Technical Note No.
32
Ventilation
Introduction Permanent ventilation
It has been estimated that up to 90 per cent of our
time is spent indoors. With the introduction of
Definitions:
more airtight building construction, and modern
lifestyles generating increased amounts of moisture Rapid ventilation
and air pollution within both domestic and Rapid ventilation is defined as “one or more
commercial buildings, ventilation has become more ventilation openings with some part at high level
of a concern. Heating, smoking, cooking and (typically 1.75m above floor level) such as an
sleeping are all sources of atmospheric pollutants opening window.”
and water vapour. It has been proven that adequate
ventilation is essential for the well being and health Background ventilation
of building occupants and to the fabric of the
building itself. Correct ventilation of domestic and Background ventilation is defined as a ventilation
commercial buildings is therefore essential. opening (or openings) e.g. trickle ventilators or
airbricks, fitted with a ‘hit-and-miss’ type grille,
In domestic properties the building façade, and where appropriate a suitably designed opening
especially the window element, provides the window (acceptable for domestic applications).
designer with the means of supplying ventilation to The ventilation openings should be reasonably
the building and its occupants. Commercial secure, adjustable and located typically 1.75m
buildings generally benefit from an integrated above floor level so as to avoid discomfort to the
approach that commonly incorporates some form of occupants from cold draughts.
air handling plant together with air conditioning.
Extract ventilation
The level of ventilation required to each building Mechanical extract ventilation operated manually
type is covered within The Building Regulations and/or by sensor or controller. For the purposes of
‘Approved Document F’ for England and Wales. this technical note refer to the Building regulations
Readers in Scotland should refer to the Building ‘Approved document F’ for guidance on the design
Standards (Scotland) Regulations for guidance on requirements for mechanical extract rates.
ventilation requirements, and readers in Northern
Ireland should refer to ‘Northern Ireland Building
It should be noted that extract ventilation is a
Regulations’.
requirement of Approved document F for kitchens
bathrooms and utility rooms even if natural
It should be noted that buildings which house ventilation is provided.
specialist activities such as schools, workspaces,
hospitals plant rooms and smoking rooms will
Passive Stack Ventilation systems are acceptable
require specialist design to the standards listed
forms of extract ventilation provided that such a
within Section 2 of ‘Approved Document F’.
system is suitably designed and certified by an
approved or recognised body.
Types of Ventilation
The ventilation types identified in the Approved Permanent ventilation
Documents are:
Permanent ventilation is that which is required to
supply combustion air to all rooms containing a
Rapid ventilation (e.g. opening windows). non-room sealed fuel-burning appliance.
Background ventilation.
Extract ventilation
© CWCT 2001
December 2001
Methods Domestic buildings
Rapid ventilation Habitable rooms 8000mm2
Kitchen 4000mm2
The most obvious means of providing rapid Utility room 4000mm2
controllable natural ventilation is via an openable Bathroom 4000mm2
window however there may be a conflict with Sanitary accommodation
security requirements especially on ground and (separate from bathroom) 4000mm2
lower floors.
Rapid ventilation requirements are set out within An acceptable alternative is to provide an average
the ‘Approved Document F’, and usually require of 6000mm2 per room with a minimum of 4000mm2
1/20th of the floor area of the room to be an in each room.
openable window, part of the ventilation opening
must be at high level (typically 1.75m above the
floor level). The ‘Approved Document’ also sets Non-domestic buildings
out which rooms require a rapid ventilation Occupiable room up to 10m2 4000mm2
provision. Due regard must be paid to BS 8213 Occupiable room over10m2 400mm2 per m2
Part1: Code of practice for safety in use and during Kitchen (domestic type) 4000mm2
cleaning of windows and doors, when designing 4000mm2 per
opening windows. Bathroom, shower room bath/shower
Sanitary accommodation 4000mm2 per WC
Attention is drawn to the need to avoid solar
overheating as detailed in Approved document L
2000 with regard to the provision of opening Rooms that are not situated on external walls or
windows in conservatories and other glazed rooms that gain ventilation via another habitable
constructions. room have a different requirement for ventilation;
these are explained in ‘Approved document F’
The effectiveness of the ventilation supplied by a
window is directly linked to its style and
Trickle ventilators within windows
proportions; deeper (taller) windows ventilate more
effectively but can cause a nuisance due to There are three principle methods of providing
draughts. Different types of window produce ventilators within windows:
varying airflows into and out of the building.
Through the frame or sash
Through a purpose-designed extension to
the window head
Glazed into the frame above the glass
Other factors may need careful consideration when
deciding which trickle vents to use, such as:
Background Ventilation Acoustic performance (sound attenuation)
Background ventilation can be provided by means Operation
of:
Humidity
Trickle ventilators within windows Aesthetic
Airbricks with hit-and-miss grilles
Trickle ventilators may need to be provided in
Windows with suitably designed hardware locations that are subject to high levels of noise
pollution e.g. adjacent to busy roads or rail tracks.
Approved document F details the individual Ventilators can be supplied which attenuate the
requirements for background ventilation for rooms noise however these tend to be larger than standard
and these are tabulated below for rooms containing ventilators due to the requirement for a sound-
windows, i.e. those located on an external wall: absorbing acoustic material. This type of ventilator
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is able to provide up to 48dB sound reduction, but If used in PVC-U windows, trickle ventilators that
may provide less. pass through the frame or sash member may need to
be sleeved if the frame is reinforced in order to
Most trickle vent manufacturers produce ventilators prevent condensation forming on the
that can be operated by remote means, i.e. cords or steel/aluminium reinforcing and causing corrosion
rigid rods. These are especially useful in locations problems. If however the reinforcement is cut back
where physically reaching the vent is difficult, such to accommodate the ventilator the strength of the
as over sinks, or for use when people have medical framing member may be compromised and the
conditions that prevent them using the conventional window performance impaired.
operating methods.
With timber windows, the frames should be
Trickle ventilators are available that monitor the pressure treated after all the machining has been
humidity within the air. They open and close carried out. If the vents are being fitted into pre-
accordingly the relative humidity within each room. treated timber frames then adequate preservative
Requiring no electrical supply, this type of must be applied to the machined area in order to
ventilator is perceived to be more energy efficient prevent decay.
and is particularly useful in kitchen and bathroom
applications where humidity can fluctuate widely. Some aluminium systems companies recommend
that cut aluminium faces should be treated with a
It should be borne in mind when specifying compatible surface treatment or air drying paint.
ventilators that BS 6375: Part 1 1989 Classification Such consideration must be given to sites in coastal
of weathertightness states that there is no locations or those subject to other aggressive
requirement to record the flow of air through a environments.
controlled ventilator when closed, however some
manufacturers have carried out weather testing. It Through purpose designed head section or
is therefore incumbent upon the designer to ensure head extension profile
that the specified product performs satisfactorily.
Through the Frame or sash Vent through
head
extension
Through the frame ventilators are manufactured in
both aluminium and plastic and are usually
comprised of two major parts, an external grille or The ventilators used in this method are the same as
hood and an internal section that houses the those used in through frame. However this method
controllable element of the vent, usually a sleeve of applying a trickle ventilator is generally more
section is also available. Most internal sections are favoured by the manufacturer especially when using
designed to deflect the airflow upwards to avoid PVC-U as it does not require the fabricator to carry
discomfort to the building occupants and to out any additional operations within the window
discourage them from blocking the vents. manufacturing process and a separate production
line is usually set up producing head vent sections.
This type of ventilator is applied over a routered The completed head vent sections are then fixed to
slot within the window framing element and clips or the finished window.
screws into place; many are now fitted with seals to
help reduce air infiltration when they are closed. A further benefit of this method is that the head
sections are generally purpose designed and afford
In all cases the system must contain an integral the ventilation opening a degree of protection
insect screen.
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against severe weather and can offer an can lead to unaligned edges, but sensible
aesthetically pleasing detail. positioning can alleviate this problem.
Purpose made sections usually make provision for Air bricks with hit and miss grilles
internal building finishes allowing for dry lining
etc. without impairing the ventilation function. Usually provided within the façade of the building
this method uses proprietary airbricks on the
outside of the building, and a controllable hit and
Glazed-in ventilators
miss grille on the inside of the building. This form
of providing ventilation is however usually more
expensive owing to the construction costs of lintels
and cavity trays, etc.
Purpose-designed window hardware
It is also possible to provide background ventilation
using window hardware that has been designed to
offer two opening positions. This is usually
achieved on casement windows by fitting strikers
which have two positions for the espagnolettes or
shoot bolts, sometimes called nightvents. Windows
fitted with cockspur handles may also have
Glazed in purpose-designed handles but these are becoming
vent obsolete due to the aesthetic considerations of such
large handles.
Sometimes called over glass ventilators these
ventilators sit on top of the sealed unit and are
glazed into the frame along with the sealed unit.
They are available for a range of standard double
glazed sealed unit thicknesses and for single
glazing. Glazed-in ventilators are constructed in
both aluminium and PVC-U. Those which are
constructed of aluminium should incorporate a
thermal break. Recent developments have seen the
introduction of vents that close positively with wind
pressure and improve the weather performance.
Care should be taken when detailing this type of
ventilator to ensure that the passing action of the
window sashes is not impaired should it be
required.
This type of ventilator can be more noticeable by Providing background ventilation by this method is
virtue that it physically increases the sight lines, and usually confined to windows above ground floor
because of the risk to security. Locking handles
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may be used but this can conflict with the need to Permanent ventilation can be supplied using
provide escape in the event of a fire, see Building through frame or glazed in methods as well as via
Regulations, ‘Approved Document B’ B1 means of permanently open airbricks.
warning and escape.
These ventilators must be provided in addition to
Colour the requirements for background ventilation
The human eye can detect small colour differences
Ventilation of non domestic buildings
therefore whichever of the above methods is
employed, the specifier must satisfy himself/herself As mentioned previously most new non-domestic
that the colour and finish of the ventilators is buildings benefit from an integrated approach to
suitably matched to the window system. ventilation that commonly incorporates mechanical
air handling equipment together with air
Extract Ventilation conditioning, however recent trends towards
sustainability are promoting natural ventilation in
The required volumes for extract ventilation are set large buildings. The Building Regulations set out
out in the Building Regulations, ‘Approved the requirements for ventilation. It is however
Document F’, and the requirements for domestic practical to provide small non domestic buildings
buildings are tabulated below. Extract ventilation with ventilation through the principles detailed in
is commonly provided by electrically operated the approved documents
ducted fans with manual and automatic switching
which ensure that polluted air is extracted. Care The design of natural ventilation for large non-
must be exercised when using extract fans and non- domestic buildings is a very complex matter.
room sealed appliances to avoid the spilling of Natural ventilation can be defined as ventilation
combustion fumes into the room. driven by the natural forces of wind and
temperature. It should not be confused with
There is an option to provide passive stack infiltration which is the uncontrolled entry of
ventilation (PSV). PSV is a ventilation system outside air through gaps and cracks in the external
using ducts from the ceiling of rooms to terminals fabric of the building.
on the roof which operate by a combination of the
natural stack effect, i.e. the movement of air due to When designing such a ventilation system building
the difference in temperature between inside and size, form, local topography, location and
outside, and the effect of air movement over the orientation have to be taken into account, this is
roof. best carried out with the aid of a specialist engineer.
Habitable room no requirement Further information may be found in BS 5925:
Kitchen 1991 Code of Practice for ventilation principles
adjacent to hob 30 litres/sec and designing for natural ventilation.
Elsewhere 60 litres/sec or by PSV
Utility 30 litres/sec or by PSV
CIBSE Application Manual AM10: 1997 Natural
Bathroom 15 litres/sec or by PSV
ventilation in non-domestic buildings
Permanent Ventilation
Some fuel burning appliances require ventilators
that supply combustion air; these ventilators must
be permanently open. They must comply with BS Further reading
5440: Part 2: 1989 Specification for the installation
of ventilation for Gas Appliances. This Standard BS 5925: 1991 Code of Practice for ventilation
ensures that there is a constant and suitable supply principles and designing for natural ventilation.
of combustion air to appliances and prevents the
build up of potentially lethal carbon monoxide. BS 5720: 1979 Code of practice for mechanical
ventilation and air conditioning in buildings.
Permanent air vents should be sited at high level, or
adjacent to appliances. BS 5250: 1989 Code of practice for the control of
condensation in buildings.
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BRE Digest 398 Continuous mechanical
ventilation in dwellings: design, installation and
operation.
BSRIA Making Natural Ventilation Work. (GN
7/2000)
BS 5440: Part 2: 2000 Specifications for
installation of ventilation for gas appliances.
BS 8213: Part 1: 1991 Code of practice for safety
in use and during cleaning of windows and doors.
BRE guide to air quality in urban areas from Centre
for Safety and health.
CIBSE Application Manual AM10 :1997 Natural
ventilation in non-domestic buildings.
NHBC Technical Standards section 6.7
© CWCT 2001
University of Bath, Claverton Down, Bath,
BA2 7AY
Tel: 01225 826541; Fax: 01225 826556;
email: cwct@bath.ac.uk; website:
www.cwct.co.uk
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