What changed in BASIX ventilation rules in 2026?
From 1 May 2026, the BASIX tool (version 4.04) removed the option to direct bathroom, kitchen, and laundry exhaust systems into the roof space. All exhaust systems must now discharge directly to the outside of the building. This change aligns BASIX with the NCC 2022 ventilation provisions (Part F8D4), which already require exhaust systems to discharge to outdoor air rather than into roof cavities. For NSW builders and designers, this means that any BASIX certificate generated on or after 1 May 2026 must specify externally ducted exhaust systems — the previous roof-space discharge workaround is no longer accepted by the tool.
This is not a minor administrative tweak. Roof-space exhaust discharge has been a common cost-saving practice in NSW residential construction for decades, particularly in project homes and renovations where running ducts to external vents adds time and expense. The removal of this option forces a design-level change: every new home and major renovation that requires a BASIX certificate must now include properly ducted exhaust systems routed to external grilles, wall vents, or soffit outlets.
Why was exhaust-to-roof-space removed?
The removal addresses three interconnected problems that have become increasingly significant as NSW homes are built to higher thermal performance and airtightness standards.
Condensation and mould risk
When warm, moisture-laden air from bathrooms, kitchens, and laundries is discharged into a roof space, the moisture condenses on cold surfaces — particularly in cooler months and in NSW's climate zones 6, 7, and 8 (which include the Sydney metropolitan area, the Blue Mountains, and the Southern Highlands). Condensation accumulation in roof spaces leads to timber rot, mould growth on ceiling linings and structural members, and degraded insulation performance. The NCC 2022 condensation management provisions (Part F8D5) explicitly require roof spaces in climate zones 6, 7, and 8 to be provided with ventilation openings to mitigate this risk. BASIX has now caught up by removing the source of the moisture at the point of entry.
Conflicts with 7-star thermal performance
Since 1 October 2023, BASIX requires 7-star NatHERS thermal performance for most new homes. Higher-performance building envelopes are more airtight, which means less natural air leakage to dilute moisture. Discharging humid exhaust air into a sealed roof space — above an airtight, well-insulated ceiling — creates a concentrated moisture load that the roof space ventilation alone may not adequately manage. The BASIX tool upgrade recognises that the 7-star standard and roof-space exhaust discharge are fundamentally incompatible.
Alignment with NCC 2022 Part F8D4
The NCC 2022 Deemed-to-Satisfy provisions for exhaust systems already state that kitchen, bathroom, and laundry exhaust systems must be discharged to the outside of a building. BASIX previously offered a pathway that was less stringent than the NCC, creating a compliance inconsistency. The v4.04 update resolves this by bringing BASIX into alignment with the national code.
What are the new exhaust requirements for NSW homes?
For bathrooms and sanitary compartments without sufficient access to outdoor air (i.e., no window), the updated requirements also mandate that exhaust fans must be controlled to turn on when lights are turned on and continue operating for at least 10 minutes after lights are turned off. Additionally, make-up air must be provided from an adjacent room — typically achieved through an undercut in a separating door.
How does this affect BASIX energy scores?
Externally ducted exhaust systems consume more energy than roof-space discharge because they require longer duct runs, which increases static pressure on the fan. The BASIX energy calculation for ventilation uses the following formula:
Ventilation Energy (kWh/day) = Qmin × Pstatic × Top / ε
Where Pstatic (static pressure) is 70 Pa for ducted systems versus 20 Pa for non-ducted systems. This means a ducted kitchen exhaust (50 L/s at 70 Pa) will consume approximately 3.5 times more fan energy than the same fan discharging directly into a roof space at 20 Pa.
However, the energy impact is relatively small in the context of whole-home energy consumption. For a typical 3-bedroom home with three exhaust fans (kitchen, bathroom, laundry), the additional energy from ducted systems amounts to approximately 0.3–0.5 kWh per day — roughly $30–$50 per year at current NSW electricity rates. This is offset by the elimination of condensation-related damage, which the CSIRO estimates costs Australian homeowners an average of $2,000–$5,000 per incident in mould remediation and structural repairs.
What does this mean for different stakeholders?
For homeowners planning renovations
If you are planning a renovation that requires a new BASIX certificate (alterations and additions valued at $50,000 or more), your builder or designer must now specify externally ducted exhaust systems for all kitchen, bathroom, and laundry ventilation. This adds approximately $200–$500 per exhaust point for ducting, external grilles, and installation labour. If your existing exhaust fans discharge into the roof space and you are only replacing like-for-like (no new BASIX certificate required), you are not obligated to upgrade — but doing so voluntarily will reduce condensation risk and protect your ceiling linings.
For builders and certifiers
Every BASIX certificate generated from 1 May 2026 will show externally ducted exhaust selections. Certifiers should verify during construction that:
1. All exhaust fans are ducted to external grilles (not terminating in the roof space)
2. Duct runs are as short and direct as possible to minimise static pressure losses
3. Bathroom exhaust fans in rooms without windows are interlocked with lighting controls
4. Make-up air pathways (door undercuts, transfer grilles) are provided where required
Builders should update their standard exhaust installation specifications and pricing schedules to include external ducting as a default, rather than treating it as an optional upgrade.
For designers and architects
The removal of roof-space discharge affects roof design and spatial planning. Designers need to accommodate:
- Duct paths from wet areas to external walls or eaves (minimum duct length preferred)
- External grille locations that do not conflict with aesthetic requirements or bushfire ember attack zones
- Raked ceilings or flat ceilings where roof-space ducting was previously the only option — alternative routing through bulkheads or service cavities may be needed
- For Passive House projects, the MVHR system already handles exhaust externally, so this change has minimal impact on Passive House-certified designs
How does this interact with Passive House and MVHR systems?
Passive House-certified homes in NSW already meet and exceed the new exhaust requirements because Mechanical Ventilation with Heat Recovery (MVHR) systems discharge all exhaust air to the exterior through a central duct with a heat exchanger. The BASIX Passive House Standard method (available since 2020, with expanded eligibility following the May 2026 tool upgrades) inherently satisfies the external discharge requirement.
For builders considering the Passive House pathway for BASIX compliance, the ventilation change is a net positive — it removes a competitive cost advantage that roof-space discharge previously held over properly ducted systems. The playing field is now more level, and the long-term benefits of MVHR (70% heat recovery, filtered supply air, no condensation) are easier to justify when the alternative is no longer simply "dump it in the roof."
What about existing projects with in-progress BASIX certificates?
The BASIX tool update (v4.04) applies to new certificates generated on or after 1 May 2026. Projects with certificates already in progress before this date are not affected — the tool retains the previous version's settings for in-progress projects. However, if you need to revise or regenerate a certificate after 1 May 2026, the new ventilation requirements will apply. The BASIX standards version is displayed on the first page of the certificate for verification.
For projects with a valid BASIX certificate submitted with a DA or CDC application before 1 May 2026, there is no need to revise. But if the design changes after submission and a new certificate is required, the v4.04 rules — including the exhaust-to-outside requirement — will apply to the new certificate.
Practical implementation guide for NSW builders
Ducting specifications
Common mistakes to avoid
1. Using flexible duct exclusively — Flexible duct has 2–3× the pressure drop of rigid duct, significantly reducing fan effectiveness and increasing energy consumption.
2. Excessive duct length — Every metre of duct adds static pressure. Plan wet-area layouts to minimise the distance to external walls.
3. No backdraft damper — Without a backdraft damper, external air enters the duct in windy conditions, creating drafts and energy losses.
4. Discharging into eaves in bushfire zones — In BAL-40 and BAL-FZ areas, ember-proof grilles are required for all external openings, including exhaust discharge points.
What other BASIX tool changes came with v4.04?
The May 2026 update included several additional changes alongside the ventilation removal:
- Data retention when switching thermal performance methods — Users no longer lose entered data when toggling between Simulation and DIY methods
- Expanded glazing options — New low solar heat gain coefficient (SHGC) glazing values are available for high-performance windows
- Low-emissions concrete grades — Users can now select different grades of concrete when low-emissions concrete is selected as the internal wall type
- Multi-dwelling capacity — Up to 1,000 dwellings can now be included on a single BASIX certificate (previously limited)
- Green Square Town Centre — Postcode 2015 now has a specific option for reticulated alternative water supply
- Straw bale panel counting — Certificates now show the number of straw bale panels where applicable
- Declaration requirement — All users must now declare that entered information is accurate before generating a certificate
Looking ahead: NCC 2025 adoption in May 2027
NSW will adopt NCC 2025 on 1 May 2027, with a 12-month transition period. While the residential energy efficiency changes in NCC 2025 are relatively limited (the major changes target commercial buildings), the ventilation and condensation provisions are expected to be further refined. Builders and designers should treat the BASIX v4.04 exhaust-to-outside requirement as a preview of the tighter ventilation standards coming with NCC 2025 adoption. Planning exhaust systems to external discharge now will reduce the need for retrofits when the new code takes effect.
FAQ
Can I still use roof-space discharge if my BASIX certificate was generated before 1 May 2026?
Yes. Certificates generated before 1 May 2026 are valid under the previous tool version. The v4.04 changes only apply to new certificates generated on or after that date. However, if you need to revise your certificate for any reason after 1 May 2026, the new requirements will apply to the revised certificate.
Does the exhaust-to-outside rule apply to renovations and additions?
Yes, if the renovation requires a new BASIX certificate. Alterations and additions valued at $50,000 or more require a BASIX certificate, and any certificate generated after 1 May 2026 must comply with the v4.04 ventilation requirements. Like-for-like exhaust fan replacements without a new BASIX certificate do not trigger the requirement.
How much does external ducting add to construction costs?
External ducting adds approximately $200–$500 per exhaust point, depending on the distance to the external wall, duct type (rigid vs flexible), and grille type. For a typical 3-bedroom home with three exhaust points, the total additional cost is approximately $600–$1,500. This is offset by reduced condensation damage risk, which averages $2,000–$5,000 per incident.
Does this affect Passive House or MVHR systems?
No. Passive House-certified homes already use MVHR systems that discharge exhaust air to the exterior through a central duct with heat recovery. The BASIX Passive House Standard method inherently satisfies the external discharge requirement. Builders using the Passive House pathway will see no change in their BASIX process.
What happens if I am in a bushfire zone (BAL-40 or BAL-FZ)?
In BAL-40 and BAL-FZ areas, all external openings — including exhaust discharge grilles — must be protected with ember-proof grilles compliant with AS 3959:2018. This adds approximately $50–$150 per grille. Standard weatherproof grilles are acceptable in lower BAL ratings.
Are there exemptions for climate zones 9, 10, and 11?
No. The exhaust-to-outside requirement applies to all BASIX certificates regardless of climate zone. The climate zone exemptions that exist for thermal performance and energy standards (zones 9, 10, 11 in north-eastern NSW) do not extend to the ventilation provisions. All new homes across NSW must comply with the external discharge requirement.
Will NCC 2025 adoption in May 2027 change these requirements further?
NSW will adopt NCC 2025 on 1 May 2027. While the residential ventilation provisions are not expected to change dramatically, the code will introduce additional condensation management requirements for roof spaces. Builders who already comply with the BASIX v4.04 exhaust-to-outside rule will be well-positioned for NCC 2025 compliance.
This article was prepared by Franz Phan, Senior Building Designer at giantA Pty Ltd, with 15+ years of experience in NSW residential design and BASIX compliance. giantA provides integrated building design, drafting, and approval services across Sydney and NSW. For project-specific BASIX advice, [book a free consultation](/appointment/58?invite_token=a93bda8bcf0341c5bfb0fa606cf84568&filter_appointment_type_ids=[58]&filter_staff_user_ids=[6]).
Sources:
- NSW Planning Portal — BASIX Tool Upgrades Now Live (5 May 2026): https://www.planningportal.nsw.gov.au/news/basix-tool-upgrades-now-live
- NSW Planning Portal — Ventilation and Exhaust (updated 25 February 2026): https://www.planningportal.nsw.gov.au/basix-energy/design-principles/ventilation-and-exhaust
- NSW Planning Portal — Passive House Standard Method (updated 9 March 2026): https://www.planningportal.nsw.gov.au/basix/thermal-performance/passive-house-standard-method
- NSW Government — NCC 2025 Adoption May 2027: https://www.nsw.gov.au/ministerial-releases/nsw-to-adopt-new-national-construction-code-may-2027
- NSW Planning Portal — BASIX Newsletter December 2025: https://www.planningportal.nsw.gov.au/news/basix-newsletter-december-2025