Internal services

Also see:

8.1.12Extract ducts

Ductwork to intermittent and continuously running mechanical extract ventilation systems shall ensure satisfactory performance and durability. Issues to be taken into account include:

  1. building integration
  2. resistance to airflow
  3. control of condensation
  4. installation
  5. terminals.

Building integration

The route of ductwork should take account of other building elements. Ductwork passing through structural elements should not adversely affect the structural or fire performance of the building. Where alterations to structural elements, such as I-joists, are required, this should only be carried out in accordance with the manufacturer’s recommendations, or be designed by an engineer in accordance with Technical Requirement R5.

The fire requirements of the building should be in accordance with relevant building regulations and standards. Issues that should be taken into account include:

  • suitable detailing of components passing through other elements of the building
  • the location and type of dampers and firestops to be used
  • the integrity of protected stairs and halls
  • the integrity of walls and floors.

Resistance to airflow

Ductwork systems should be designed to minimise the resistance to airflow, and be formed from compatible components.

Rigid duct is preferable to flexible, but where flexible duct is used, it should be restricted in length to ensure that the airflow resistance does not prevent the designed ventilation rate from being achieved. Flexible duct should be installed:

  • straight
  • in accordance with the manufacturer’s recommendations.

Bends should generally be formed with proprietary rigid components. Where flexible duct is used to form bends on an intermittent extract system, they should be restricted to a maximum of:

  • two for systems up to 30 L/s
  • one for extract rates higher than 30 L/s.

Control of condensation

Where extract ductwork passes through unheated spaces, it should be continuously insulated to achieve a thermal resistance equivalent to a minimum of 25mm of insulating material with a thermal conductivity of 0.04W/(mK). This can be achieved by using:

  • suitable pre-insulated ductwork, or
  • a proprietary insulation system.

Alternatively, the ductwork can be fitted with a condensate trap that discharges to the outside or installing the duct to slope to the outside.


Ductwork should be installed in a neat and workmanlike manner, be securely fixed, and have:

  • adequate support throughout its length
  • sealed mechanically fixed joints and connections.

Where ductwork passes through an external wall, it should be positioned to slope slightly outwards to prevent water entering
the building. Clips and supports for ductwork should be spaced at equal distances and in accordance with the ductwork
manufacturer’s recommendations. For rigid ductwork, they should not generally be more than 750mm apart.

Ductwork should not be in direct contact with other surfaces, such as plasterboard ceilings, that may transfer noise to the home.


Ventilation systems should terminate freely to open air.

The air flow resistance of terminals should not adversely affect the performance of the ventilation system. Airflow resistance of terminals can be obtained through testing in accordance with BS EN 13141-2.