Ductwork - New construction and the design implications of the envelope line and ceiling space

Oct 24, 2026

Upload by Patrick

Peer-reviewed technical case study examining ductwork compliance under NCC 2019 Section J for new construction, focusing on how envelope line and ceiling space design choices affect system performance. It reviews DtS pathways for component-based and whole-of-system-based fan and ductwork requirements, including pressure-drop considerations, insulation R-value thresholds, and sealing provisions aligned with referenced standards. The paper connects duct aspect ratio and insulation thickness to sizing efficiency impacts across supply and return ducts.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages4

Reading Time10 min

AccessFree to download

Introduction

Applicable NCC 2019 Clauses

DtS for fan and ductwork system (Parts J5.4)

DtS for ductwork insulation (Part J5.5)

DtS for ductwork sealing (Part J5.6)

Defining the Building Envelope

Application of Part J1 and building envelope meaning

F O R U M PEER-R EVIEWED TECH N ICAL PAPERS Section J Case Study series: Ductwork New construction and the design implications of the envelope line and ceiling space By Ken Thomson, M.AIRAH INTRODUCTION For the NCC 2019 the new fan performance requirements are based around the design of the total fan and ductwork system. The 2019 NCC Section J became mandatory on May 1, 2020, with the ductwork Deemed-to-Satisfy (DtS) provisions covered under Part J5.4 as part of the fan and ductwork system, and ductwork has additional requirements under Part J5.5 and J5.6. The code is a performance-based code, and the DtS provisions are just one way of complying with it. A Performance Solution that demonstrates compliance with the performance requirements of the code by using other methods of assessing the total duct and fan system can also be used. Determining the ductwork pressure drops is dependent on other factors in the design and construction of a building. Duct pressure drop allowances of 1Pa/m for straight duct can be exceeded due to these and other constraints. Ductwork aspect ratios can affect efficiency of the ductwork, resulting insignificant pressure losses that are not always accounted for. This opens an opportunity for ductwork component designs tobe improved and innovation to occur in the area of duct design. This case study will investigate some of the complexities in the application ofthe DtS provisions and the impacts it has on ductwork sizing and overall efficiency of the ductwork. The NCC 2019 provisions apply to additions, alterations and new construction; in this instance we are looking at new construction and the design implications of the envelope line and ceiling space. The ductwork aspect ratio and insulation thickness for each type of ductwork (supply and return) will be investigated to understand what impacts on the overall efficiency of the ductwork and hence fan and ductwork system. APPLICABLE NCC 2019 CLAUSES The applicable clauses in the NCC 2019 are for compliance assessment for the ductwork performance, based on a DtS solution using Parts J5.4, J5.5 and J5.6 . The ductwork must comply with clauses in all of these parts, and hence the following summarises the key aspects covered in this case study. To meet the performance requirements using the DtS solution in Part J5.4, there are two ways to comply: • Component-based compliance: fan efficiency complies with minimum required system static efficiency, and meets pressure-drop requirements along the index run, for the straight duct and ductwork fittings. • Whole-of-system-based compliance: fan motor power per unit offlowrate (W per L/s) of the proposed system is lower than the reference fan system W per L/s. To meet the DtS provisions for the ductwork insulation, Part J5.5 requirements are: • Ductwork and fittings in an air conditioning system must be provided with insulation – (i) Complying with AS/NZS 4859.1; and (ii) Having an insulation R-Value greater than or equal to –(A) for flexible ductwork, 1.0; or (B) for cushion boxes, that of the connecting ductwork; or (C) that specified in Table J5.5. | Location of ductwork and fittings | Climate zone 1, 2, 3, 4, 5, 6 or 7 | Climate
zone 8 | | --- | --- | --- | | Within a conditioned space | 1.2 | 2.0 | | Where exposed to direct sunlight | 3.0 | 3.0 | | All other locations | 2.0 | 3.0 | Table J5.5: Ductwork and fittings – Minimum insulation R-Value There are other clauses in Part J5.5 that have additional details and exclusions that need to be reviewed by the designer; however, these are not the focus of this case study so have been left for readers to follow up themselves. To meet the DtS provisions for the ductwork sealing, Part J5.6 Ductwork sealing requires: • Ductwork in an air conditioning system with a capacity of 3000L/s or greater, not located within the only or last room served by the system, must be sealed against air loss in accordance with the duct-s

FAQs

What does NCC 2019 Section J require for fan and ductwork systems?

It sets new fan performance requirements based on the total fan and ductwork system design, with DtS provisions available as one compliance route under Parts J5.4, J5.5, and J5.6.

How can compliance be demonstrated using the DtS solution in Part J5.4?

Compliance can be shown via component-based compliance (fan efficiency and pressure-drop requirements along the run and fittings) or whole-of-system-based compliance (lower fan motor power per unit flow rate than the reference system).

What key requirements are specified for ductwork insulation and sealing under DtS provisions?

Ductwork and fittings must be insulated with an insulation R-value meeting AS/NZS 4859.1 requirements and specified minimum R-values by duct location and climate zone. Large-capacity systems also require duct sealing against air loss according to AS 4254.1 and AS 4254.2 for system static pressure.

Oct 24, 2026

Upload by Patrick