In this article, Premier Guarantee explores how to insulate extraction ducts in buildings, and the risks of installing them incorrectly
Extraction ducts are used to remove stale air from a room to the outside via a duct. The stale air inside the duct is warm, and when the duct passes through a cold space (such as a cold pitched roof), the warm air can condensate into moisture. This can lead to damp issues in and around your home if the ducting is incorrectly installed.
Extraction ducts should be insulted to avoid condensation build-up
Where extraction ducts are used and pass through a cold space, they should be insulated to avoid issues of condensation build-up. This should be considered at the design stage and checked on site prior to the closing up of works.
Particular care needs to be taken when installing ducts vertically, for example, through a roof space to connect to a tile vent. In these situations, a suitable condensation trap should be provided as close to the base of the duct as possible, ensuring that a suitable condensate pipe is provided which discharges at eaves level.
The condensate pipe should also have a sufficient fall to ensure that water does not run back towards the base of the duct.
Installation of flexible ducts in dwellings
Incorrect installation of ducting can result in an ineffective ventilation strategy affecting the health and convenience of occupiers or damage the building fabric.
If ducts are not fitted correctly, the required extract rates might not be achieved upon testing/ commissioning.
This may lead to costly and intrusive remedial works to correct non-compliance with Requirement F1 of the Building Regulations in England or Wales or Standard 3.14 within the Building Standards Technical Handbook – Domestic (6),that supports the Building (Scotland) Regulations 2004.
Poor installation can also result in the collection of dirt/debris, condensation and excess energy use. Therefore, it is critical that ducts for any ventilation system are in accordance with Building Regulation statutory guidance and any additional requirements of the Premier Guarantee Technical Manual.
Note: The following information is based on guidance provided in support of Building Regulations valid from June 2022 in England.
Although similar, different minimum requirements may apply to Wales and Scotland and should be reviewed following relevant guidance.
Where they pass through a cold space, flexible ducts should be factory insulated to avoid condensation. Guidance on the standards to be achieved can be found in BSRIA BG 43/2013: A guide to specification, procurement, installation and maintenance
Solid ducts should be used for all installations, and flexible ducts may be used as a final connection if necessary.
Where necessary, flexible ducts must be installed
1a. Flexible ducts should be used as a final connection only and not exceed 1500mm in length.
1b. Connections should be mechanically secured and sealed. Rigid connectors and jubilee clips are needed to ensure a good seal and prevent leaks.
1c. Adequate support is required at centres not exceeding 600mm, ensuring the duct is straight without any peaks or troughs.
1d. Flexible ducting must be pulled taught so that it is at least 90% of its maximum length
1e. Bends should be avoided. If necessary, the minimum radius to the centreline of the duct is twice the nominal diameter of the duct (Radius/Duct Diameter is greater than 2)
Additional guidance for installation in a cold roof space or void
If a duct is installed within or through a cold (unheated) space; the guidance in diagram 2 should be followed in addition to that of diagram 1:
2a. BR 262(1) states that ducts slope towards the outside to allow safe removal of condensate. If the duct is installed vertically, a condensate trap with a separate pipe to drain is required B.
2b. A suitable thickness of insulation should be provided to the duct where the dew point temperature of the surrounding air in the void is below that of the air temperature C in the ductwork. BRE Report 262 (1) recommends that material giving a thermal resistance of at least 0.6 m²K/W is used.
2c. Insulation should be factory applied and not fitted to the duct on site
Incorrect installation of flexible ducting
Diagram 3 shows some incorrect installation methods:
3a. Leaving gaps between a duct and any surrounding construction compromises the airtightness and moisture resistance. All gaps should be sealed between the duct penetrations and surrounding construction in any wall, floor or roof to ensure the air and moisture barrier is maintained.
3b. The length of the flexible duct must not exceed 1500mm and only used for short final connections.
3c. If the full diameter of the duct is not maintained, resistance to air flow is created which can impact on the performance of the ventilation system. Ensure that the ductwork does not pass through holes with a smaller diameter than the duct.
3d. Ensure that the duct is pulled taught, do not install the duct compressed. The duct should be pulled taught so that it is a minimum 90% of its maximum length D.
3e. The duct should be installed straight so that it does not sag between joists or other constructions. Ducting should be suitably supported at maximum 600mm centres
