星空传媒

A revised version of Part F of the 星空传媒 Regulations is due to come into effect, alongside Part L, in April next year. It has been 10 years since the regulations, which cover natural and mechanical ventilation, were last updated and although the draft approved document has not yet been approved by ministers, it gives a clear indication of what鈥檚 to come.

The biggest change to the draft concerns dwellings and the shift from prescriptive regulations to performance-based compliance, providing designers with greater flexibility and opportunities for innovation.

The three methods of approach

There are three approaches that a designer can follow to ventilate dwellings. The first is to follow the guidance given on acceptable levels of moisture in the air to avoid mould growth, as well as acceptable levels of many other indoor air pollutants such as combustion products from cooking. These can affect an occupant鈥檚 health and are causing increasing public concern. Designers are free to come up with any solution that addresses these issues.

For many designers, however, more detailed guidance is helpful, and therefore the second approach is to follow the air supply and extract rates, specified in the draft approved document, that need to be used throughout a dwelling to meet, in normal circumstances, the acceptable levels of indoor air quality.

Table 1, over the page, shows the extract rates necessary to remove moisture and other pollutants from 鈥渨et鈥� rooms 鈥� such as bathrooms 鈥� using an intermittent or continuous extract system. Table 2, meanwhile, shows the necessary supply rates for the habitable rooms. This is based on the number of bedrooms, as the more people present, the more moisture and other pollutants are released into the air. It also takes account of factors such as chemical off-gassing from construction products, as well as occupant density, to ensure, for example, sufficient ventilation in cases where overcrowding is thought likely to occur.

In addition, purge ventilation provisions (previously called rapid ventilation), such as an openable window, are required in each habitable room to aid removal of high concentrations of pollutants and water vapour released from occasional activities such as painting and decorating. Purge ventilation provisions may also be used to improve thermal comfort and/or over-heating of buildings in summer, although this comes under Part L. The purge ventilation provisions should be capable of extracting a minimum of four air changes an hour per room directly to outside.

The draft approved document also specifies detailed design guidance for a range of ventilation systems, and the third approach entails following this guidance. While this approach is similar to that currently used in the 1995 version of approved document F, the new draft significantly expands the number of ventilation systems described, including both natural and mechanical solutions. It includes guidance for the following systems:

• system one: background ventilators and intermittent extract fans
• system two: passive stack ventilation
• system three: continuous mechanical extract
• system four: continuous mechanical supply and extract with heat recovery.

Currently, system one recommends background ventilator free areas of 4000 mm2 and 8000 mm2 for wet and habitable rooms respectively. This was based on a broad-brush approach, treating all types of dwelling similarly. The new draft approved document, however, relates background ventilator area directly to the individual dwelling requirements. Linking to table 2, a total dwelling ventilator area is specified for each dwelling depending on the number of bedrooms (in other words occupants). Minimum equivalent areas of

2500 mm2 and 5000 mm2 for wet and habitable rooms respectively are recommended to achieve good air distribution.

The draft approved document also considers in more detail the impact of dwelling type on ventilation. In particular, additional guidance is given in two cases:

• For single storey buildings such as flats and bungalows, there is less natural stack ventilation through the rise of air through the dwelling (as a result of temperature differences between inside and outside). Hence, for systems one and two, which are based on natural ventilation, additional background ventilators are recommended.
• For buildings with a single exposed facade, it is more difficult to achieve cross-ventilation (basic ventilation strategy). In this case, it is preferable to have the habitable rooms on the exposed facade. Cross ventilation can still be achieved for systems two and three if the wet rooms with extract ventilation are on the other side of the dwelling. For system one, it is not possible to achieve cross ventilation, and additional background ventilators are recommended to achieve significant single-sided ventilation (in other words, ventilators provided at both high and low positions in each room on the external facade). In this latter case, the dwelling should be no greater than 6 m in depth.

What it all means

The forthcoming revision of Part L of the 星空传媒 Regulations and its approved documents will improve the air tightness of dwellings to help meet future energy targets. This will result in a reduction in uncontrolled ventilation through the building fabric (known as infiltration). From the point of view of ventilation this is good as infiltration alone results in poorly distributed air flow. It is better to build-tight and then ventilate-right by installing manually or automatically controlled ventilators where they are needed.

In practice, the necessary changes to approved document F are small. The 1995 version already recommends ventilation provisions sufficient for many reasonably air-tight properties. However, there are notable exceptions. In particular, the new draft approved document recommends a significant increase of ventilation provisions (by up to 200%) for naturally ventilated flats. Furthermore, to ensure good transfer of air through all dwellings, there should be an undercut of effectively 10 mm in all internal doors above the floor finish.

There has been concern that some ventilation products do not perform as intended. For example, a poorly specified fan which provides sufficient air flow in free air may significantly under-ventilate when installed due to pressure losses along ductwork. Trickle ventilators having the same free area may perform very differently depending on how they are designed (especially true of sound-attenuating ventilators, which contain materials to absorb noise, and can restrict the air flow). The draft approved document addresses this issue by recommending that ventilators should be tested under realistic conditions and refers to a new set of European test standards which both BRE and the UK ventilation industry took part in developing. One consequence is the change in terminology for trickle ventilators from 鈥渇ree area鈥� to 鈥渆quivalent area鈥� 鈥� two ventilators with the same equivalent area will provide the same air flow at a given pressure difference.

For the first time, guidance has been provided in the draft document for the ventilation of basements. This has been developed and expanded from an industry-written approved document on basements for dwellings, which was prepared by the National House 星空传媒 Council and British Cement Association in 1997. In particular, as basements are at least partially below ground, partially airtight and less exposed to the wind, it includes new guidance that a ventilation pathway of at least 2000 mm2 equivalent area per 10 m2 of basement floor area should also be provided between the basement and any floor above (taking care of fire risks etc) to ensure sufficient air flow within the basement. Essentially, if there is a permanently open access between the basement and the rest of the dwelling above, the basement can be treated normally as part of a multistorey building, with mechanical ventilation solutions preferred if the basement has only one single-exposed facade. If there is no permanent access between the basement and the floors above ground, the basement is to be treated as a stand-alone single-storey building.

Turning to other building types, there has been less change in the draft approved document. For offices, the air supply rate has increased from 8 to 10 litres/s per person to reflect research relating to ventilation and health. In addition, the draft document now recommends that printers and photocopiers in substantial use should be located in a separate room and an extract rate of 20 litres/s per machine provided during use to remove ozone and organic chemical emissions.

Overall, there has been a significant expansion in the number of building types addressed in the new draft document. However, generally, it refers to other commonly used design guidance to avoid duplication. There is a new appendix which provides general guidance on minimising the ingress of external pollution into buildings in urban areas.

Finally, a small section has been included for work on existing buildings. It is primarily focused on dwellings but considers other building types as well. Probably the most significant addition is the recommendation that all replacement windows should include trickle ventilators (at least 5000 mm2 equivalent area for habitable rooms and 2500 mm2 for wet rooms) or an equivalent opening should be provided in the same room. This is to ensure that the building ventilation is not reduced on installing more airtight modern windows.