airtight than a conservatory and therefore the rate of air change will be compromised. 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 and the extension.

Moisture producing areas - if the conservatory or 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, or a passive stack ventilation system 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 is still to the outside air.

3.14.9 Mechanical ventilation

Where a dwelling is mechanically ventilated it should be provided in accordance with the recommendations of Section 3, Requirements of CIBSE Guide B2: 2001, Ventilation and air conditioning.

Mechanical ventilation provided in line with this guidance should be to the outside air but it may be via a duct or heat exchanger.

Where a mechanical ventilation system serves more than 1 dwelling it should have a duplicate motor and be separate from any other ventilation system installed for any other purpose. Where the mechanical ventilation system gathers extracts into a common duct for discharge to an outlet, no connections to the system should be made between any exhaust fan and the outlet. The use of non-return valves is not recommended.

Open-flued appliances - care should be taken when installing mechanical extract systems where there is an open-flued combustion appliance in the dwelling. Further guidance is provided in clause 3.17.8.

3.14.10 Control of legionellosis

An inlet to, and an outlet from, a mechanical ventilation system should be installed such that their positioning avoids the contamination of the air supply to the system. The system should be constructed and installed in accordance with the recommendations in Legionnaires' Disease: The control of legionnella bacteria in water systems – approved code of practice and guidance - HSE L8, in order to ensure, as far as is reasonably practicable, the avoidance of contamination by legionalla.

3.14.11 Mechanical ventilation and systems

The design, installation and commissioning of a mechanical ventilation system should mean that it is capable of performing in a way that is not detrimental to the health of the occupants of the building and when necessary, is easily accessible for regular maintenance. Very few dwellings are air-conditioned but the use of continuously operated balanced supply and extract mechanical ventilation systems and of heat recovery units are becoming more popular as a result of the need to conserve energy and reduce greenhouse gas emissions. As building are constructed to lower infiltration rates, mechanical ventilation may be necessary to deliver the effective ventilation needed to provide a healthy living environment.

Simpler and more efficient systems are steadily being introduced that augment, complement and/or improve the natural ventilation of dwellings.Where infiltration rates of less than 5m3/h/m2@ 50 Pa are intended, such a system should be used. The following are examples of mechanical systems that will aid ventilation in a dwelling:

  • continuously operating balanced supply and extract mechanical ventilation systems. When combined with heat recovery these installations are known as Mechanical Ventilation and Heat Recovery (MVHR) systems. Installations should be in accordance with the guidance in BRE Digest 398. In hot weather windows can be opened to cool the dwelling while the system is not operating. Openable windows may also be needed for fire escape purposes

  • centralised mechanical extract ventilation (MEV) installed in accordance with the guidance in BRE Digest 398

  • where the infiltration rate is not less than 3m3/hr/m2 @ 50 Pa, decentralised mechanical extract ventilation units (dMEV), in rooms where there is likely to be high humidity such as kitchens, bathrooms and shower rooms. A dMEV should be designed, installed and commissioned to provide minimum continuous extraction rates in accordance with the following table:

    Table 3.6. Minimum continuous extraction rates for dMEVs

    Kitchen 6 litres/sec with 13 litres/sec boost
    Utility room 4 litres/sec with 8 litres/sec boost
    Bathroom 4 litres/sec with 8 litres/sec boost
    Toilet 3 litres/sec with 6 litres/sec boost

Where dMEVs are located in rooms adjacent to bedrooms the noise generated by them on a continuous rate should not exceed 30 dBL Aeq,T calculated in accordance with BS 8233: 1999.

Positive input systems - mechanical input air ventilation systems have been successfully installed in existing dwellings with the objective of overcoming problems of surface condensation and mould growth. They can also improve air quality and remove musty odours. The general principle of building tighter to reduce the amount of uncontrolled air movement through the building fabric may have a detrimental effect on the operation of input air ventilation systems and therefore they may not be appropriate for installation in new dwellings. Further information should be obtained from the product manufacturer.

  • In houses air is supplied to the hall via a low speed fan unit located in the roof space where it is allowed to circulate throughout the house before being extracted to the outside normally through the building fabric or trickle ventilators.

  • In flats and maisonettes the air is drawn direct from the outside through the fan unit before being discharged into the dwelling. The air supplied will normally have a lower relative humidity than the air in the dwelling thus removing harmful surface condensation and eliminating mould growth.

  • Where an input ventilation is proposed it should be installed in accordance with the 'conditions of certification by a notified body'.

3.14.12 Ventilation of 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 dwelling, the separating construction should be as air tight as possible. Where there is a communicating door airtight seals should be provided or a lobby arrangement may be appropriate.

Large garages - few domestic garages over 60m2 in area are constructed but guidance on such structures is provided in the non-domestic Technical Handbook.

Small garages - 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.

Open-flued appliances - although not considered good practice, open-flued combustion appliances are installed in garages. Ventilation should be provided in accordance with the guidance to Standards 3.21 and 3.22.

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 provided 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.

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