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Building Services. By Dr David Johnston – licensed under the Creative Commons Attribution – Non-Commercial – Share Alike License ENVIRONMENTAL SCIENCE & SERVICES - LEVEL 1. Ventilation Dr. David Johnston. Objectives.

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Building Services

By Dr David Johnston – licensed under the Creative Commons Attribution – Non-Commercial – Share Alike License

environmental science services level 1


Dr. David Johnston


By the end of this lecture you should be able to:

  • Define what is meant by the term ‘ventilation’.
  • Understand why buildings require ventilation.
  • Define air leakage and understand how it occurs.
  • Be able to identify common air leakage paths in buildings.
  • Understand what is meant by the term ‘airtightness’.
  • Be aware of different methods of ventilating buildings and their associated advantages.

This lecture is structured as follows:

  • Introduction.
  • Air leakage and airtightness.
  • Examples of airtight buildings.
  • Natural ventilation.
  • Mechanical ventilation.
the purpose of ventilation
The purpose of ventilation

Ventilation is simply defined as the process of changing air in an enclosed space.

In order to maintain optimum air quality in buildings, a proportion of the air contained

within any enclosed space should be continuously removed and replaced with fresh

outside air.

All buildings require ventilation for:

  • Human respiration.
  • The health and comfort of the occupants.
  • The control of condensation and humidity.
  • Fuel burning appliances.
  • The dilution and disposal of pollutants.

Ventilation can also be used to control thermal comfort.

If the incoming ‘fresh’ air is contaminated by pollutants, then measures have to be

taken to remove them.

types of ventilation
Types of ventilation

Buildings are ventilated via a combination of:

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

Methods of achieving sufficient purpose-provided ventilation are contained

within the Building Regulations 2000 Approved Document Part F 2006 edition

(ODPM, 2006a).

Ventilation requirements for UK dwellings are typically:

  • Recommended - between 0.5 and 1.0 ac/h.
  • Minimum - between 0.3 to 0.5 ac/h.

Infiltration can be reduced by reducing the air leakage from the building

air leakage
Air leakage

Air leakage is defined as the uncontrolled exchange of air both into (infiltration) and out

of (exfiltration) a building through cracks, gaps and other unintentional openings in the

building envelope.

It is driven by the same physical processes that drive natural ventilation, namely:

  • Wind effect.
  • Stack effect.

Wind effectStack effect

The rate of air leakage is dependent upon:

  • The air permeability of the construction.
  • The wind speed and direction.
  • The temperature difference between the inside and outside of the building, as well as within the building.

Airtightness is the measurement criteria used to evaluate the air leakage of a


The airtightness of a building determines the uncontrolled background ventilation or

leakage rate of a building which, together with purpose-provided ventilation, makes

up the total ventilation rate for the building.

Total Ventilation Rate=Uncontrolled Background Ventilation + Purpose Provided Ventilation

Traditionally, airtightness was expressed in air changes per hour (ac/h or h-1).

However, nowadays air permeability (m3/(h.m2) is more commonly used as it takes

into consideration the effects of shape and size.

The more airtight a building, the lower the air permeability.

In the UK, airtightness is measured at an artificially induced pressure of 50Pa (n50).

measurement of airtightness
Measurement of airtightness

The airtightness of a building envelope can be measured using

  • Fan pressurisation (blower door) technique.
  • Tracer gas technique.

Fan pressurisation is the simplest, quickest and most

widely used technique in the UK and involves the use of:

  • Portable variable speed fan.
  • Adjustable door frame and panel.
  • Fan speed controller.
  • Pressure and flow gauge.

Pressurisation tests are the basis for guidelines and legislation in a number of


measurement of airtightness1
Measurement of airtightness

A series of multiple fans or trailer mounted fans are used for pressure testing

largedomestic and non-domestic buildings.

leakage identification
Leakage identification

The most widely used technique for identifying the main areas of air leakage within a

domestic building is smoke detection.This technique involves either pressurising or

depressurising the building, and then locating the areas of air leakage using a manual or

electronically operatedhand-held smoke puffer.


In most instances, detection is undertaken from inside the dwelling under pressurisation,

as it is much easier to identify where the smoke leaks out of the habitable space.

Important point about smoke detection is, that in most cases, it is only possible to identify the point where the smoke leaks out of the habitable space, and not the path that the smoke takes from the inside to the outside of the building.

leakage identification1
Leakage identification

Infrared thermal imaging using an infrared camera can also be used to identify the

main areas of air leakage within the building fabric. It can provide additional information

which is not always possible to recognize purely by smoke detection.

However, this technique is considerably more complex and problematic than smoke


Limitations as to when and where it can be used as a detection technique often prohibit

its use. Also, without appropriate skills and knowledge of the construction, it is

possible to misinterpret the images obtained.

main air leakage paths
Main air leakage paths

The main air leakage paths in UK dwellings are illustrated below:

Important to realise that air leakage can occur both directly and indirectly.

1. Gaps at ceiling-to-wall joint at the eaves

2. Gaps around windows

3. Leaky windows

4. Leaky doors

5. Leaks at threshold

6. Open chimneys

7. Leaks around flue penetration of ceiling

8. Gaps in and around suspended timber floors

9. Open fire/stove

10. Gaps around skirting board and floor

11. Gaps around internal partition/ceiling junction

12. Gaps in and around electrical fittings

13. Gaps around loft hatch

14. Gaps around soil stack

15. Gaps around ceiling light fittings

16. Vents penetration roof/ceiling

17-21. Gaps around waste pipe and flue penetrations

22. Gaps around wall-to-floor join

direct air leakage points
Direct air leakage points

These are points in the building envelope where air leakage occurs directly through the

primary air barrier from inside the insulated envelope to outside or vice versa. Common direct

air leakage points include:

  • Around trickle ventilators and through poorly closing trickle ventilators.
  • Around and through the loft hatch.
  • Around poorly fitting windows and doors.
  • Through gaps at bay windows and around sliding mechanism of patio doors.
  • At thresholds.
  • Around services at the point where they penetrate through the primary air barrier.
indirect air leakage points
Indirect air leakage points

These are points in the building envelope where air leakage occurs indirectly through the

primary air barrier via a series of interconnected voids from inside the insulated envelope to

outside or vice versa. Common indirect air leakage points include:

  • On external and party walls at the ground floor/external wall junction.
  • Under kitchen & utility room units.
  • Around staircases.
  • Into intermediate floor voids and at intermediate floor perimeters.
  • Into service voids (e.g. behind bath panels).
  • At service penetrations where they penetrate the dry-ling and/or internal finish.
quantifying air leakage
Quantifying air leakage

In UK dwellings, experience indicates that the majority of air leakage tends to occur indirectly

rather than through easily identifiable directgaps and cracks in the building envelope.

Work undertaken in the late 1990’s by the BRE suggested that the vast majority of component

air leakage could not be attributed to a single component. Instead, it could be attributed to the

numerous “hidden paths”, through cracks and gaps that exist throughout the building.

Component air leakage in dwellings [After: Stephen, 2000]

These “hidden” air leakage paths are often complicated, making it very difficult, if not

impossible, to trace and seal effectively. Therefore, it is much more effective to design and

construct airtight dwellings in the first instance, rather than try to carry out post construction

tightening (most commonly taking the form of secondary sealing) once the dwelling is built.

airtightness and ventilation
Airtightness and ventilation

The level of airtightness achieved within a building will have an important influence

on the overall ventilation rates that will be achieved and the type of ventilation

strategy that should be adopted.

However, irrespective of the ventilation strategy adopted, the aim of good

ventilation design should always be to minimiseuncontrolled infiltration by

making the building envelope as airtight as possible, and then ventilate the

building appropriately by providing sufficient purpose-provided ventilation.

In other words:

‘build tight, ventilate right’

thermal bypassing and airtightness
Thermal bypassing and airtightness

Thermal bypassing is complex and is often confused with airtightness.

A thermal bypass occurs where air is allowed to move through, around and

between the insulation, in effect bypassing the benefit of the insulation.

Therefore, it is possible to have a very airtight dwelling but still have thermal

bypassing resulting in lower thermal performance.

The important issues in relation to thermal bypassing are:

Location of the primary air barrier.

  • The location of the thermal insulation and its relationship with the air barrier.
  • If there is separation of the thermal insulation from the air barrier, then a thermal bypass can exist.

Position of the insulation.

There are no air gaps between the thermal insulation and the primary air barrier.

By constructing a building with a high level of airtightness and ensuring that the

air barrier is kept in contact with the thermal insulation layer, thermal

bypassing can be effectively eliminated from the structure.

airtightness and the building regulations
Airtightness and the Building Regulations

Airtightness in England and Wales is currently addressed in Approved Document Part

L1A 2006 (ADL1A 2006). ADL1A 2006 requires that the building fabric should be

constructed to a reasonable quality of construction so that the air permeability is

within reasonable limits (ODPM, 2006b).

A reasonable limit for the design air permeability is given as 10 m3/(h.m2) @ 50Pa.

In the majority of cases, compliance with the regulation will require some degree of

compulsory pressure testing. Details of the pressuretesting regime associated with

each method of compliance are detailed within ADL1A 2006. A more onerous testing

regime is required if accredited construction details have not been adopted.

Compliance with ADL1A 2006 also requires that the pressure tests are undertaken in

accordance with the procedure set out in the Air Tightness Testing and Measurement

Association Technical Standard 1 (ATTMA, 2007).

Compulsory pressure testing (and re-testing, if required) will be performed at the

house builder’s expense.

airtightness and the building regulations1
Airtightness and the Building Regulations

In a recent consultation document on ADL1A 2006, it is proposed that an air

permeability target is introduced for those dwellings that are not tested (CLG, 2009).

In such cases, the assessed air permeability of the non-tested dwellings is the

average air permeability obtained from other tested dwellings of the same type

increased by 2 m3/(h.m2) @ 50Pa.

This takes into account the likely variability of air leakage that would be achieved by on-

site testing.

airtightness and energy performance
Airtightness and energy performance

Airtightness can have a significant impact on the energy use and CO2 emissions

attributable to buildings.

In the UK, any exchange of air from the inside to the outside is likely to result in:

  • A significant reduction in the thermal resistance of the thermal insulation, due to thermal bypassing, leading to increases in realised fabric U-values.
  • An increase in the building’s ventilation and fabric heat losses, resulting in an increase in space heating requirement.
  • Increased energy usage and higher carbon emissons.

Impact of poor airtightness on a dwelling.

airtightness and energy performance1
Airtightness and energy performance

It is also important to realise that as the fabric performance of new dwellings improves, the

proportion of total heat losses attributable to ventilation is likely to increase, unless air

leakage is addressed.

For instance, in reasonably well insulated but relatively leaky dwellings (those built to Part

L 2006 with an air permeability of 10m3/(h.m2)), ventilation heat losses can account for up

to one third of the dwellings’ total heat loss.

Comparison of ventilation and fabric heat losses for a ‘notional’ (80m2) semi-detached house

airtightness of new uk dwellings
Airtightness of new UK dwellings

Recent measurements undertaken on a sample of 750 dwellings,of various construction

types and forms, the majority of which were built to conform to ADL1 A 2006 showed:

Mean air permeability of new UK dwellings [Source: BSL, 2009]

  • A very wide range of airtightness within the sample – 0.75 to 24 m3/(h.m2) @ 50Pa, with a mean of 6.13 m3/(h.m2) @ 50Pa.

Sample of 750 dwellings

Mean of 6.13 m3/(h.m2) @ 50Pa

Dwellings built to Part L 2002

Sample of 99 dwellings

Mean of 9.2m3/(h.m2) @ 50Pa

Existing stock

Sample of 384 dwellings

Mean of 11.5m3/(h.m2) @ 50Pa

airtightness of new uk dwellings1
Airtightness of new UK dwellings

In terms of construction type:

[Source: BSL, 2009]

Sample of 750 dwellings

airtightness of new uk dwellings2
Airtightness of new UK dwellings

In terms of building form:

[Source: BSL, 2009]

Sample of 534 dwellings

airtightness of new uk dwellings3
Airtightness of new UK dwellings

In terms of main construction type and building form:

[Source: BSL, 2009]

Sample of 534 dwellings

Timber frame


airtightness of new uk dwellings in context

Zone of Current



(~ 3

15ach @ 50Pa)

Kronsberg P

assive House Estate, Hannover, Germany

32 Dwellings (Feist, Peper & Gorg, 2001)

Stamford Brook, Altrincham, UK

44 dwellings


Shenton, Wingfield & Bell, 2007)

Dwellings built to Part L 2002

Dwellings built to Part L 2006

750 dwellings

99 dwellings

(BSL, 2009)

(Grigg, 2004)


Existing UK stock (pre




71 dwellings

(Stephen, 2000 & 2004)







Mean air change rate (ach

@ 50Pa)

Airtightness of new UK dwellings incontext







factors influencing airtightness
Factors influencing airtightness

A number of factors are known to influence the airtightness of dwellings. These include:

  • Age of the dwelling.
  • Construction type
  • Location and continuity of the primary air barrier.
  • Number of storeys.
  • Size and complexity.
  • Seasonal variation.
  • Longevity.
  • Sequencing of construction processes.
  • Site supervision and workmanship.
  • Quality of construction.
  • Communication.
size and complexity
Size and complexity

Other things being equal, the larger and more complex the floor plan and the more

complex the construction techniques used, the greater the number of junctions

between the elements of the thermal envelope.

This increases the potential for air leakage.

This does not mean that complexity should be avoided. Instead, designers and

constructors need to understand the airtightness problems that may be introduced by

adopting complex detailing and devise appropriate and robust solutions.

sequencing of construction processes
Sequencing of construction processes

Sequencing can have an important impact on the airtightness of a dwelling., as the

build sequence adopted can often make it difficult to gain access to and maintain

continuity of the primary air barrier.

  • Approach 1 - Top floor ceiling installed prior to the installation of the metal stud partitioning.
  • Approach 2 - Metal studwork partitioning installed first. Timber head plate then installed over the top of the head channel in metal partitioning in an attempt to reduce air movement through this channel.
construction observations
Construction observations

Plasterboard dry lining

Experience suggests that it is extremely difficult, if not impossible, to achieve a completely

airtight seal around the edges of plasterboard dry lining on external and separating walls

and all openings when using adhesive dabs.

construction observations1
Construction observations

Built-in joists

Achieving an airtight seal is difficult with built-in joists.

Excess mortar around built-in joists and gaps at perpends.

Offset joist running parallel with the wall.

construction observations2
Construction observations

Window sills

There is potential for air movement under window sills.

Window sill at various stages of construction.

construction observations3
Construction observations

Service penetrations

Service penetrations are often left unsealed and are then hidden behind boxing or panels.

improving airtightness
Improving airtightness

High levels of airtightness (low air permeabilities) are only likely to be achieved by

understanding and adopting a number of basic principles throughout the design,

procurement and construction of the building..

These principles relate to the following:

  • Design stage.
  • Sequencing of construction processes.
  • Site supervision and workmanship.
  • Quality control.
  • Communication.
improving airtightness1
Improving airtightness

Design stage

Defining a continuous and robust primary air barrier at the design stage by:

  • Identifying a line through the building that will act as the main barrier to air leakage. This is known as the dwelling’s ‘primary air barrier’.
  • Ensure that the primary air barrier is continuous around the thermal envelope and, where possible, in contact with the thermal insulation layer. This will not only minimise air leakage but also the possibility of thermal bypassing.
  • Check the continuity of the primary air barrier by undertaking a ‘pen-on-section’ test. This involves using a line to mark the location of the primary air barrier on a set of General Arrangement drawings. The line should be continuous and separate the heated (conditioned) spaces from the unheated (unconditioned) spaces.

Red line indicates location of the primary air barrier and yellow shading the position of the insulation.

improving airtightness2
Improving airtightness

Design stage (continued)

  • From the ‘pen-on-section’ test, identify areas where additional detailing will be required and identify those trades that are responsible for the design and construction of the air barrier.
  • Produce large scale drawings (1:5) of any areas of complexity or changes in plane identified by the ‘pen-on-section’ test and identify how continuity of the primary air barrier will be maintained at these areas.
  • Minimise the number of service penetrations through the primary air barrier. Consider the adoption of service zones or voids that may group services together.
  • Try and make the primary air barrier as simple as possible. Try and avoid or at least minimise changes of plane and complex detailing.
  • Consider the impact that materials with different tolerances may have on the primary air barrier. Ensure that any issues are resolved at the design stage prior to commencing construction.
  • Ensure that the primary air barrier is robust, impermeable and durable.
  • Do not rely on secondary sealing, for example using sealant to seal the junction between intermediate floors and the skirting board, to provide part of the primary air barrier.
improving airtightness3
Improving airtightness


Give explicit consideration to sequencing during design, procurement and construction by:

  • Attempting to install the primary air barrier over as large an area as possible in one single operation. For example, installation of the top floor ceiling prior to the erection of the internal partitions minimises the number of junctions and penetrations through the ceiling.
  • Ensuring that the primary air barrier can be completed, inspected, tested and repaired prior to any part of it being covered up by other materials or finishes. For example, where a parging coat forms the primary wall air barrier, it should be applied to walls before any subsequent trades commence.
  • Sleeve and seal service penetrations through the primary air barrier during installation wherever possible, to avoid the need to break out subsequent new construction.
  • Ensure that the method of sealing service penetrations through the primary air barrier is robust enough to enable later fitting-out work to take place without compromising the installed seal. For example, electricity cables that penetrate the primary air barrier should be fitted with an appropriate seal that allows for the cables to be manipulated during and after the installation of the terminal fitting without detriment to the seal.
improving airtightness4
Improving airtightness

Site supervision and workmanship

Ensure that there are high standards of site supervision and workmanship on-site by:

  • Providing airtightness training as an integral part of site induction. Both generic and trade-specific airtightness training should be provided to all operatives on-site. Training should explain why airtightness is important, how it is being tested, what quality control processes are in place and what happens when things go wrong.
  • Ensuring that operatives know what they are required to achieve and what constitutes an acceptable standard. The definition and visibility of the air barrier is crucial.
improving airtightness5
Improving airtightness

Quality control

Testing, monitoring, and feedback are essential to any quality control process. Specific

ways in which process can be improved include:

  • Formally describing the quality control process and clearly setting out the different roles and responsibilities with the lines of reporting, recording, investigation and action established and applied consistently.
  • At key stages of the construction, check the integrity of the primary air barrier and undertake airtightness measurements before the construction progresses to a stage where it becomes impossible to efficiently undertake remedial action.
  • Maintain a photographic record of observations made during the construction process. This not only allows a more precise retrospective analysis in the event of future investigations, but also provides useful material for training and improving the awareness among site staff of the impact of their actions.
  • As far as possible, construction specifications should ensure standardisation of detailing to enable site teams to become familiar with the materials, components and tolerancing needs. Where modifications are required these should be undertaken in a controlled way accompanied by effective detailed documentation.
improving airtightness6
Improving airtightness


Communication of detailed design information and feedback on airtightness

performance is crucial if high standards of airtightness are to be achieved. Effective

communication requires:

  • Design information to be provided to all subcontractors and trades that may have an impact on the integrity of the primary air barrier, through an appropriate mixture of documentation and detailed briefings. The design information should include procedural specifications as well as drawings depicting the final form. In particular, all drawings and specifications should define the primary air barrier and detail drawings should show how the air barrier is to be maintained at junctions and penetrations.
  • Any modifications or deviations from the design made on site (including ad-hoc design alterations, product substitutions and procedural changes) should be fed back to the designers to be included in the final “as-built” detailed drawings. These amended details will need to be reassessed where necessary – particularly where there may be implications for the air barrier integrity, thermal performance or condensation risk.
improving airtightness7
Improving airtightness

There are a number of benefits to be gained from improving the airtightness of a

particular building. These are:

  • Energy and CO2 emission savings.
  • Improved thermal comfort.
  • Reduced risk of deterioration.

However, care should be taken to ensure that the recommended ventilation rates can still be achieved!

The aim should be to:

‘build tight - ventilate right’

That is, to minimise uncontrolled (and, usually, unwanted) infiltration by making

the building envelope airtight while providing the required ventilation with ‘fresh’

air in a controlled manner.

It should be emphasised that a building cannot be too tight - but it can be under-ventilated!

examples uk
Examples - UK

The Denby Dale Passivhaus [Source: Green Building Store, 2010]

  • A 3 bedroom detached property designed to PassivHaus standards.
  • Wet plastered dense concrete block and natural stone masonry cavity external walls.
  • Considerable attention was given to airtightness during the design and construction.
  • Probably the tightest masonry cavity dwelling recorded in the UK, with an air permeability of 0.41 m3/(h.m2) @ 50Pa (0.38 ac/h).
examples uk1
Examples - UK

Field Trial at Stamford Brook, Cheshire

  • A development of over 700 dwellings, designed to an energy efficiency standard some 10% to 15% in advance of the 2006 building regulations for England and Wales.
  • Masonry cavity construction.
  • Application of a thin (2-4mm) plaster-based parging coat to the interior surfaces of the dwellings external walls, prior to the installation of the plasterboard dry-lining.
  • Designed to have an air permeability target of 5m3/h.m2 @ 50Pa.
  • Workforce trained and briefed about the purpose and principles of the air barrier.
  • High level of supervision and workmanship on-site.
  • Of 44 dwellings tested, air permeability ranged from 1.8 to 9.7 m3/(h.m2) @ 50Pa, with a mean of 4.5mh-1 @ 50Pa.
examples uk2
Examples - UK

The Hockerton Housing Project, Southwell, Nottinghamshire

  • The UK’s first earth-sheltered, self-sufficient ecological housing development.
  • Construction:
    • Masonry cavity and reinforced concrete rear and side walls.
    • All of the internal walls are wet plastered.
  • Considerable attention was given to airtightness during the design and construction.
  • Air permeability of between 0.95 to 1.23 m3/(h.m2) @ 50Pa (1.09 to 1.40 ac/h).
examples uk3
Examples - UK

Stenness, Orkney Islands [Source: Olivier, 1994]

  • 2 pairs of semi-detached single-storey dwellings.
  • Timber-frame construction.
  • Workforce was briefed about the purpose and principles of the airtightness measures both before and during installation.
  • One of the tightest houses recorded in the UK when originally tested, with an air leakage rate of 1 ac/h @ 50 Pa.
examples uk4
Examples - UK

Boundary Close, York

  • A development of eight 2½ storey 2 bedroom developments with a sleeping deck, designed to near PassivHaus standards.
  • Composite timber-frame joist panel.
  • Designed to have an air permeability target of 1 m3/(h.m2) @ 50Pa.
  • Dwellings had a well thought through, properly designed and properly executed primary air barrier.
  • Mean air permeability of 1.58 and 1.94 m3/(h.m2) @ 50Pa.
examples uk5
Examples - UK

The Green Man House

  • A 3 bedroom detached property designed to near PassivHaus standards.
  • Balloon frame timber external walls, comprising 450mm thick glulam beams filled with Warmcel, 19mm OSB, 12mm panel vent and clad externally in timber.
  • Walls were full height and intermediate floor was inserted after the external walls were in place.
  • Air permeability of 1.09 m3/(h.m2) @ 50Pa (1.12 ac/h).
examples overseas
Examples - Overseas

The Self-sufficient Solar House, Freiburg, Germany

[Source: Fraunhofer Institute For Solar Energy Systems, 2010]

  • Two-storey dwelling with a living area of 145m2.
  • Externally insulated masonry construction.
  • Air leakage of 0.3 ac/h @ 50 Pa.
examples overseas1
Examples - Overseas

Passive House Programme

  • CEPHEUS (Cost Efficient Passive Houses as European Standards) Programme.
    • Various construction types.
    • Air leakage rates ranged from 0.3 to 0.6 ac/h @ 50Pa in 9 of the projects.
    • Passive House estate in Hannover-Kronsberg the air leakage of the 32 dwellings ranged from 0.17 to 0.4 ac/h with a mean of 0.29 ac/h @ 50Pa.
examples uk retrofit
Examples – UK retrofit

12 Field Trial Dwellings at Derwentside, Co. Durham

The results of undertaking the airtightness and refurbishment work were:

  • Air leakage reduced from an average of 25 ac/h @ 50 Pa to between 8.5 ac/h and 13.5 ac/h.
  • Most significant airtightness measure undertaken was sealing the external walls.
  • The nature of the refurbishment prevented some measures from being carried out.
further information on airtightness
Further information on airtightness

Further information on airtightness can be found at:

natural and mechanical ventilation
Natural and mechanical ventilation

There are three separate approaches to ventilation. These are:

  • Natural ventilation.
  • Mechanical ventilation.
  • Acombination of natural and mechanical (mixed-mode).

The majority of UK dwellings are naturally ventilated, or naturally ventilated with

some mechanical assistance (i.e. mechanical extract).

It is important to note that:

  • In naturally ventilated airtight dwellings ventilation rates may be inadequate for much of the time.
  • All naturally ventilated dwellings will be prone to under-ventilation during periods of relatively high external air temperature and low wind speed.
natural ventilation
Natural ventilation

Natural ventilation is driven by the wind effect and the stack effect. Air is brought

through the building facade via opening windows or purpose provided

ventilation openings.

Wind and stack effect

The use of natural ventilation imposes restrictions on the plan form of the

building, in that all of the ventilated spaces must be within a certain maximum

distance from an opening window or ventilation opening.

However, this spatial constraint does permit the following:

  • The provision of natural lighting to much of the space.
  • Allows the occupants good views of the outside world.
natural ventilation strategies
Natural ventilation strategies

The basic natural ventilation strategies that can be employed in dwellings are as


  • Single-sided ventilation.
  • Cross-ventilation.
  • Passive Stack Ventilation (PSV).

It should be noted that each of these ventilation strategies may be applied to

different parts of a building as appropriate.

natural ventilation strategies1
Natural ventilation strategies

Single-sided ventilation

Single opening - where a single ventilation opening is provided.

Double opening - where ventilation openings are provided at different heights within the


Single-sided opening ventilation Stack-induced flow through a double opening

[Source: CIBSE, 2005]

Cross ventilation

With cross ventilation, openings exist on both sides of a space and air flows from one

side of the building to another.

Cross ventilation [Source: CIBSE, 2005]

natural ventilation strategies2
Natural ventilation strategies

Passive Stack Ventilation (PSV)

These systems consist of vents located in the ‘wet’

areas of a dwelling that are connected via vertical

or near vertical ducts to ridge or other roof

terminals. The vents are usually fitted to the ceiling.

Fresh air is drawn into the dwelling via background

ventilators (i.e. trickle ventilators) and air leakage.

PSV systems are suitable for installation in houses and blocks of flats up to four

storeys in height.

PSV system [Source: BRECSU, 1993]

The rate at which the air flows within the ducts depends upon:

  • The difference between the indoor and outdoor temperatures.
  • The wind speed and direction.
  • The airtightness of the dwelling.
  • The type and position of the air leakage paths.
  • The type and position of the duct outlet terminal.
passive stack ventilation psv
Passive Stack Ventilation (PSV)

The performance of PSV systems can be enhanced by:

  • Incorporation of an extract fan into the passive stack duct (termed assisted passive stack).
    • Extract fan operates only when required.
    • Extract fan runs constantly within the duct (similar to mechanical extract ventilation).
  • Installation of humidity controlled extract grilles.

The performance of PSV systems is highly dependant on the quality of the


Simple PSV systems are not recommended for installation in dwellings with an air

permeability of less than 3 m3/(h.m2) @ 50Pa. In such situations, an assisted

passive stack system should be used.

dwellings with psv systems
Dwellings with PSV systems

Roaf Residence

  • Passive stack ventilation system with extract fans in ‘wet’ rooms and trickle vents in other windows.
natural ventilation strategies3
Natural ventilation strategies

The basic natural ventilation strategies that can be employed in non-domestic

buildings are as follows:

  • Single-sided ventilation.
  • Cross-ventilation.
  • Stack ventilation.
  • Night ventilation.

Single-sided and cross-ventilation

Principles are the same as those mentioned for the domestic situation.

Single-sided opening ventilation Cross ventilation

[Source: CIBSE, 2005] [Source: CIBSE, 2005]

natural ventilation strategies4
Natural ventilation strategies

Stack ventilation

Operate on the same principal as domestic PSV systems, except that a chimney or

atrium is used instead of a series of ducts.

  • Chimney ventilation - The chimneys form no other functional purpose than to provide ventilation. They can take the from of a single linear chimney or several smaller chimneys distributed around the building to suit the required ventilation flowpath.
  • Atrium ventilation - An atrium is a variant of the chimney ventilation principle. The essential difference is that the atrium serves many more functions than the chimney, i.e. it can provide usable space. In addition, the atrium can be used to provide daylight to the adjoining spaces, and it can act as a buffer space during winter.

Night ventilation

Takes advantage of the natural diurnal variations in temperature to promote cooling.

Can reduce the maximum daytime temperature by 2 to 3OC (CIBSE, 1997).

chimney ventilation systems
Chimney ventilation systems

The Queens Building, De Montfort University & BRE’s Office of the Future [Source: BRECSU, 1997, 1995a & 1995b]

atrium ventilation systems
Atrium ventilation systems

Anglia Polytechnic University’s Learning Resource Centre [Source: Bunn, 1994]

night ventilation systems
Night ventilation systems

BRE’s Office of the Future [Source: BRECSU, 1995a & 1995b]

  • In summer, the control systems open ventilation paths through the concrete slab to cool it at night.
utilising natural ventilation
Utilising natural ventilation

A number of issues should be borne in mind when utilising natural ventilation.

These include:

  • Capital cost - cost comparisons should always be carried out on the whole building. This is because savings in costs due to the reduced requirement for mechanical equipment may result in increased fabric costs due the requirement for increased thermal mass within the building.
  • Noise - natural ventilation systems do allow externally generated noise into the building. If there are particular problems with external noise on a particular facade, special acoustic ventilators or a mixed-mode approach may be appropriate.
  • Air quality - ventilation inlets must be carefully sited away from sources of pollution.
advantages of utilising natural ventilation
Advantages of utilising natural ventilation

The main advantages of a well-designed naturally ventilated building are:

  • Simpler and more manageable environmental control systems - due to the building envelope acting as the primary climate modifier.
  • Robustness - naturally ventilated buildings have fewer failure modes due to a reduction in the number of components that are susceptible to malfunction.
  • Less space required for equipment - less space is required in naturally ventilated buildings for plant rooms and the distribution of services.
  • Lower construction costs - due to a reduction in the requirement for environmental services such as ventilation and air-conditioning equipment
  • Lower operating costs (energy and maintenance) - no need for chillers and fans. Also the narrower plan form means than naturally ventilated buildings can utilise natural daylight.
  • Enhanced user satisfaction - through greater potential for occupant control over the environment
  • Environmental benefits - through reduced electrical energy consumption and the possible elimination of mechanical refrigeration.
mechanical ventilation
Mechanical ventilation

Mechanical ventilation is usually provided where:

  • The building has an airtight construction.
  • The plan form of the building means that natural ventilation cannot penetrate to the core areas.
  • Where the building is subject to unacceptable levels of external noise.
  • Where filtration or other air treatment is required.
  • Where the client requires a ‘prestige’ office development.

In mechanical systems, an electrically driven fan or fans are used to move air

within the building. The main advantage of utilising such a system is controllability.

A mechanical ventilation system, used in conjunction with an airtight

construction, can allow more control over the supply of fresh air and result in better

indoor air quality compared with natural ventilation.

The mechanical ventilation systems which are typically employed in domestic

buildings are as follows:

  • Localised mechanical extract.
  • Continuous mechanical extract ventilation (MEV).
  • Continuous mechanical ventilation with heat recovery (MVHR).
mechanical extract systems
Mechanical extract systems

Localised mechanical extract systems

These are the most widely used form of mechanical ventilation in UK dwellings.

They consist of an extract fan or fans which are typically

located in the kitchen and bathroom, and are window, ceiling

or wall-mounted.

They operate intermittently, and are normally used in

combination with trickle ventilators to provide the higher

ventilation rates required in moisture producing areas.

Control of the fans is generally provided either manually, semi-automatically (light

switch coupled to timer or humidistat sensor) or automatically (using a passive infra red

detector). They are not recommended for installation in dwellings with an air permeability

of less than 3 m3/(h.m2) @ 50Pa

mechanical extract systems1
Mechanical extract systems

MEV systems

These systems comprise either a central extract system,

individual room fans or a combination of both. They

operate continuously and are designed to offer extract

from the wet areas of a dwelling.

Systems typically have two speed settings. Continuous

trickle ventilation is provided on the low speed setting

and boostventilation is provided on the high speed setting.

The boost setting can be operated either manually or automatically (using a humidistat


Fresh air is drawn into the dwelling via background ventilators (i.e. trickle ventilators)

and air leakage.

mvhr systems
MVHR systems

A typical balanced MVHRsystem consists of:

  • An intake and an extract fan.
  • A heat exchanger to transfer heat from the extract air to the inlet stream.
  • Ductwork to distribute air around the dwelling.

MVHR systems operate by extracting

warm moist air from the wet areas of the

dwelling, such as the kitchen and the

bathroom, and supplying fresh air to the

living room and the bedrooms. Both airflow's

are ducted through the heat exchanger,

where 70% or more of the heat from the

outgoing air can be transferred to the

incoming air. Systems typically have two

speed settings - low speed trickle ventilation

setting and a high speed boostventilation


Whole house MVHR system [Source:BRECSU,1993]

mvhr systems1
MVHR systems

MVHR systems have been incorporated into a number of low-energy dwellings in

the UK and abroad.

In principle, such systems can reduceoverall energy use, however, they are still

comparatively expensive and can be a difficult retrofit option.

MVHR systems also require high levels of airtightness - certainly better than 3

ac/h @ 50 Pa, and ideally better than 1 ac/h @ 50 Pa (Liddament, 1993).

In dwellings with an air leakage of 3 - 5 ac/h @ 50 Pa, MEV or PSV systems may

perform better overall than a MVHR system (Lowe, Bell & Johnston, 1996).

Problems associated with MVHR systems include:

  • Occupant behaviour, i.e. window opening.
  • Noise in poorly designed systems.
  • Problems due to incorrect installation of MVHR systems.
  • Over or under specification of the system.
mechanical extract systems2
Mechanical extract systems

The Building Regulations 2000 require that all kitchens, utility rooms and

bathrooms, even if adjacent to an external wall, are to have extract fans fitted, or

be connected to a mechanical extract or PSV system.

However, there are some drawbacks to mechanical extract systems:

  • Manually controlled fans may be switched off or even removed by occupants.
  • Humidistat controlled fans may operate when not required, i.e. during warm, humid conditions.
  • Extract systems slightly depressurise the building, resulting in increased infiltration if the envelope is not airtight.
dwellings with mvhr systems
Dwellings with MVHR systems

Lower Watts House [Source:Olivier & Willoughby, 1996]

  • MVHR systems using a counterflow heat exchanger.
dwellings with mvhr systems1
Dwellings with MVHR systems

The Self-sufficient Solar House, Freiburg, Germany

[Source: Fraunhofer Institute For Solar Energy Systems, 2010]

  • Fresh air enters the MVHR system via a system of earth-buried tubes which are located 4m under the ground.
  • Air passing through the tubes is pre-heated by the ground then passes onto a heat exchanger, where it recovers heat from the extract air.
dwellings with mvhr systems2
Dwellings with MVHR systems

12 Field Trial Dwellings at Derwentside, Co. Durham

  • Retrofitted MVHR units installed in 6 of the 12 field trial dwellings.
mechanical ventilation systems
Mechanical ventilation systems

The mechanical ventilation systems which are typically employed in non-

domestic buildings are as follows:

  • Supply-only.
  • Extract-only.
  • Balanced.

Supply-only ventilation

These systems slightly pressurise the space, therefore resisting infiltration.

However, if there are significant amounts of moisture in the room, the increased

internal pressure can force moist air from the space into the building fabric,

increasing the risk of interstitial condensation. This can be minimised if the

building envelope is airtight, and by providing purpose designed exhaust vents.

Mechanical supply-only ventilation[Source: CIBSE, 1997]

mechanical ventilation systems1
Mechanical ventilation systems

Extract-only ventilation

In extract only systems, air is extracted at high level by fans with natural inlets for

fresh air at low level. To ensure fresh air at all parts of the room, the distance

between the inlets and the outlets should not be excessive, since the fresh air

becomes progressively contaminated as it travels across the room.

Mechanical extract-only ventilation[Source: CIBSE, 1997]

The primary advantage of such systems is the ability to extract internally generated

pollutants at source (e.g. ozone generated by a photocopier).

The disadvantage of extract-only ventilation is that it depressurises the building,

resulting in increased infiltration if the building is not airtight.

mechanical ventilation systems2
Mechanical ventilation systems

Balanced ventilation

Offers the combined benefits of the previous two systems. However, the need for

two independent ductwork systems increases both the initial and running costs

for fan energy and maintenance. In order to retain the advantage of slight

pressurisation of the space, it is customary to arrange for the inlet fans to deliver

approximately more volume flow rate than the output fans.

Balanced mechanical ventilation[Source: CIBSE, 1997]

Balanced ventilation also allows the application of heat recovery devices to

produce a Mechanical Ventilation with Heat Recovery (MVHR) system.

mechanical ventilation systems3
Mechanical ventilation systems

The main advantages of a mechanically ventilated building are:

  • They can provide positive ventilation at all times, irrespective of outside conditions.
  • They ensure a specified air change rate.
  • The air under fan pressure can be forced through an air filter.
  • Ventilation rate is controllable and ventilation air can be introduced more uniformly.
mixed mode ventilation
Mixed-mode ventilation

Not all parts of a building have to be ventilated in the same way. Natural ventilation

can be combined with mechanical ventilation and/or air conditioning in a ‘mixed-

mode’ system.

A number of strategies can be employed, the two main strategies being:

  • Zonal mixed-mode - Different parts of a building are serviced in different ways to cater for different uses of the space
  • Seasonal mixed-mode - Both natural and mechanical ventilation systems are installed to service a particular zone, with the alternative systems being used at different times of the year.

Zonal and seasonal mixed mode strategy [Source: CIBSE, 1997]


ATTMA (2007) Technical Standard 1. Measuring Air Permeability of Building Envelopes [Internet]. Airtightness Testing

and Measurement Association. Issue 2, 13th July 2007. Available from: [Accessed 28th April


BRECSU (1997) The Queens Building De Montfort University - Feedback for Designers and Clients. New Practice

Case Study 102, London, HMSO.

BRECSU (1995a) A Performance Specification for the Energy Efficient Office of the Future. General Information

Report 30, London, HMSO.

BRECSU (1995b) Avoiding or Minimising the Use of Air-conditioning - A Research Report from the EnREI

Programme.General Information Report 31, London, HMSO.

BRECSU (1993) Domestic Ventilation. General Information Leaflet 9. Department of the Environment, London, HMSO.

BSL (2009) Air Permeability Database. Oxon, UK, Building Sciences Limited.

BUNN, R. (1995) Teaching Low Energy. Building Services, Volume 17, No. 4, April 1995, pp.19-23.

BUNN, R. (1994) Making Light Work. Building Services, Volume 16, No. 11, November 1994, pp.20-24.

CIBSE (2005) Natural Ventilation in Non-domestic Buildings. CIBSE Applications Manual AM 10: 2005 Edition,

London, Chartered Institute of Building Services Engineers.

CIBSE (1997) Natural Ventilation in Non-domestic Buildings. CIBSE Applications Manual AM 10, London, Chartered

Institute of Building Services Engineers.

CLG (2009b) Proposals for amending Part L and Part F of the Building Regulations – Consultation. Volume 2: Proposed

technical guidance for Part L [Internet]London,Communities and Local Government. Available from|: [Accessed 20th July 2009].

references continued
References (continued)

EST (2006) Energy Efficient Ventilation in Dwellings – A Guide For Specifiers. Good Practice Guide 268 (GPG 268).

London, Energy Saving Trust.

FRAUNHOFER INSTITUTE FOR SOLAR ENERGY SYSTEMS (2000) The Solar House in Freiburg. [Internet]

Freiburg, Germany, Fraunhofer Institute for Solar Energy Systems.Available from:


solar-house-in-freiburg> [Accessed 8th February, 2010].

GREEN BUILDING STORE (2010) Denby Dale Passivhaus. [Internet]. Huddersfield, UK, Green Building Store.

Available from <> [Accessed 1st February


GRIGG, P. (2004) Assessment of Energy Efficiency Impact of Building Regulation Compliance. A Report Prepared for

the Energy Savings Trust/Energy Efficiency Partnership for Homes. Client Report Number 219683, Garston, Watford,

Building Research Establishment.

JAGGS, M. and SCIVYER, C. (2006), Good Building Guide GBG67 Part 1. Achieving Airtightness: General Principles.

Garston, Watford, Building Research Establishment.

LOWE, R. BELL, M. and JOHNSTON, D. (1996) Directory of Energy Efficient Housing. Coventry, Chartered Institute of


ODPM (2006a) The Building Regulations 2000 Approved Document Part F: Ventilation. 2006 Edition Office of the Deputy

Prime Minister, London, HMSO.

ODPM (2006b) The Building Regulations 2000 Approved Document L1A: Conservation of Fuel and Power in New

Dwellings. 2006 Edition, Office of the Deputy Prime Minister, NBS, London.

ODPM (2003) The Building Regulations 2000 Approved Document Part F: Ventilation. Office of the Deputy Prime Minister,

London, HMSO.

OLIVIER, D .(1994) Build Tight: The Orkney Experience. Building Services, Volume 16, No. 7, July 1994, pp.33-34.

OLIVIER, D. and WILLOUGHBY, J. (1996) Review of Ultra-low-energy Homes: Ten UK Profiles in Detail. General

Information Report 39, Department of the Environment, London, HMSO.

STEPHEN, R. K. (2000) Airtightness in UK Dwellings. BRE Information Paper IP 1/00. Garston, Watford, Building

Research Establishment

references continued1
References (continued)

OLIVIER, D .(1994) Build Tight: The Orkney Experience. Building Services, Volume 16, No. 7, July 1994, pp.33-34.

OLIVIER, D. and WILLOUGHBY, J. (1996) Review of Ultra-low-energy Homes: Ten UK Profiles in Detail. General

Information Report 39, Department of the Environment, London, HMSO.

STEPHEN, R. K. (2000) Airtightness in UK Dwellings. BRE Information Paper IP 1/00. Garston, Watford, Building

Research Establishment