Part 3 airtightness requirements are easy to misread, because the answer to “what do we have to hit” is not a number published in the code. It is a number your own energy model committed you to, verified by a test that has to happen before occupancy, documented in a form the Authority Having Jurisdiction accepts, and signed by someone with the right professional designation. Miss any one of those four and the building does not get its permit, regardless of how airtight it actually is.
This guide sets out what the BC Energy Step Code requires of Part 3 projects, what has changed since 2023, who is responsible for which piece, and where municipalities diverge. It is written for developers, project managers, architects and Registered Professionals rather than for homeowners. If you need the procedural side of the test itself, our large building airtightness testing service page covers boundary definition, fan capacity and reporting.
What Counts as a Part 3 Building
The division is structural, not a matter of building type. Part 3 of the BC Building Code applies to buildings over three storeys or with a building footprint greater than 600 square metres. Everything smaller falls under Part 9.
In practice, Part 3 covers most of what gets built in Metro Vancouver above townhouse scale:
- Group C residential occupancies including apartment buildings, condominiums and hotels
- Group D business and personal services occupancies such as office buildings
- Group E mercantile occupancies including retail
- Institutional buildings, schools and care facilities
- Mixed-use buildings with commercial podiums under residential floors
The consequence of crossing that threshold is significant. Part 9 projects are handled by a Certified Energy Advisor working in HOT2000. Part 3 projects require a Registered Professional, whole-building energy modelling, and a compliance route with its own article number in the code.
The Step Requirement: What Changed in 2023
This is the single most common source of outdated advice on Part 3 projects, because the floor moved and older guidance did not.
Effective 1 May 2023, Step 1 in Article 10.2.3.3 of Division B was marked as reserved. At the same time, the Step 2 standard for Part 3 buildings became universally applicable across the province as the new minimum acceptable standard for compliance with the BC Building Code. Before that change, a municipality had to adopt a step by bylaw for it to apply. After it, Step 2 applies whether or not the local government has passed anything.
| Building class | Province-wide minimum since May 2023 | Code article |
|---|---|---|
| Part 9 residential | Step 3 | 9.36.6 |
| Part 3 buildings | Step 2 | 10.2.3 |
Municipalities remain free to require more. Several Lower Mainland local governments have adopted higher steps for particular occupancy types or project scales, which is why the step that applies to your project is a function of both the provincial floor and your municipality’s bylaw, and needs to be confirmed at the permit application stage rather than assumed.
The Zero Carbon Step Code sits on top of this
Since 10 March 2025, Step EL-1 of the Zero Carbon Step Code applies to all new building permit applications for Part 3 multi-family residential, commercial, office and retail projects. At EL-1 the requirement is to measure and disclose operational greenhouse gas emissions rather than to reduce them to a target, so it changes the reporting burden more than the design.
It matters here because the energy report submitted at building permit stage now has to include the data needed for that disclosure. A team that prepares its energy documentation to the pre-2025 template will have an incomplete submission.
Is Airtightness Testing Mandatory for Part 3 Buildings?
Yes, and the wording used by the authorities is unambiguous. All Part 3 buildings must demonstrate compliance with the 10.2.3 pathway under the BC Building Code, including a post-construction airtightness test. The Province requires all Step Code buildings to conduct a final airtightness test in order to receive an occupancy permit.
Two clarifications that come up repeatedly on site:
- The test is post-construction. The requirement is tied to the completed enclosure, not to an interim check. A mid-construction test is not mandated by code, which is a separate question from whether it is a good idea.
- The result feeds the model, not a standalone table. At Step 1, before it was reserved, airtightness only had to be good enough for overall building energy performance to equal the reference building. That logic carries through the higher steps: the measured value goes back into the energy model and the model demonstrates compliance.
This is why “what is a passing number for Part 3” has no general answer. The passing number is whatever your model assumed. Which leads directly to the next point.
The Target Is Set by Your Own Energy Model
On a Part 9 house you can look up 1.5 ACH50 for Step 4 and know where you stand. Part 3 does not work that way.
The airtightness value used in the energy model at permit stage becomes the value the building has to achieve at completion. If the modeller assumed an optimistic figure to make the energy targets work on paper, the construction team inherits that assumption as a hard requirement without necessarily being told.
Part 3 leakage is normalized against enclosure surface area rather than interior volume, and reported in litres per second per square metre at a 75 pascal reference pressure. The reason is geometric: dividing by volume flatters tall buildings, which have large interior volumes relative to their skin. Dividing by enclosure area measures what the enclosure actually does.
For reference, the National Energy Code of Canada for Buildings sets a maximum prescribed envelope infiltration of 1.5 L/s per square metre at 75 pascals in its prescriptive path. NECB is referenced under certain BC compliance pathways, so on some projects that figure is directly relevant and on others it is not. Confirm which applies before treating any number as your target.
Where NECB 2020 fits, and where it does not
NECB 2020 introduced whole-building airtightness testing as an option for complying with air leakage requirements, using ASTM E3158. The word doing the work in that sentence is “option”: under NECB itself, no building is compelled to undertake whole-building airtightness testing to demonstrate code compliance.
That is a national code provision, and it is frequently confused with the BC situation. In British Columbia, the Step Code pathway makes the final test a condition of occupancy. A team reading NECB guidance in isolation can reach the wrong conclusion about whether testing is mandatory on a BC project. Our explainer on ASTM E3158 covers the test method itself in detail.
Who Is Responsible for What
Part 3 compliance is a coordination problem more than a technical one, because the responsibilities split across parties who often have no contractual relationship with each other.
| Role | Responsibility |
|---|---|
| Registered Professional | Whole-building energy model, stamped and signed energy report, the airtightness assumption that becomes the target, and the compliance forms submitted to the AHJ |
| Owner and developer | Attestation letter committing the project to the required step, signed by the owner and prepared by a Registered Professional |
| Architect and envelope consultant | Air barrier design, continuity at transitions, and the detailing that determines whether the target is reachable |
| Construction team | Installation quality at interfaces and penetrations, plus building preparation for the test |
| Airtightness testing provider | Test plan, enclosure area calculation, boundary definition, calibrated testing and the measured result |
| Building official | Review of submitted documentation and, at their discretion, attendance at the test |
The point worth internalizing is that the testing provider does not replace the engineer or the energy modeller. The testing provider supplies the measured airtightness data that confirms whether the enclosure meets the target the model assumed. That data then goes back to the Registered Professional, whose reporting demonstrates compliance.
Engineering firms that produce the energy model are often not set up to perform field testing, so a third party is commonly engaged for the testing scope. That arrangement is normal and frequently more cost-effective, but it puts a coordination obligation on the project manager: the party calculating enclosure area for the model and the party calculating it for the test have to agree, because it is the denominator of the reported result.
Documentation the AHJ Expects
Requirements vary by municipality, but a Part 3 submission across the Lower Mainland generally involves the following at building permit stage:
- Energy report for the Energy and Zero Carbon Step Code for Part 3 buildings, with complete contact information and the professional section completed.
- Printed energy model report, stamped, signed and dated by a Registered Professional. The city may request the underlying model files for auditing.
- Plan drawings clearly identifying all energy efficiency upgrades beyond minimum code, and the type of air barrier.
- Attestation letter prepared by a Registered Professional and signed by the owner, committing to the required step and any applicable low-carbon energy system and benchmarking obligations.
Then, at completion:
- Airtightness test results, in the format the AHJ accepts.
- Verification of the energy efficiency upgrades that the model claimed.
A practical scheduling note: at least one Lower Mainland municipality requires applicants to notify the Building Inspections section at least 48 hours before a scheduled airtightness test, so that a building official may attend at their discretion. Build that notice period into the construction schedule rather than discovering it the week of the test.
Where Municipalities Diverge
The most expensive assumption a project team can make is that a process which worked in one municipality transfers to the next one over.
City of Vancouver
Vancouver operates under the Vancouver Building By-law rather than the BC Building Code, and publishes its own airtightness testing expectations in Building Bulletin 2023-004. That bulletin addresses test method, enclosure area calculation, pressurization and depressurization, and how results are coordinated with the energy model. Requiring both pressurization and depressurization is a meaningful difference: it changes fan planning, test duration and cost.
The rest of the Lower Mainland
Everywhere else, Part 3 projects follow the BC Energy Step Code and the applicable Part 3 compliance pathway, with NECB referenced under certain routes. Individual municipalities publish their own bulletins layering on submission requirements, notice periods and in some cases higher step requirements.
A developer with concurrent projects in Vancouver, Burnaby and Surrey is working to three overlapping sets of expectations. The airtightness target may be similar; the documentation, the test protocol and the timing of the notice are not.
Why the Timing of Engagement Decides the Outcome
By the time a Part 3 building is finished, every decision that affects its airtightness result has already been made. The test at the end measures consequences, not causes.
| Stage | Still influenceable |
|---|---|
| Design and specification | Target selection, test standard, enclosure boundary, whether the design is testable at all |
| Air barrier installation | Detailing at transitions, penetrations and interfaces, before they are covered |
| Mid-construction | An optional interim test that finds defects while they are still cheap to reach |
| Substantial completion | Preparation quality and scheduling only |
| Post-completion | Remediation cost and retest scheduling |
Two items on that list are worth pulling out.
Testability is a design decision. A building with a parkade connected to the residential floors through stairs, elevator shafts and vestibules has to have those connections isolated deliberately during the test. If nobody thought about it at design stage, the isolation becomes an improvised and expensive site problem on the test day.
Enclosure area has to be agreed early. Different interpretations of below-grade surfaces, shared boundaries, roofs and parkade interfaces change the normalized result without anything about the building changing. Settling this after the test is how disputes start. If the engineer cannot supply building volume and enclosure area figures, the testing provider will calculate them from the architectural set, and both parties should be working from the same numbers.
Common Misreadings of the Part 3 Requirements
- Our municipality has not adopted a step, so nothing applies.
Step 2 for Part 3 became province-wide in May 2023 without needing local adoption. - The energy model demonstrates compliance, so the test is a formality.
The occupancy permit depends on the measured result, and the model is only valid if the building matches its assumptions. - We can convert our ACH50 result to the Part 3 metric.
Not without both the interior volume and the enclosure area of that specific building, and even then the two are not interchangeable as compliance figures. - NECB says whole-building testing is optional.
True of NECB itself. Not true of the BC Step Code occupancy requirement. - Passing means the enclosure has no problems.
A building can meet its overall leakage target while containing a concentrated leak at a roof transition or curtain wall interface that matters for moisture and comfort.
Getting a Part 3 Project Through Compliance
The projects that pass without drama share one characteristic: the airtightness target, the test standard, the enclosure boundary and the reporting format were settled in writing before construction rather than negotiated after the fans arrived.
Monolith Housing Solutions performs whole-building, multizone and guarded airtightness testing for Part 3 projects across Vancouver and the Lower Mainland, and works with project teams from drawing review onward rather than at substantial completion. Send the architectural set and the airtightness section of your specification and you get a scoped proposal rather than a generic quote.
Frequently Asked Questions
Is airtightness testing mandatory for Part 3 buildings in BC?
Yes. All Part 3 buildings must demonstrate compliance with the 10.2.3 pathway of the BC Building Code, and that includes a post-construction airtightness test. The Province requires a final airtightness test on Step Code buildings in order to issue an occupancy permit.
What step applies to a Part 3 building in British Columbia?
Step 2 is the province-wide minimum and has been since 1 May 2023, when Step 1 was marked reserved in Article 10.2.3.3. Individual municipalities may require a higher step for particular occupancies or project types, so confirm the local bylaw at permit application.
What is the airtightness target for a Part 3 building?
There is no single published figure. The target is the airtightness value assumed in the energy model submitted for the project, expressed in litres per second per square metre of enclosure area at 75 pascals. Confirm it in writing before testing so there is no ambiguity about what passing means.
Can ACH50 be used for a Part 3 building?
No. ACH50 normalizes leakage by interior volume, which distorts comparisons between buildings of different shapes and flatters tall buildings. Part 3 requirements normalize by enclosure surface area instead, and the two metrics cannot be converted without knowing both the volume and the enclosure area of the specific building.
Does the Zero Carbon Step Code affect Part 3 airtightness?
Not directly. Step EL-1 has applied to new Part 3 multi-family residential, commercial, office and retail permit applications since 10 March 2025, and requires operational greenhouse gas emissions to be measured and disclosed. It changes what the energy report must contain rather than the airtightness target itself.
Who is allowed to perform Part 3 airtightness testing?
The energy model and compliance reporting for a Part 3 building require a Registered Professional. Field airtightness testing is commonly performed by a third-party testing provider, which is often more practical because engineering firms producing the model frequently do not offer testing. Where a signed or sealed report is required, confirm reporting responsibility in writing before testing begins.
Is a mid-construction test required on a Part 3 building?
No, the code requirement is a post-construction test. An interim test is nonetheless the difference between sealing an exposed transition detail and opening finished assemblies to reach it, which is why many teams choose to run one while the air barrier is still accessible.
What happens if a Part 3 building fails its airtightness target?
The shortfall is documented, leakage sites are identified where diagnostics are in scope, and the team seals and retests. Because the requirement is tied to the occupancy permit, discovering the shortfall at completion compresses the remediation into the most expensive possible window.
