7.1 When building work is carried out on an existing building, the work should comply with the applicable requirements of Schedule 1 to the Building Regulations, and the rest of the building should not be made less satisfactory in relation to the requirements than before the work was carried out (see regulations 3 and 4 of the Building Regulations). Further, when a building undergoes a material change of use, as defined in regulation 5 of the Building Regulations (see paragraph 3.17 above), Part F applies to the building or that part of the building which has been subject to the change of use, in accordance with regulation 6. Therefore, the guidance in other sections of this Approved Document may be applicable.
7.2 Windows are a controlled Regulation 4(1) of the Building Regulations requires that, when windows in an existing building are replaced, the replacement work should comply with the applicable requirements of Schedule 1, i.e. Parts L and N. In addition, the building work once completed should not have a worse level of compliance with other applicable requirements of Schedule 1 than before commencement of the work. Other applicable requirements of Schedule 1 may include Parts B, F and J.
7.3 Where the original windows were fitted with trickle ventilators the replacement windows should include them and they should be sized as set out in paragraph 6.
7.4 Where the original windows were not fitted with trickle ventilators and the room is not ventilated adequately by other installed provisions, it would be good practice to fit trickle ventilators (or an equivalent means of ventilation) to help with control of condensation and improve indoor air quality. Ventilation devices should be fitted with accessible controls.
See A guide to trickle ventilators at www.ggf.org.uk
7.5 When windows are replaced as part of the work connected with a material change of use, Sections 5 and 6 of this Approved Document
7.6 In all cases where trickle ventilators (or an equivalent means of ventilation) are to be fitted, the new ventilation opening should not be smaller than was originally provided, and it should be controllable. Where there was no ventilation opening, or where the size of the original ventilation opening is not known, the following minimum sizes should be adopted.
- habitable rooms – 5000 mm² equivalent area
- kitchen, utility room and bathroom (with or without WC) – 2500 mm² equivalent area.
Buildings other than dwellings:
- occupiable rooms: for floor areas up to 10 m² – 2500 mm² equivalent area; greater than 10 m² – at the rate of 250 mm² equivalent area per m² of floor area
- kitchens (domestic type) – 2500 mm² equivalent area
- bathrooms and shower rooms – 2500 mm² equivalent area per bath or shower
- sanitary accommodation (and/or washing facilities) – 2500 mm² equivalent area per WC.
Addition of a habitable room (not including a conservatory) to an existing dwelling
7.7 The requirements will be met by following the guidance in paragraphs 7.8 to 7.10.
7.8 The general ventilation rate for the additional room and, if necessary, adjoining rooms could be achieved by one of the following options.
a. Background ventilators could be used as follows:
i. if the additional room is connected to an existing habitable room which now has no windows opening to outside, the guidance in paragraph 5.15 should be followed; or
ii. if the additional room is connected to an existing habitable room which still has windows opening to outside but with a total background ventilator equivalent area less than 5000 mm², the guidance in paragraph 5.15 should be followed; or
iii. if the additional room is connected to an existing habitable room which still has windows opening to outside and with a total background ventilator equivalent area of at least 5000 mm², there should be background ventilators of at least 8000 mm² equivalent area between the two rooms and background ventilators of at least 8000 mm² equivalent area between the additional room and outside.
b. A single room heat recovery ventilator could be used to ventilate the additional habitable room. The supply rate to that room should be determined as follows. First, determine the whole building ventilation rate from Table 5.1b. Second, calculate the room supply rate required from:
(Whole building ventilation rate × Room volume) /(Total volume of all habitable rooms)
7.9 For purge ventilation, follow the guidance given in Table 5.2a.
7.10 Guidance on location and controls is also given in Table 5.2a and guidance on performance testing standards is given in Table 5.3.
Addition of a wet room to an existing dwelling
7.11 The requirements for the additional wet room will be met by following the guidance in paragraphs 7.12 to 7.15.
7.12 Whole building and extract ventilation can be provided by:
a. intermittent extract, as given in Table 5.2a, and a background ventilator of at least 2500 mm² equivalent area; or
b. single room heat recovery ventilator, as given in Table 5.2a; or
c. passive stack ventilator, as given in Table 5.2b; or
d. continuous extract fan, as given in Table 5.2c.
7.13 To ensure good transfer of air throughout the dwelling, there should be an undercut of minimum area 7600 mm² in the internal door between the wet room and the existing dwelling. This is equivalent to an undercut of 10 mm for a standard 760 mm width door. This should be achieved by making an undercut of 10 mm above the floor finish if the floor finish is fitted, or by a 20 mm undercut above the floorboards, or other surface, if the finish has not been fitted.
7.14 For purge ventilation, follow the guidance for the appropriate system given in Tables 5.2a to 5.2c.
7.15 Guidance on location and controls is given in Tables 5.2a to 5.2c and guidance on performance standards is given in Table 5.3.
Addition of a conservatory to an existing building
7.16 The guidance applies to conservatories with a floor area over 30 m².
7.17 The requirements will be met by following the guidance in paragraphs 7.18 to 7.20.
7.18 The general ventilation rate for the conservatory and, if necessary, adjoining rooms could be achieved by the use of background ventilators. Follow the guidance in paragraph 5.16 whatever the ventilation provisions in the existing room adjacent to the conservatory.
7.19 For purge ventilation, follow the guidance given in Table 5.2a.
7.20 Guidance on location and controls is also given in Tables 5.2a and guidance on performance standards is given in Table 5.3.
Refurbishing a kitchen or bathroom in an existing dwelling
7.21 If any of the work being carried out in the kitchen or bathroom of an existing building is ‘building work’, as defined in regulation 3 of the Building Regulations, the Regulations require that you comply with the appropriate requirements of the Regulations, and in doing so you do not make compliance with other requirements of the Regulations, including ventilation, worse than before. The Regulations also require that before you start work, the work is notified to a BCB, except in certain circumstances. (See paragraphs 3.4 to 3.10 inclusive above for details of notification requirements and exemptions.)
7.22 The definition of ‘building work’ in regulation 3 of the Building Regulations includes a range of building activities in existing buildings, and includes all work on controlled services. For more information see the Building Regulations 2010 at: www.planningportal.gov.uk.
7.23 If you carry out any ‘building work’, and there is an existing extract fan or passive stack ventilator (or cooker hood extracting to outside in the kitchen), you should retain or replace it. However, if there is no existing ventilation system you need not provide one. Replacing an extract fan or cooker hood with a similar type, and using the existing cabling, need not be notified to a BCB.
7.24 If a combustion appliance is installed, Approved Document J: Combustion appliances and fuel storage systems should be consulted. It is available at: www.planningportal.gov.uk/approveddocuments.
Appendix A: Performance-based ventilation
As specified in the section on Performance, this Approved Document recommends ventilation provisions to control both moisture and pollutants in buildings. In order to do this, acceptable levels of moisture and other pollutants need to be defined. This Appendix sets out the levels of moisture and other pollutants that the provisions in this Approved Document are designed to control.
Note that the guidance within this Approved Document may not be adequate to address pollutants from flueless combustion space heaters or from occasional, occupant-controlled events such as painting, smoking, cleaning or other high-polluting events. It does not address the airborne spread of infection or contamination from outdoor sources. While many of these considerations could be important factors in achieving acceptable indoor air quality, solutions are not ready for inclusion in this guidance, and indeed indoor air quality may be better controlled at source (e.g. avoidance, isolation or use of lower emitting products).
Performance criteria for dwellings
The performance criterion for moisture is as follows:
- There should be no visible mould on external walls in a properly heated dwelling with typical moisture generation.
The principal performance criteria used for other indoor air pollutants are as follows.
Exposure to the following levels of nitrogen dioxide (NO2) should not be exceeded:
- 288 µg/m³ (150 ppb) – 1 hour average (DOH, 2004)
- 40 µg/m³ (20 ppb) – long-term average (DOH, 2004).
Exposure to the following levels of carbon monoxide should not be exceeded:
- 100 mg/m³ (90 ppm) – 15 minute averaging time (DOH, 2004)
- 60 mg/m³ (50 ppm) – 30 minute averaging time (DOH, 2004)
- 30 mg/m³ (25 ppm) – 1 hour averaging time (DOH, 2004)
- 10 mg/m³ (10 ppm) – 8 hours averaging time (DOH, 2004).
Exposure to total volatile organic compound (TVOC) levels should not exceed 300 µg/m³ averaged over 8 hours (ECA, 1992).
Control of bio-effluents (body odours) for adapted individuals (reduction in perception due to being exposed to the environment for a period of time) will be achieved by an air supply rate of 3.5 l/s/person (ASHRAE, 2003).
Mould growth can occur whether the dwelling is occupied or unoccupied, so the performance criteria for moisture (as set out in Table A2) should be met at all times, regardless of occupancy. The other pollutants listed above are harmful to the occupants only when the dwelling is occupied.
Assumptions used in applying performance criteria for dwellings in Section 5
- For the default option in which the design air permeability can be any value, dwellings with ventilation System 1 or 2 are assumed to have an infiltration of 0.05 ach (air changes per hour); and dwellings with ventilation System 3 or 4 are assumed to have no infiltration.
- For the alternative option in which the design air permeability is >5 m³/(h.m²) at 50 Pa, dwellings with ventilation System 1, 2 or 4 are assumed to have an infiltration of 15 ach, and dwellings with ventilation System 3 are assumed to have negligible infiltration, as discussed in Table 5.2c.
- The ventilation effectiveness is 1.0.
- For the purpose of this Approved Document, for all dwellings (new, and existing where Part F applies), the moisture criterion is likely to be met if the moving average surface water activity of the internal surfaces of external walls is always less than the value noted in Table A1 during the heating season, evaluated over each of the stated averaging periods. Table A1 is the primary basis for demonstrating compliance with the moisture criterion.
Table A1 Surface Water Activity
As a guide, for new dwellings, for the purpose of this Approved Document, the moisture criteria in Table A1 are likely to be met if the moving average relative humidity (RH) in a room is always less than the value given in Table A2 during the heating season, evaluated over each of the stated averaging periods.
Table A2 Indoor air relative humidity
The performance criterion for moisture (given above) is the same as that used in the 2006 edition of Approved Document F. However, research carried out since 2006 (Altamirano- Medina et al, 2009) has indicated that the values of relative humidity needed to satisfy the criterion could be expressed in a different and more practical way as shown in Table A2. These RH values may, in general, be slightly less onerous than those used in the 2006 edition. This may not be true in all cases and so the ventilation rates necessary to satisfy the criterion have not been changed.
- The principal pollutant to be removed by extract ventilation is moisture. The source rates were taken from BS 5250:2002 Table B.1.
- For intermittent extract:
– Historically, a ventilation rate of 60 l/s has been specified in the kitchen for the removal of moisture and there is no strong justification to amend it. The ventilation rate removes moisture generated at a production rate of 2000 g/h. A reduced ventilation rate of 30 l/s is used for a cooker hood, owing to the greater ventilation effectiveness.
– Historically, a ventilation rate of 15 l/s has been specified in the bathroom for the removal of moisture and there is no strong justification to amend it. The ventilation rate removes moisture generated at a production rate of 400 g/h.
– In the utility room, it is assumed that the ventilation rate required is 50% of that in the kitchen.
– In WCs, the main pollutant is odour. Historically, a ventilation rate of 6 l/s has been specified and there is no strong justification to amend it.
- For continuous extract:
– No reason has been found to change the extract rates, so the rates used in Table 5.1a are the same as used in the 2006 edition of AD F.
Whole dwelling ventilation
- The principal pollutant to be removed by whole dwelling ventilation is moisture. The source rates were taken from BS 5250:2002 Table B.1.
- It was assumed that local extract removes 100% of the moisture generated in the bathroom and 50% of the moisture generated in the kitchen.
- The rates given in Table 5.1b are the same as in the 2006 edition.
- The calculations were based on winter weather conditions. During warmer spring and autumn periods, the moisture removal capacity of the outdoor air will be less (i.e. the outdoor air on being heated to the internal temperature within the dwelling will have a higher relative humidity in the spring and autumn periods) and additional ventilation may be required. The provisions for purge ventilation (e.g. windows) may be used for this purpose.
- There are other pollutants which must also be adequately controlled. These are particularly important in homes of low occupant density where moisture production is low for the size of the Levels of volatile organic compounds were monitored in a BRE study of UK homes (Dimitroulopoulou et al, 2005). From these data, the total source production rate of volatile organic compounds was determined to be 300 µg/h per m² of floor area. To meet the performance criterion of 300 µg/m³, it requires a minimum whole dwelling ventilation rate of 0.3 l/s per m² of internal floor area.
- A value of 4 ach has been selected as:
– it provides a purge ventilation rate an order of magnitude above whole building ventilation;
– it is similar to the ventilation rate provided by windows in the 1995 edition of Approved Document The calculation assumes single-sided ventilation for a dwelling in an urban environment and an internal/external temperature difference of 3°C.
Basis of Table 5.2 – Whole dwelling ventilation rates
- In determining the ventilation rates, the air supply rates in Table 5.1b have been used.
- For dwellings having an air permeability of > 5 m³/(h.m²) at 50 Pa, the air supply rate has been reduced by 0.15 ach to allow for infiltration.
- To determine the equivalent areas, the standard air flow equation has been used as below:
Note that the total actual equivalent area required (A ) is double that derived from the equation above, which provides the equivalent area only for air supplied to the dwelling. A similar equivalent area is required for air to exit the dwelling. The total equivalent area determined in this way is given in the guidance for Systems 1 and 2. Note that in determining these pressure differences, a meteorological wind speed of 4 m/s at 10 m height was taken (based on BS 5925:1991) and an internal/external temperature difference of 15°C.
Performance criteria for buildings other than dwellings
The main guidance within this document has focused on offices. For this, the main criteria have been:
- A supply rate, in the absence of tobacco smoking or other excessive pollutants, of 10 l/s/person, based upon surveys which indicate that below this level the incidence of health effects becomes increasingly This will also satisfy the requirement of 8 l/s/person needed to control bio-effluents for unadapted individuals.
- There should be no visible mould on external walls in a properly heated building with typical moisture generation.
- Exposure to the following levels of nitrogen dioxide (NO ) should not be exceeded:
- 288 µg/m³ (150 ppb) – 1 hour average (Department of the Environment, 1996)
- 40 µg/m³ (21 ppb) – annual mean (WHO, 2005).
- Exposure to the following levels of carbon monoxide should not be exceeded:
- 100 mg/m³ (90 ppm) – 15 minute averaging time (WHO, 2000)
- 60 mg/m³ (50 ppm) – 30 minute averaging time (WHO, 2000)
- 30 mg/m³ (25 ppm) – 1 hour averaging time (WHO, 2000)
- 10 mg/m³ (10 ppm) – 8 hours averaging time (Department of the Environment, 1994a).
- Exposure to the following levels of carbon monoxide for occupational exposure should not be exceeded:
- 35 mg/m³ (30 ppm) – 8 hours averaging time (HSE, 2003).
- Exposure to total volatile organic compound (TVOC) levels should not exceed 300 µg/m3 averaged over 8 hours (ECA, 1992).
- Ozone levels should not exceed 100 µg/m3 (Department of the Environment, 1994b).
Note that the guidance within this Approved Document may not be adequate to address pollutants from occasional, occupant-controlled events such as painting, smoking, cleaning or other high-polluting events. While these could be important factors in achieving acceptable indoor air quality, solutions are not ready for inclusion in this guidance, and indeed they may be better controlled at source (e.g. avoidance, isolation or use of lower emitting products).
Mould growth can occur whether the building is occupied or unoccupied, so the performance criteria for moisture (as set-out in Table A1) should be met at all times, regardless of occupancy. The other pollutants listed above are harmful to the occupants only when the building is occupied.
Where the Health and Safety Executive gives guidance for specific situations, it should be followed in preference to the guidance given here.
Assumptions used in applying performance criteria for offices in Section 2
- The office has an air permeability of 3 m³/ (h.m²) at 50 Pa.
- At this level of air permeability, in large buildings (low ratio of surface area to volume contained), infiltration can be assumed to be negligible compared with the purpose- provided ventilation.
- The ventilation effectiveness is 0.9 (for Table 6.1b).
- For the purposes of this Approved Document, the moisture criterion will be met if the surface water activity in a room does not exceed the values given in Table A1 during the heating season.
- Office equipment can emit pollutants including ozone and organic For example, a study by Black and Wortham (1999) suggests the following emission rates for laser printers and dry paper copiers assuming 30 minutes use in an hour:
- 25 mg/h for TVOCs
- 3 mg/h for ozone.
To meet the performance criteria for these pollutants requires an extract rate of 20 l/s per machine during use.
- For sanitary accommodation, the extract rates used for dwellings have been applied.
- For food and beverage preparation areas, the extract rates used for dwellings have been applied.
Whole building ventilation
- A number of studies have investigated ventilation and health in offices (principally sick building syndrome). Although there is no clear threshold ventilation rate below which health suddenly worsens, a number of sources have identified 10 l/s/p as a significant This can probably be traced back to an analysis of experimental studies of office buildings by Mendell (1993). Hence the recommendation within the Approved Document is for 10 l/s/p for buildings with no smoking and no significant pollutant sources.
- Increasing the ventilation rate above 10 l/s/p may improve health (results unclear), but there are diminishing returns (i.e. the improvement in health per l/s/p increase in ventilation rate becomes smaller as the ventilation rate increases). This suggests that there is little advantage in increasing the whole building ventilation rate above 10 l/s/p. Increased ventilation has a cost in economic and environmental Having set a ventilation rate of 10 l/s/p, if further improvements in indoor air quality are necessary, alternative approaches should be considered first, e.g. use of low-emission materials.
- There are normally more options for the removal of high concentrations of pollutants from office spaces than for dwellings (e.g. leaving rooms unoccupied until acceptable pollutant levels are achieved). Hence, general guidance has been provided rather than specifying any ventilation rate(s).
Appendix B: Purge ventilation
Adequate purge ventilation may be achieved by the use of openable windows and/or external doors. This Appendix provides details of necessary window and door sizes. The diagrams highlight the window dimensions of importance.
- For a hinged or pivot window that opens 30º or more or for parallel sliding windows (e.g. vertical sliding sash windows), the height x width of the opening part should be at least 1/20th of the floor area of the room.
- For a hinged or pivot window that opens between 15º and 30º, the height x width of the opening part should be at least 1/10th of the floor area of the room.
- If the window opens less than 15º it is not suitable for providing purge ventilation and other arrangements should be made.
- If the room contains more than one openable window, the areas of all the opening parts may be added to achieve the required proportion of the floor area. The required proportion of the floor area is determined by the opening angle of the largest window in the room.
- Note that Approved Document B includes provisions for the size of escape The larger of the provisions in Approved Document B or F should apply in all cases.
External doors (including patio doors)
- For an external door, the height x width of the opening part should be at least 1/20th of the floor area of the room. If the room contains more than one external door, the areas of all the opening parts may be added to achieve at least 1/20th of the floor area of the room.
- If the room contains more than one external door, the areas of all the opening parts may be added to achieve at least 1/20th of the floor area of the room.
- If the room contains a combination of at least one external door and at least one openable window, the areas of all the opening parts may be added to achieve at least 1/20th of the floor area of the room.
The aim of this guidance is to achieve a purge ventilation rate of 4 ach.
- It provides a purge ventilation rate of an order of magnitude above the whole building ventilation rate.
- It is similar to the purge ventilation rate provided by windows historically in ADF.
The guidance contained within this Appendix is a simplification of guidance in BS5925:1991 (AMD 8930, 1995) Code of practice for ventilation principles and designing for natural ventilation. This Appendix has assumed single-sided ventilation for dwellings in an urban environment (local wind speed of 2.1m/s) and a summer-time internal/external temperature difference of 3°C. It has considered and simplified variations in air flow rates caused by factors including window type and window height.
This design guidance should deliver 4 ach in most cases. Depending on the dwelling design or the external climate, it may be possible to achieve this ventilation rate through a smaller window opening area. BS5925 provides a good starting point for determining the window openings required. It may be beneficial to also seek expert advice.
Appendix C: Example calculations for ventilation sizing for dwellings
This Appendix provides example calculations for each ventilation system set out in paragraph 5.10. A ground-floor flat and a semi-detached house have been considered for each system type. Thus there are eight examples as follows.
Example C1 – Background ventilators and intermittent extract fans
Example C2 – Passive stack ventilation
Example C3 – Continuous mechanical extract
Example C4 – Continuous mechanical supply and extract
Example C5 – Background ventilators and intermittent extract fans
Example C6 – Passive stack ventilation
Example C7 – Continuous mechanical extract
Example C8 – Continuous mechanical supply and extract
It has been assumed that the intended measured air permeability is > 5 m³/(h.m²) in examples C1, C2, C5 and C6, and the design air permeability is ≤ 3 m³/(h.m²) in the other examples.
Details of ground-floor flat
The flat contains the following rooms:
- combined living/dining room
- one double bedroom
- internal bathroom containing WC, and in addition
- all rooms have an external wall except for the bathroom.
The floor plan is given in Diagram C1.
- cooker hood adjacent to cooker hob
- gross internal volume of the heated space of 83 m³
- total floor area of 36 m²
- two-person occupancy and
- side-hinged windows 1.0 m high and openable to 60°.
Diagram C1 Ground floor flat plan
Example C1 Background ventilators and intermittent extract fans (this is based on Table 5.2a)
Example C2 Passive stack ventilation (this is based on Table 5.2b)
Example C3 Continuous mechanical extract (this is based on Table 5.2c)
Example C4 Continuous mechanical supply and extract with heat recovery (this is based on Table 5.2d)
Details of semi-detached house
The semi-detached house contains the following rooms:
- entrance hall/stairway
- dining room
- living room
- three bedrooms
- bathroom containing WC, and in addition
- all rooms have an external
The floor plans are given in Diagrams C2 and C3.
- cooker hood adjacent to cooker hob
- gross internal volume of the heated space of 210 m³
- total floor area of 84 m²
- four-person occupancy
- side-hinged windows 1.0 m high and openable to a fixed position of 20º.
Diagram C2 Semi-detached ground floor plan example
Diagram C3 Semi-detached house first floor plan example
Example C5 Background ventilators and intermittent extract fans(this is based on Table 5.2a)
Example C6 Passive stack ventilation (this is based on Table 5.2b)
Example C7 Continuous mechanical extract (this is based on Table 5.2c)
Example C8 Continuous mechanical supply and extract with heat recovery (this is based on Table 5.2d)
Appendix D: Minimising ingress of external pollution into buildings in urban areas
Typical urban pollutants that need to be considered include those covered by the UK Air Quality Strategy (www.defra.gov.uk/environment/ airquality/strategy/index.htm (2007)). These are:
- carbon monoxide, CO
- nitrogen dioxide,NO2
- sulphur dioxide, SO2
- ozone, O
- particles(PM )
- polycylic aromatic hydrocarbons (PAHs)
Although nitrogen oxide, NO, is not included in the UK Air Quality Strategy, it is a normal constituent of combustion discharges and in many cases (for example, from gas-fired plant) the largest polluting emitter. Therefore, it also needs to be taken into account.
Typical pollution emission sources that need to be considered include:
- road traffic, including traffic junctions and underground car parks
- combustion plant (such as heating appliances) running on conventional fuels, most commonly natural gas
- other combustion processes (for example, waste incineration, thermal oxidation abatement systems)
- discharges from industrial processes
- fugitive (i.e. adventitious/not effectively controlled) discharges from industrial processes and other sources
- building ventilation system exhaust discharges
- construction and demolition sites which are a source of particles and vapourous discharges.
In urban areas, buildings are exposed simultaneously to a large number of individual pollution sources from varying upwind distances (long range, intermediate range and short range) and heights and also over different timescales. The relationship between these and their proportionate contribution under different circumstances governs pollutant concentrations over the building shell and also internally.
Internal contamination of buildings from outdoor pollution sources therefore depends upon the pollutant sources, the physical characteristics of the building and its relation to its surroundings, the ventilation strategy employed and the location of the air intake. Whatever type of ventilation system is used, it is important to ensure that the intake air is not contaminated.
This is especially important in air quality management areas where, by definition, pollution levels of at least one pollutant are already close to the air quality standards. Simplified guidance on ventilation intake placement for minimising ingress of pollutants may be summarised, as in Table D1.
Control of ventilation intakes
For pollutant sources such as urban road traffic, whose concentration fluctuates with the time of day, reducing the flow of external air or closing ventilation intakes during peak periods of high external pollutant concentrations, for example during rush hours, for up to an hour may be an option.
Air intakes located on a less polluted side of the building may then be used for fresh air, or air may be fully recirculated within the building. Alternatively, the building may be used as a ‘fresh air’ reservoir to supply air during these short periods. The use of atria as a source of ‘fresh air’ for this purpose may be an option.
However, care must be taken since, for example, reducing the inflow of external air will also reduce the outflow of internal air, resulting in a build-up of internally generated pollutants that need to be removed. Most modern buildings have low ceiling heights and therefore the concept of a substantial ‘fresh air’ reservoir available within the building may not apply. Further details of this principle with examples may be found in Liddament (2000).
Table D1 Guidance on ventilation intake placement for minimising ingress of pollutants
Location of exhaust outlets
The location of exhausts is as important as the location of air intakes. These should be located such that re-entry to a building, or ingestion into other nearby buildings, is minimised (for both natural and mechanical intakes) and such that there is no adverse effect on the surrounding area. Guidance on outlet placement may be summarised as follows:
- Exhausts should be located downstream of intakes where there is a prevailing wind direction
- Exhausts should not discharge into courtyards, enclosures or architectural screens as pollutants tend to build up in such spaces and do not disperse very readily
- It is recommended that stacks should discharge vertically upwards and at high level to clear surrounding buildings and so that downwash does not occur
- Where possible, pollutants from stacks should be grouped together and discharged vertically The increased volume will provide greater momentum and increased plume height. This is common practice where there are a number of fume cupboard discharges; greater plume height dispersion can be achieved by adding the general ventilation exhaust.