Air Tightness and Draught Sealing Requirements

What This Requirement Covers

Air tightness and draught sealing requirements set minimum standards for how well a building envelope prevents uncontrolled air movement between the inside and outside. The rules aim to reduce heat loss and gain, improve energy efficiency of heating and cooling systems, control moisture movement that can cause condensation or mould, and ensure mechanical ventilation performs as designed. In practice this covers sealing gaps at junctions, penetrations, and around windows, doors, services and cavity interfaces, and verification of overall envelope leakage by pressure testing when required.

These requirements apply to designers, builders and certifiers of new and substantially altered buildings across residential and commercial classes regulated by the National Construction Code (NCC) / Building Code of Australia (BCA). Specific limits, test methods and verification paths are set out in the NCC (Volume One and Volume Two), with references to Australian Standards for test procedures such as AS/NZS ISO 9972 and AS/NZS 4284. State and territory NCC schedules may add or modify obligations for particular building classes or situations.

Key Requirements

1. Air permeability limits (building envelope): Verification of building envelope sealing is required by NCC Volume One, J1V4. The maximum permitted air permeability rates tested in accordance with Method 1 of AS/NZS ISO 9972 are:
2. Class 2 building or Class 4 part: not more than 10 m3/hr.m2 at 50 Pa reference pressure (NCC Volume One, Clause J1V4(1)(a)).
3. Class 5, 6, 8 or 9a or 9b (other than ward areas) in climate zones 1, 7 and 8: not more than 5 m3/hr.m2 at 50 Pa reference pressure (NCC Volume One, Clause J1V4(1)(b)).
4. Class 3 or Class 9c, or Class 9a ward area in climate zones 1, 3, 4, 6, 7 and 8: not more than 5 m3/hr.m2 at 50 Pa reference pressure (NCC Volume One, Clause J1V4(1)(c)).
5. Sole-occupancy units mechanical ventilation trigger: Where a sole-occupancy unit of a Class 2 or Class 4 part achieves 5 m3/hr.m2 at 50 Pa or lower, a mechanical ventilation system must be provided meeting the requirements in J1V4(2) (NCC Volume One, Clause J1V4(2)). The ventilation must be manually overridable and supply outdoor air at a minimum flow rate calculated per the formula set out in J1V4(2)(a)-(b).
6. Test methods and pressure levels:
7. Air permeability testing must follow Method 1 of AS/NZS ISO 9972 (NCC Volume One, Clause J1V4).
8. For facade and window water penetration tests and cyclic/static pressure regimes, AS/NZS 4284 procedures and the pressures stated in NCC Volume Two are referenced (for Class 1 and 10 buildings and specific wall/window tests) (NCC Volume Two, clauses referencing AS/NZS 4284).
9. Where specified in NCC Volume Two for certain façade/cavity tests, apply static pressures of 300 Pa or 30% serviceability wind pressure (whichever is higher) and cyclic pressures per specified tables (NCC Volume Two, relevant test clauses).
10. Sealing and thermal break requirements:
11. The NCC requires building sealing “to the degree necessary” to facilitate efficient energy use appropriate to building function and location (NCC Volume One, J1P1, J1P2 and NSW-specific clauses such as NSW J1P6 for Class 2 and Class 4 parts). NSW schedule explicitly requires a level of building sealing against air leakage to facilitate efficient use of energy (NCC Volume One, NSW J1P6).
12. Thermal breaks and sealing between framing and external cladding are required where necessary to achieve efficient thermal performance (NCC Volume One, Clause J1P5 and related explanatory information).
13. References to Australian Standards and test procedures:
14. AS/NZS ISO 9972 - Air permeability of buildings - Fan pressurization method (Method 1) (referenced in NCC Volume One, J1V4).
15. AS/NZS 4284 - Testing of building facades - referenced for water penetration and cyclic/static pressure regimes in NCC Volume Two (see specific test procedures in Volume Two clauses).
16. Other related standards referenced by the NCC for energy verification or systems include ANSI/ASHRAE Standard 140 (modelling verification), and standards for mechanical ventilation testing where relevant (NCC Volume One, related clauses).
17. Applicable building classes:
18. Primary prescriptive air permeability limits in NCC Volume One, J1V4 apply to Class 2 and Class 4 parts, Class 3, and various Class 5-9 categories depending on climate zone and occupancy type. (See NCC Volume One, J1V4 for the detailed mapping.)

Residential vs Commercial

1. Residential (Class 1 and Class 10): The NCC Volume Two provisions govern Class 1 (detached houses, townhouses) and Class 10 buildings. Volume Two sets facade and window test regimes (including water penetration and cyclic/static pressure tests) and references AS/NZS 4284 for glazed assemblies and curtain walls. Specific pressure sequences, static pressure magnitudes (for example 300 Pa or 30% serviceability wind pressure) and cavity wall test protocols are prescribed in Volume Two clauses for these classes.
2. Residential multi-unit (Class 2 / Class 4 parts): Air permeability verification in Volume One (J1V4) sets a target of 10 m3/hr.m2 at 50 Pa for Class 2 buildings or Class 4 parts. If a sole-occupancy unit achieves tighter performance (<= 5 m3/hr.m2 at 50 Pa), additional mechanical ventilation requirements are triggered (NCC Volume One, Clause J1V4(2)).
3. Commercial (Class 3-9): For many non-residential classes the limit is 5 m3/hr.m2 at 50 Pa for specified classes and climate zones (NCC Volume One, Clause J1V4(1)(b)-(c)). Verification and modelling routes (J1V1-J1V5) are often used for larger or complex commercial buildings; energy modelling, verification of services, and additional Specification S33C1/S33C2 requirements may apply (NCC Volume One, S33C and J1 verification clauses).

Exceptions and Exemptions

1. Climate or system-based exemptions: Some NSW schedule applications exclude buildings where the only air-conditioning is an evaporative cooler from NSW J1P6 (NCC Volume One, NSW J1P6 Applications). Similar practical exemptions may exist where parts cannot be fully enclosed or where permanent openings are required for gas appliance safety.
2. Alternative compliance paths: The NCC allows verification by reference-building modelling, Deemed-to-Satisfy (DTS) provisions, or other verification methods (J1V1-J1V5). Where envelope sealing is not practicable to meet a specific air permeability limit, designers may use modelling (reference building or annual greenhouse gas emissions pathway) to demonstrate compliance with energy performance objectives (NCC Volume One, J1V1-J1V5 and NSW J1V3).
3. Testing waivers and partial scopes: Where only certain parts of an envelope are altered, or where a deemed-to-satisfy detail provides guaranteed performance, full building leakage testing may not be required. Local certifiers may accept specific component tests or robust details in lieu of whole-building fan pressurization, subject to evidence.

State and Territory Variations

1. New South Wales: NSW schedule includes specific clauses for building sealing (NSW J1P6) and verification (NSW J1V4 and NSW J1V3). NSW J1P6 requires a level of sealing for Class 2 and Class 4 parts except in specified exclusions and sets the same verification method and numerical limits as J1V4 for NSW (NCC Volume One, NSW J1P6; NSW J1V4).
2. Other jurisdictions: The NCC is national, but each state and territory can add or modify requirements via its schedule (Schedules 4-12 in NCC Volume One). Designers should check the relevant state schedule in NCC 2022 Volume One for local modifications or additional compliance requirements. For example, some states may emphasise thermal breaks, particular sealing details for metal framed construction, or additional verification requirements for high-rise residential. Always verify against the state schedule applicable to the project.

Practical Compliance Tips

1. Seal continuity is critical: focus on continuous sealing at junctions - wall-to-window, wall-to-roof, wall-to-slab, service penetrations and around cavity barriers. Small gaps add up; use gaskets, compressible seals, tapes and sealants compatible with substrates.
2. Design for testing early: if whole-building fan pressurization (AS/NZS ISO 9972 Method 1) will be required, design and sequence construction so the envelope is complete and accessible for testing before internal finishes obstruct access to critical junctions.
3. Control penetrations: plan service routes (electrical, plumbing, ventilation ducts) to minimise the number of penetrations through the thermal and airtight envelope and use purpose-designed airtight service collars or sleeves sealed with compatible sealants.
4. Provide mechanical ventilation where needed: if airtightness targets approach or exceed the stricter thresholds (for example 5 m3/hr.m2 at 50 Pa in sole-occupancy units), include an engineered mechanical ventilation solution that meets NCC J1V4(2) requirements and provides controlled outdoor air with override controls.
5. Use tested window and door systems: specify windows/doors with documented weather and air infiltration performance (tested to AS/NZS 4284 and/or relevant product standards) and install them to the manufacturer’s airtightness flashing and sealing details.
6. Document and retain evidence: keep records of design details, product data, installation photos, and test reports (fan pressurization and component tests). Certifiers will expect clause and table references from the NCC and relevant Australian Standards in supporting documentation.
7. Check state schedule early: review the applicable state or territory schedule in NCC Volume One at project start to identify any additional sealing or verification obligations (for example NSW J1P6/J1V4) and avoid rework.

References cited inline: NCC 2022 Volume One - J1V4 and related J1P clauses; NCC 2022 Volume Two - clauses referencing AS/NZS 4284; AS/NZS ISO 9972; AS/NZS 4284; and relevant NCC Specification S33C1/S33C2 for additional energy verification details.

Could the answer be determined from the context? true