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Scottish Building regulations

3.14 Ventilation

  • 3.14 Functional standard
  • 3.14.0 Introduction
  • 3.14.1 Ventilation generally
  • 3.14.2 Ventilation of dwellings
  • 3.14.3 Ventilation of conservatories
  • 3.14.4 Ventilation of areas designated for drying of washing
  • 3.14.5 Trickle ventilators
  • 3.14.6 Passive stack ventilation systems
  • 3.14.7 Conservatories and extensions built over existing windows
  • 3.14.8 Mechanical ventilation
  • 3.14.9 Control of legionellosis
  • 3.14.10 Mechanical ventilation and systems
  • 3.14.11 Ventilation of garages

3.14 Functional standard
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.

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 Filtration
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.

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).
Additional ventilation

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 4 m².This is not intended to include a kitchen or utility room where ventilation should be in accordance with the recommendations in table to clause 3.14.2.

Ventilation should be to the outside air. However clauses 3.14.5 and 3.14.7 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 3 m above any floor level in that space may be disregarded.

3.14.2 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.

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 10 m3/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 5 m3/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 3.14.10).

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. 12,000 mm2
Kitchen either:
a. mechanical extraction capable of at least 30 l/sec (intermittent) above a hob [3]; or
b. mechanical extraction capable of at least 60 l/sec (intermittent)
if elsewhere [3]; or
c. a passive stack ventilation system [4].
10,000 mm2
Utility room either:
a. mechanical extraction capable of at least 30 l/sec (intermittent) [3]; or
b. a passive stack ventilation system [4].
10,000 mm2
Bathroom or shower room (with or without a WC) either:
a. mechanical extraction capable of at least 15 l/sec (intermittent);
b. a passive stack ventilation system [4].
10,000 mm2
Toilet either:
a. a ventilator with an opening area of at least 1/30th of the floor area it serves; or
b. mechanical extraction capable of at least 3 air changes per hour.
10,000 mm2


  1. Where the trickle ventilator is ducted, the recommended areas in the table should be doubled (see clause 3.14.5).
  2. The overall provision of trickle ventilation in a dwelling may be provided at an average of 11,000 mm2 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 10 m3/h/m2 @ 50 Pa, which may often be the case in existing buildings, the size of trickle ventilation may be reduced to 8000 mm2 for apartments and 4000 mm2 for all other rooms. Alternatively, the overall provision of trickle ventilation in a dwelling may be provided at an average of 6000 mm2 per room, with a minimum provision of 4000 mm2 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 1.75 m above floor level.

3.14.3 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 1.75 m for ventilators, high level or roof vents are best placed to minimise the effects of heat build-up and reduce stratification.

3.14.4 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 3.14.6.

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

3.14.5 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. The area recommendations in the table to clause 3.14.2 may, in some situations, be difficult to achieve in the window frame and other options may need to be considered. A permanent ventilator is not recommended since occupants like control over their environment and uncontrollable ventilators are usually permanently sealed up to prevent draughts.

Because dwellings produce so many variables and differing life styles, ventilation of such buildings is not an exact science. The recommended areas in the table to clause 3.14.2 therefore are based on the geometric method (actual size of the opening formed in the window frame, in the case of a slot vent) as being a simple method that will satisfy most conditions.

Where the trickle ventilator has to be ducted, e.g. to an internal room, the geometric area of the trickle ventilator should be increased to double that shown in the table to clause 3.14.2, 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.

Non-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 3.14.2;
  • 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.

Height of trickle ventilators 
A trickle ventilator should be so positioned that a part of it is at least 1.75 m above floor level. This will allow at least some movement of air within the dwelling 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 area is fitted with a trickle ventilator in accordance with the guidance in clause 3.14.2. In these cases, noise transmission may need to be limited.

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.

3.14.6 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 8 m maximum length of stack) as the stack effect will diminish as the air cools.
Every passive stack ventilation 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 25 mm thick material having a thermal conductivity of 0.04 W/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 125 mm 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 10,000 mm2.

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.

3.14.7 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.

3.14.8 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.

3.14.9 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.

3.14.10 Mechanical ventilation and systems 
A mechanical ventilation system should be designed, installed and commissioned to perform 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 buildings 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 5 m3/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:

  1. 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 operating. Openable windows may also be needed for fire escape purposes;
  2. continuously operating mechanical extract ventilation installed in accordance with the guidance in BRE Digest 398;
  3. mechanical extract ventilation units (extract fans), either window or wall mounted, in rooms where there is likely to be high humidity such as kitchens, bathrooms and shower rooms installed in accordance with the recommendations in clause 3.14.2. Fans can be switched manually or automatically via a humidistat control. Humidistat control is not recommended for rooms containing a watercloset as odours may not be removed.

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'.

3.14.11 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 60 m2 in area are constructed but guidance on such structures is provided in the non-domestic Technical Handbook.

Small Garages
Garages of less than 30 m2 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 30 m2 but not more than 60 m2 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:

Natural Ventilation
a. 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 600 mm above floor level; or

Mechanical ventilation 
b. 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 600 mm above floor level.

3.13 Heating

  • 3.13 Functional standard
  • 3.13.0 Introduction
  • 3.13.1 Heating recommendations
  • 3.13.2 Alternative heating systems

3.13 Functional standard
Every building must be designed and constructed in such a way that it can be heated and maintain temperatures at a level that will not endanger the health of the occupants.

This standard applies only to a dwelling.

3.13.0 Introduction 
Heating in a building is necessary to provide suitable conditions in which to live. Heating, ventilation and thermal insulation should be considered as part of a total design that takes into account all heat gains and losses. Failure to do so can lead to inadequate internal conditions, e.g. condensation and mould and the inefficient use of energy due to overheating.

Whole house ‘central heating’ is now almost universal, particularly in new buildings and is regarded as almost essential in combating problems such as condensation and mould growth.

Normal activities within a dwelling add both heat and water vapour to the air. If the heating maintains comfort levels in the whole at all times, condensation problems will be minimised, but costs will be high. A reasonable compromise needs to be given to heating and ventilation to reduce the possibility of such problems and guidance is provided for both these issues in this sub-section. Section 6, Energy, provides guidance on the third issue, thermal insulation.

This guidance covers dwellings only as the heating of buildings other than dwellings is covered by the Workplace (Health, Safety and Welfare) Regulations.

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).

3.13.1 Heating recommendations 
The layout of a dwelling, the size and orientation of the windows, the thermal mass, level of insulation, airtightness, and ventilation can have a significant affect on the demand for heat. The performance of a heating system will also have a major affect on energy efficiency. Section 6, Energy provides guidance on these issues.

Heating a dwelling will normally be tailored to personal comfort taking cost into consideration. However in addition to comfort, the heating should reflect the combined effects of occupancy pattern, ventilation provision, building mass and insulation to reduce the possibility of producing excess condensation that might damage the building fabric.

Every dwelling should have some form of fixed heating system, or alternative that is capable of maintaining a temperature of 21º C in at least 1 apartment and 18º C elsewhere, when the outside temperature is minus 1º C.

There is no need to maintain these temperatures in storage rooms with a floor area of not more than 4 m2.

3.13.2 Alternative heating systems 
Alternative heating systems may involve a holistic design approach to the dwelling and can include the use of natural sources of available energy such as the sun, wind and the geothermal capacity of the earth. Passive design, such as use of the orientation of glazing for solar gain and of the building mass to store heat with controlled heat release may only need minor supplementation from a lower output fixed heating system. Active heating systems, such as heat pumps that extract heat from ground, air, water or geo-thermal sources, can limit emissions of carbon dioxide and reduce the use of fossil fuels. Complementary systems can also be used, to heat water using solar energy or generate electricity using solar or wind power.

Elderly and infirm
Where there are elderly or infirm occupants in a dwelling the capability of the heating system to maintain an apartment at a temperature higher than 21º C is a sensible precaution. Since it is not possible to determine the occupants at design stage the heating system should be designed with the capability of being easily upgraded at a later date.

If an existing heating system is to be upgraded to provide higher temperatures the boiler size may not need to be replaced, provided it was correctly sized originally. The upgrading may necessitate the replacement of some pipes and radiator in one or two rooms and accessibility will need to be considered. Such modification could result in a small increase in the recovery time of the hot water supply for bathing and washing.