How to comply with the heat transfer requirements of insulated ductwork
In this blog you’ll learn about the standards prescribed for insulating ductwork, which is installed into new and existing non-domestic buildings.
The ductwork system efficiency claimed should be based on the appropriate method of calculation, as detailed in the 2013 edition of supporting document the ‘Non-domestic Building Services Compliance Guide’ (NDBSCG), published by DCLG. The NDBSCG is the supporting document of both Approved Documents ADL2A and ADL2B. It is important to check that the thermal conductivity of the ductwork insulation you are intending to use is calculated in accordance with BS EN ISO 12241: 2009 (Thermal insulation for building equipment and industrial installations), rather than a test method which applies only to insulation suitable for walls, floors and roofs for example. As a result, insulation that has not been calculated to this standard may be non-conformant.
Section 11 of the NDBSCG details the maximum heat transfer requirements for insulated heating, cooling and dual purpose ducts in new and existing buildings. For heating ductwork the maximum permissible heat transfer is 16.34 W/m2 and for cooling and dual purpose ductwork the maximum permissible heat transfer is -6.45 W/m2.
The maximum permissible heat transfer requirements can be met through using a specific insulation thickness, the correct thickness is determined by the thermal conductivity of the insulation.
|Thickness of insulation for heated ductwork*|
|Thermal Conductivity at 25oC (W/m.K)||0.020||0.023||0.025||0.030||0.035||0.040||0.045||0.050||Max permissable heat loss (W/m2)|
|Thickness Required (mm)||17||20||21||25||29||33||38||42||16.34|
|Thickness of insulation for chilled / dual purpose ductwork**|
|Thermal Conductivity at 19oC (W/m.K)||0.020||0.0216||0.025||0.030||0.035||0.040||0.045||0.050||Max Permissible heat gain (W/m2)|
|Thickness Required (mm)||29||30||36||43||50||57||64||71||-6.45|
For clarity, 0.0216 W/m.K is the thermal conductivity of Kingspan KoolDuct panels. The thermal conductivity for an insulation material may change subject to the different temperature at which it is being tested. For example, for heating ductwork you are required to produce a thermal conductivity where the mean temperature of insulation is 250C, and for cooling and dual purpose ductwork the temperature should be 190C. It is vital that the thermal conductivity that you use to work out the insulation thickness is calculated correctly. The correct thermal conductivity can be found in a product’s Declaration of Performance. Please visit www.kingspaninsulation.co.uk/dop for details on Kingspan’s duct insulation products and pre-insulated ductwork.
Fortunately, it is relatively clear how to comply with the heat transfer requirements for insulated ductwork. Using the above tables and knowing the correct thermal conductivity of the insulation you are intending to use, you can easily work out the thickness of the insulation required for the specific type of ductwork you are insulating. Lastly, the ductwork should be insulated along its whole length to ensure the maximum permissible heat transfer requirement is met throughout the whole of the system.
* Thickness of insulation should be calculated according to BS EN ISO 12241 using the following standardised assumptions 35oC, with 60 mm vertical sidewall in still air 15oC.
*** Thickness of insulation should be calculated according to BS EN ISO 12241 using the following standardised assumptions 13oC, with 60 mm vertical sidewall in still air 25oC.
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