On teaching a course for Passive House Canada for Building Officials last week and today, I was re-introduced to the numerous disparate adoptions of the NECB and NBC model National Codes by the provinces. I have never yet in my career (30yrs) seen such broad disparity. Ontario, my jurisdiction, is clinging hard to the actually pretty good SB12 (Part 9) and SB10 (Part 3) standards for thermal and energy performance. I do a lot of Part 9 work (small buildings) and a few Part 3 buildings every year, and have developed an efficient process for submissions for each, where I create a dedicated page for Building Officials where we clearly walk through the approach chosen (typically prescriptive packages) and then how our model exceeds these packages. Easier than doing a full performance path and as we’re never taking short-cuts (ie. lowering one metric while increasing another aka. ‘trade-offs’), this is the simplest approach to design and review. I attach a sample here you can check out.
Now I should mention many Architects don’t actually do energy modelling, and that’s OK. In those instances EA’s or Engineers can do the work for them, but we’re allowed to do this work, provided we self qualify. And we should be proficient at this before we start offering services to the public. There’s an article over here that delineates who can do what with respect to Energy Modelling to Code requirements: https://openbuilding.ca/2020/09/10/architects-as-energy-experts/
As you can see, we provide the thermal zone map for Ontario (two basic zones for Ontario for Part 9 buildings), and show how that informs the selection of the appropriate table (yes we use little read threads to connect relevant information like a crime map!), then we highlight the relevant prescriptive table column we are using, and we then show how we may be going ABOVE these code-required minimums. Then we often provide dynamic hourly analysis summary pages (from ArchiCad EcoDesigner in this instance) as well as a simplified stamp showing how OBJECTIVE matches up with these values. The EEDS or Energy Efficiency Design Summary form expected by most plans examiners and reviewing Building Officials carries our Architect (or EA/BCIN) license number, and matches the design data. In our case we also show the targeted ACH50 number of 0.6ACH@50Pa in this instance, as a design Target, but not as a requirement. This is an important distinction. Targeted ACH values and blower door test results DO NOT need to be reviewed when the prescriptive table is complied with. This is spelled out in the PDF Cheat Sheet I made for the Building Officials in our PHC course today. (Note: It has NOT yet been peer reviewed, but contains relevant code citations and has been verified by yours truly).
So: is mandatory airtightness testing in Ontario required? No, not under the Prescriptive path. Under the Performance Path, absolutely, yes it is, the data must be modelled (typically in Hot2000) and then validated against a field test, and reviewed by the AHJ/Municipal building official.
What about Part 3 Buildings?
I’m so glad you asked! Here is a second Cheat sheet with the SB12 and SB10 requirements baked-in.
And for good measure here is a broader treatment of the subject:
Again, these have not yet been peer reviewed, and so cannot (yet) be taken as any kind of legal or practice advice – you are on your own to fact check as is good professional practice in any case, but this at least highlights the relevant sections and requirements.
Now we can dive in to why the ACH50 metric applied to larger buildings (Part 3) is complicated at best. 0.6ACH@50Pa might be a fantastic number for a PHPP/PHI certified house, but a lousy measure of airtightness on a warehouse – here’s why, in a nutshell:
Why ACH50 applied to Part 3 Buildings is a bad idea.
ELA formula: Q = Cd · A · √(2ΔP/ρ) → ELA = Q(m³/s) ÷ 5.578
NLR formula: NLR (L/s·m²) = Q(L/s) ÷ envelope surface area
ACH (Air Changes per Hour at some pressure) divides the measured air flow through the envelope by the building’s volume. For a house, that’s an entirely reasonable thing to do. The problem is that as buildings get larger, volume grows much faster than exterior surface area, and the envelope surface is the thing the air actually leaks through. A compact two-storey house has a surface-area-to-volume (SA:V) ratio of roughly 0.77 m²/m³. A single-storey warehouse has about 0.28. What about a six-storey office tower? Around 0.15, or roughly one-fifth of the house. This geometric divergence means the same ACH number represents completely different envelope quality depending on what you’re examining.
The table below works through four representative building types assuming square footprints and standard ceiling heights (house 2.7m/storey, small commercial 4.5m, warehouse 8m, office 4.0m floor-to-floor). For each one, it calculates what 1 ACH @ 50 Pa actually means in air flow, in Equivalent Leakage Area (ELA, the hypothetical hole that produces the same flow under the orifice equation), and in leakage intensity per square metre of built floor area. Then it asks: what ACH would each building actually need to achieve to meet NECB 2020’s surface-normalized whole-building limit of 1.50 L/(s·m²) @ 75 Pa?
| Building Type: | House | Small Commercial | Warehouse | 6-Storey Office |
|---|---|---|---|---|
| GFA | 200 m² | 2,000 m² | 20,000 m² | 20,000 m² |
| Storeys | 2 | 1 | 1 | 6 |
| Footprint | 10 × 10 m | 44.7 × 44.7 m | 141.4 × 141.4 m | 57.7 × 57.7 m |
| Volume | 540 m³ | 9,000 m³ | 160,000 m³ | 80,000 m³ |
| Envelope area | 416 m² | 4,805 m² | 44,525 m² | 12,205 m² |
| SA:V ratio | 0.77 m²/m³ | 0.53 m²/m³ | 0.28 m²/m³ | 0.15 m²/m³ |
| Q at 1 ACH | 150 L/s | 2,500 L/s | 44,444 L/s | 22,222 L/s |
| ELA at 1 ACH | 0.027 m² | 0.448 m² | 7.97 m² | 3.98 m² |
| ELA per m² GFA | 1.34 cm²/m² | 2.24 cm²/m² | 3.99 cm²/m² | 1.99 cm²/m² |
| NLR at 1 ACH | 0.36 L/(s·m²) | 0.52 L/(s·m²) | 1.00 L/(s·m²) | 1.82 L/(s·m²) |
| Equiv. ACH to meet NECB | ~3.1 ACH | ~2.2 ACH | ~1.1 ACH | ~0.62 ACH |
Look at the ELA row. At 1 ACH, the warehouse has a hole in it roughly the size of a standard door. The house has a gap about the size of an A4 sheet of paper. Same metric. Same number. Completely different physical reality. The normalized leakage rate (NLR) which divides flow by envelope surface area rather than volume makes this visible: 0.36 L/(s·m²) for the house, 1.82 for the office tower. NECB’s pressure-corrected limit at 50 Pa is approximately 1.13 L/(s·m²). The six-storey office tower at 1 ACH already fails the NECB normalized standard. The house passes it with room to spare.
The last row is the kicker. To actually meet NECB’s surface-normalized limit, the office tower would need to hit approximately 0.62 ACH — which is almost exactly the Passive House Institute’s residential airtightness target. Achieving that in a house requires a level of trade coordination, detailing, and site discipline that most builders describe as demanding. The question that should follow immediately is:
Is it actually harder to achieve 0.6 ACH in a large building than in a house?
Counterintuitively — no. And here’s why that matters.
| Building Type: | House | 6-Storey Office |
|---|---|---|
| Volume | 540 m³ | 80,000 m³ |
| Q at 0.6 ACH | 90 L/s | 13,333 L/s |
| Envelope area | 416 m² | 12,205 m² |
| NLR at 0.6 ACH | 0.22 L/(s·m²) | 1.09 L/(s·m²) |
To achieve 0.6 ACH, the house must reach 0.22 L/(s·m²) — five times tighter per square metre of actual envelope than the office tower needs. Every window buck, every electrical penetration, every rim joist has to be executed nearly perfectly. The office tower at 0.6 ACH only needs to hit 1.09 L/(s·m²) of NLR which modern tested curtain wall systems and professional trade coordination can achieve without heroics. So when PHI certifies a Passive House commercial building at 0.6 ACH and says it meets the same standard as a PH certified house, they’re not comparing equivalent levels of effort to detail the envelope, the bigger building gets a free pass based solely on geometry, not necessarily rigorous attention to air barrier detailing.
Three takeaways:
1. The same ACH number is not the same standard. It looks equivalent on paper but demands dramatically different envelope quality depending on building form. Applying 0.6 ACH uniformly across building types quietly lets large buildings off the hook while punishing small tight ones. (obvious to anyone that has done a door-fan test on a tiny home, sure, smallest ring, but also still often lousy numbers).
2. The absolute leakage consequence is much worse at scale. It’s geometrically easier for the large building to achieve 0.6 ACH but when it fails, the ELA is measured in square metres, not centimetres!. That 13,333 L/s of infiltration at 0.6 ACH in the office tower represents a continuous heating load that would make your energy model weep. The scale of consequence demands a normalized metric even when the normalized number looks good.
3. The test pressure means something different for tall buildings. A house at 50 Pa is being depressurized to roughly five times its actual winter stack effect — about 10 Pa for a 5.4m building in an Ontario winter. It’s a deliberate stress test well above real operating conditions. A six-storey building in the same winter conditions experiences roughly 46 Pa of stack pressure just from thermal buoyancy — essentially equivalent to test conditions, without a blower door in sight. The envelope on a tall building is living the test every cold day. That’s another reason why a surface-normalized metric, calibrated to envelope area rather than volume, is a more physically honest representation of what the building is actually managing.
ACH@50 Pa is a useful, practical metric for houses because the geometry, construction method, relationship between test pressure and real operating conditions, and scale of consequence all line up reasonably well for that building type. Apply it to large buildings and every single one of those factors breaks down simultaneously. It’s not that ACH is a bad metric. It’s that it was never designed to travel – and we’ve been quietly asking it to for years.
by: Andy Thomson, M.Arch, OAA. April 24, 2026 with peer review & comments by Stephen Pope, BES, B.Arch, OAA, FRAIC.
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