Two different air barrier numbers, and most pages quote them interchangeably.

The preceding Article sets the purpose. Article 9.25.3.1.(1) requires assemblies separating conditioned from unconditioned space or from the ground to include an air barrier system providing a continuous barrier to air leakage — outward, to prevent excessive moisture condensation in wall, floor, attic or roof spaces during winter; and inward, to prevent room-side condensation, ensure comfort, and minimise the ingress of soil gas. NBC(AE) · 9.25.3.1.(1)

Continuity is handled in Article 9.25.3.3: panel-type joints sealed; flexible sheet joints sealed, or lapped not less than 100 mm and clamped. NBC(AE) · 9.25.3.3

The assembly criterion arrives the same way, one Article earlier: 9.36.2.9.(1)(b) references CAN/ULC-S742, “Standard for Air Barrier Assemblies – Specification,” and it is that standard’s Type A4 classification — ≤ 0.20 L/(s·m²) at 75 Pa — that a tested assembly has to reach. NBC(AE) · 9.36.2.9.(1)(b)

Both standards are published by ULC and are paywalled, which is a large part of why the two numbers circulate detached from what they measure. The clause numbers above are free to read; the standards behind them are not.

Note that route (a) is the material route on its own. Under (a) no assembly test is required at all — a continuous system built to 9.25.3 out of S741-conforming materials satisfies the Article. The 0.20 figure only becomes a requirement if a designer elects route (b) or (c). That is the second half of the conflation problem: the assembly number is frequently written into specifications as though it were mandatory, when it is one of three alternatives.

Route (c) has a location gate, and Red Deer clears it. NECB Table C-1 gives Red Deer a 1-in-50 hourly wind pressure of 0.40 kPa, comfortably below the 0.65 kPa ceiling in 9.36.2.9.(1)(c)(i). Whether the route applies to a given project is the designer’s and the authority’s call; the 0.40 kPa is code-table fact. NECB 2020 · Table C-1

Two further sentences govern how the system is built:

• The air barrier system shall be continuous across construction, control and expansion joints, across junctions between different materials and assemblies, and around penetrations through all building assemblies. NBC(AE) · 9.36.2.9.(2)
• Where the airtight material sits toward the exterior, its location and properties must conform to Subsection 9.25.5 — the outboard-to-inboard thermal resistance ratio rule. That is the clause that catches exterior foam, and it is set out on the vapour barrier page. NBC(AE) · 9.36.2.9.(6)

CAN/ULC-S705.1 makes air permeance of ≤ 0.020 L/(s·m²) a mandatory material property for medium-density closed-cell foam — so a conforming product clears the S741 material criterion by definition of its own material standard. Published product results sit an order of magnitude below it. CCMC technical criteria · CAN/ULC-S705.1

Municipal enforcement in Alberta is real and worth reading before specifying. Rocky View County’s spray foam guideline requires a CCMC listing, a certified installer and a product label at the electrical panel — and where a non-CCMC-listed product is used as an air or vapour barrier, it requires a building-envelope P.Eng. report against NBC(AE) 9.25.3 and 9.25.4. Rocky View County guideline

Commercial work is governed by a different subsection with a third scale of number. Article 3.2.4.1.(1) requires the envelope to be designed and constructed with a continuous air barrier system by complying with either 3.2.4.2 or 3.2.4.3 — two routes, not a single mandate.

Note that the assembly route in 3.2.4.3 still requires at least one material conforming to CAN/ULC-S741. The material criterion does not go away on a Part 3 building; it sits underneath the assembly number, exactly as it does in Part 9. NECB 2020 · 3.2.4.3

The Part 9 position is the same in substance. On the prescriptive path, no test is required. On the performance path, Alberta’s amended Sentence 9.36.5.10.(9) lets the modeller assume 2.5 ACH50 at a pressure exponent of 0.67 where the air barrier system is built per Subsection 9.25.3 and Articles 9.36.2.9 and 9.36.2.10 — or use a tested value. A test is needed only to claim something better than the default. STANDATA 23-BCB-001

That is worth reading as an opportunity rather than a loophole. Because the test is optional, a measured result is a real differentiator — and the code supplies the vocabulary for it. The Airtightness Levels in Table 9.36.6.4.-A give a project a code-defined badge: AL-1A at 2.5 ACH50, AL-2A at 2.0, AL-3A at 1.5, AL-4A at 1.0, AL-5A at 0.6. “This house tested at AL-3A” is a considerably stronger statement than “we sealed it well.” NBC(AE) · Table 9.36.6.4.-A

Two testing details that get conflated in Canadian content, worth stating precisely while we are here. The current test method is CAN/CGSB-149.10-2019, which supersedes the 149.10-M86 edition most web pages still cite. CAN/CGSB-149.10-2019 And CSA F326 is not an airtightness test standard — in the NBC it appears as CAN/CSA-F326-M, Residential Mechanical Ventilation Systems. Different document, different subject. NBC · CAN/CSA-F326-M (referenced documents)

• “Air barrier assemblies shall not exceed 0.02 L/(s·m²) at 75 Pa.” Wrong by a factor of ten. 0.02 is the material criterion in CAN/ULC-S741, reached through 9.36.2.10.(1). The assembly criterion is 0.20 L/(s·m²) — Type A4 under CAN/ULC-S742, per 9.36.2.9.(1)(b). Specifying the material number at the assembly level asks for a wall no tested assembly classification exists for.
• “NBC(AE) 9.25.3.2 requires air barrier materials to test at or below 0.02 L/(s·m²) at 75 Pa.” The number is right; the clause is not. Article 9.25.3.2 is performance-worded and carries no figure. Cite 9.36.2.10.(1) and CAN/ULC-S741 instead — that is the citation that survives a plan-review question.
• “A tested air barrier assembly is required.” Not on its own. Sentence 9.36.2.9.(1) offers three routes, and route (a) — a continuous system per 9.25.3 built from S741-conforming materials — requires no assembly test at all.
• “Alberta requires a blower door test on commercial buildings.” NECB 2020 offers whole-building testing as one of two routes.
• “Contractor shall be CUFCA-licensed.” There are four CCMC-recognised site quality assurance programmes — CUFCA, Caliber, BPC and UFC — and each manufacturer designates one for its product. A spec naming a single body excludes every contractor spraying a product designated to one of the other three, which is most of the Canadian market. The National Master Construction Specification was revised for exactly this reason: the previous version referenced only one of the four and created “quality assurance compliance confusion in the building industry.” The workable wording is “licensed under a CCMC-recognised site quality assurance programme (CUFCA, Caliber, BPC or UFC), as designated by the product manufacturer.” ACC / SFC, NMS Section 07 21 29
• Any clause requiring the contractor to hold a CCMC certification. The Canadian Construction Materials Centre evaluates and lists products, not contractors — so the requirement as written cannot be met by anyone. The checkable version is a CCMC-listed product with an active status, plus installer certification under the programme that product’s manufacturer designates. NRC CCMC 14230-R — a product evaluation report