You have the report in front of you. It has a number on it, possibly several, and nobody has told you whether that number is something to celebrate or something to fix. This is the most common moment of confusion in the whole process, and it is entirely reasonable: good blower door test results depend on the age of the building, the code path it is following, and which of four or five different metrics you happen to be looking at. This guide takes each number on a typical report, explains what it measures, and places it against the targets that actually apply in British Columbia, using a real project checklist you can download and read alongside the explanation. If you want a refresher on how the measurement itself is taken, our overview of blower door testing covers the procedure.
The Short Answer: What Counts as a Good Result
For a newly built home in British Columbia, a result at or below 2.5 ACH50 meets the current province-wide minimum, and anything at or below 1.5 ACH50 is a strong result by any standard. For an existing home, the honest benchmark is different, because the question is not whether you passed a code target but how much room you have to improve.
|
Result |
New construction | Existing home |
|---|---|---|
| Below 1.5 ACH50 | Excellent, high-performance territory | Exceptional, rarely seen without a deep retrofit |
| 1.5 to 2.5 ACH50 | Compliant and solid | Very good, above average for any vintage |
| 2.5 to 4.0 ACH50 | Falls short of current BC requirements | Reasonable, with worthwhile gains still available |
| Above 4.0 ACH50 | Requires remediation |
Typical of older stock, significant savings available |
Everything below explains how to read your paperwork well enough to know which row you are in and why.
Download a Real BC Compliance Checklist
Before going through the metrics one at a time, it helps to see the document they land on. The file below is a BC Step Code Compliance Checklist from a Monolith project: a duplex in North Vancouver targeting Step 4 of the Energy Step Code and EL-3 of the Zero Carbon Step Code. Client identifiers have been removed, and everything technical is exactly as submitted.
[DOWNLOAD PDF: Sample BC Step Code Compliance Checklist]
Two things are worth knowing before you open it.
First, this is the Pre-Construction checklist, which is the design-stage document. The airtightness figures on it are targets that the energy model assumes, not measurements. There are three checklists in the BC process, and they serve different purposes:
|
Checklist |
Stage | Airtightness figure represents |
|---|---|---|
| Pre-Construction | Building permit application | The target assumed in the energy model |
| Mid-Construction | Air barrier complete, before finishes | A measured result while correction is still cheap |
| As-Built | Occupancy permit |
The final measured result submitted to the AHJ |
Second, the airtightness section of this particular checklist illustrates something most guides never mention, and it is worth walking through in full. That comes after the metrics are defined.
Reading Your Blower Door Test Results: What Each Number Means
A blower door report usually carries four or five figures derived from the same underlying measurement. They are not alternative opinions about your building. Each one normalizes the raw leakage in a different way, and each answers a different question. Confusion almost always comes from comparing two buildings using the wrong one.
CFM50, the Raw Measurement
CFM50 is the volume of air, in cubic feet per minute, that the fan must move to hold the building at 50 pascals of pressure difference. It is the direct output of the test and everything else is calculated from it. The critical thing to understand is that CFM50 on its own says nothing about construction quality, because a large building will always have a higher CFM50 than a small one built to the identical standard.
Consider two homes that both test at 1,200 CFM50. The first has a conditioned volume of 12,000 cubic feet. The second has 28,800 cubic feet. The raw number is identical, but the first home is leaking at 6.0 ACH50 and the second at 2.5 ACH50. One needs serious work and the other meets current BC requirements. This is exactly why a contractor quoting a CFM50 figure without context is not giving you usable information.
ACH50, the Normalized Metric
ACH50 stands for air changes per hour at 50 pascals. It expresses how many times the entire volume of air inside the building would be replaced in one hour if the 50 pascal pressure difference were sustained. Because it divides leakage by volume, it lets you compare buildings of different sizes on a level footing, which is why codes and certification programs are written around it.
The calculation is straightforward:
ACH50 = (CFM50 × 60) ÷ conditioned volume in cubic feet
Worked through with real figures: a 2,400 square foot home with nine foot ceilings has a conditioned volume of roughly 21,600 cubic feet. If the fan reads 900 CFM50, then 900 multiplied by 60 gives 54,000 cubic feet per hour, divided by 21,600 gives 2.5 ACH50. That home meets Step 3.
One caveat worth knowing: the volume figure matters as much as the fan reading. If the technician measures conditioned volume generously, including spaces that are not really inside the pressure boundary, the ACH50 result improves without the building getting any tighter. A credible report states clearly how the volume was determined.
NLR50 and NLA10, the Metrics BC Actually Uses
ACH50 has a known weakness. It normalizes by volume, but air leaks through surfaces, not through volume. A compact three storey home and a sprawling single storey home of the same volume have very different amounts of exterior surface, and the sprawling one is penalized for a geometry decision rather than a workmanship one. Small homes are penalized hardest of all.
The BC Energy Step Code addresses this by defining airtightness levels, labelled AL-1 through AL-4, that specify not only an ACH50 value but also a Normalized Leakage Rate at 50 pascals (NLR50) and a Normalized Leakage Area at 10 pascals (NLA10). Both of these divide leakage by the surface area of the building enclosure rather than by its volume.
- NLR50, reported in L/s per square metre, expresses leakage as a flow rate per unit of enclosure area. This is the fairer comparison between buildings of different shapes.
- NLA10, reported in square centimetres per square metre, expresses leakage as an equivalent open area per unit of enclosure area, referenced to a lower and more realistic 10 pascal pressure.
These are not competing opinions. They are the same leakage divided by different denominators, which is why one number can move you across a threshold while another does not.
ELA, Putting the Number in Physical Terms
Equivalent Leakage Area translates the whole result into a single figure: if every crack, gap and penetration in the building were combined into one clean opening, how big would that opening be? Your report gives this in square inches or square centimetres.
This is the metric to reach for when explaining a result to someone who does not work in building science. Telling a homeowner their house is at 6 ACH50 rarely lands. Telling them the leakage adds up to the equivalent of leaving a window open all winter usually does, and it makes the case for air sealing without any technical argument at all.
A Worked Example From the Sample Checklist
Open the downloaded checklist to Section F and you will find the airtightness block. Here is what it says for that North Vancouver duplex, targeting Step 4.
| Metric | Unit | Step 4 requirement | This project |
|---|---|---|---|
| ACH50 | ACH at 50 Pa | 1.50 maximum | 1.50 |
| NLA10 | cm²/m² | 0.72 maximum | 0.80 |
| NLR50 | L/s·m² | 0.53 maximum | 0.59 |
Read that carefully. The project sits exactly on the ACH50 limit and exceeds the limit on both of the other two metrics. The checklist nevertheless records Step 4 as achieved, and it is correct to do so.
The reason is that the BC Energy Step Code lets the design team demonstrate airtightness compliance through one of the three metrics rather than requiring all three simultaneously. The code includes multiple metrics precisely because they are not equivalent for every building shape, and a building penalized by one may be fairly represented by another. This project complies on the ACH50 path.
You can verify that the three figures describe the same building. The checklist reports a building volume of 452.90 cubic metres and an enclosure surface area of 318.70 square metres. Working from the ACH50 figure:
- 1.50 ACH50 × 452.90 m³ = 679.35 m³ of air per hour
- 679.35 m³/h converted to litres per second = 188.7 L/s
- 188.7 L/s ÷ 318.70 m² of enclosure = 0.59 L/s·m²
That matches the NLR50 on the checklist exactly. Nothing about the building changed between the two numbers. Only the denominator did.
This is the practical takeaway: a building can pass on one metric and miss on another purely because of its shape. A duplex has a high ratio of enclosure surface to interior volume compared with a large detached house, which flatters its ACH50 and penalizes its NLR50. If you are holding a report where the metrics disagree, that is not an error in the testing. It is geometry. Confirm with your Energy Advisor which metric your compliance path uses before assuming you have a problem, and equally before assuming you do not.
Blower Door Test Results Chart: Where Your Number Sits
The table below places the common targets side by side. These are whole-building targets for Part 9 residential buildings, which covers single-family homes, duplexes and townhouses.
| Standard or step | ACH50 target | What it means in practice |
|---|---|---|
| BC Energy Step Code, Step 2 | 3.0 | Below the current provincial minimum, no longer a compliance path for new Part 9 builds |
| EnerGuide reference house | 2.5 | The modelling baseline used in the EnerGuide Rating System |
| BC Energy Step Code, Step 3 | 2.5 | The current province-wide minimum. Achievable with careful caulking, foam and tape |
| BC Energy Step Code, Step 4 | 1.5 | Requires a deliberate air sealing strategy planned from the design stage. This is the target on the sample checklist |
| R-2000 and Net Zero | 1.5 | Aligns with Step 4 on airtightness, with additional requirements elsewhere |
| BC Energy Step Code, Step 5 | 1.0 | Approaches Passive House performance. Rarely reached without aerosol sealing |
| Passive House | 0.6 | The most demanding widely used target in residential construction |
Existing homes sit in a different world, and it helps to know that before opening a retrofit report. Across the Lower Mainland, homes built before the 1980s commonly test somewhere between 6 and 12 ACH50, while housing stock from the 1990s and 2000s more often lands between 4 and 7. A 1970s house testing at 8 ACH50 is not a failure of anything. It is a normal starting point, and it means the first round of air sealing will deliver a larger return than almost any other upgrade available.
What Else the Checklist Tells You
The airtightness block is only one section. Section D of the sample lists the building characteristics the model depends on, and reading it shows how a 1.5 ACH50 target is actually reached. On this project the air barrier strategy is stated explicitly across three planes:
- Walls: exterior taped weather resistive barrier with all penetrations sealed
- Ceiling: taped ceiling poly, or another smart vapour and air barrier
- Foundation: the WRB tied into the concrete wall and the sub-slab poly
Notice that the strategy is described at the junctions, not at the field of each assembly. That is deliberate, and it reflects where leakage actually happens. Any specification that names a membrane without naming how it connects to the roof and the foundation is leaving the hardest part undefined.
Section D also records the ventilation strategy, in this case an HRV, which matters for the reason covered in the next section.
Can a Building Be Too Airtight?
This question comes up almost every time a project pushes below 1.5 ACH50, and the short answer is no. A building cannot be too airtight, but it can absolutely be under-ventilated, and the two get confused because they used to arrive together.
In a leaky building, fresh air arrives by accident through gaps in the enclosure. It is uncontrolled, it costs energy to condition, and it brings moisture into wall assemblies along the way. Tightening the enclosure removes that accidental supply, which means ventilation has to be provided deliberately through a heat recovery ventilator or energy recovery ventilator. This is why the sample checklist lists an HRV alongside the airtightness target rather than as an unrelated line item.
The failure mode people worry about is real, but it is a mechanical design failure rather than an airtightness failure. A tight house with a properly sized HRV has better indoor air quality than a leaky one, because the air is filtered, balanced and continuous instead of arriving whenever the wind happens to blow.
Good Results by Building Type
A single ACH50 threshold cannot serve every building, because the test itself behaves differently depending on what is being tested and how the pressure boundary is defined.
Single-Family Homes and Duplexes
This is where the numbers in the chart above apply directly, and it is the category the sample checklist belongs to. The pressure boundary is unambiguous and the volume calculation is simple. For a new build in BC, aim to know your number well before the drywall goes on, not after.
Townhouses and Multi-Unit Buildings
Here the result depends heavily on test method, and two technicians can produce very different numbers from the same building. In an unguarded test, a single unit is depressurized while its neighbours stay at ambient pressure, so air leaking through party walls counts as leakage. In a guarded test, adjacent units are depressurized simultaneously, which removes party wall transfer from the measurement and isolates leakage to the exterior.
Neither is wrong. They answer different questions. Unguarded results reflect what the occupant actually experiences, including odour and sound transfer between units. Guarded results reflect the performance of the exterior enclosure alone. Before comparing your building against a target, confirm which method produced the number. On multi-building strata sites this becomes a project planning question as much as a technical one, which is the kind of work covered by our large building airtightness testing service.
Commercial and Part 3 Buildings
ACH50 loses most of its usefulness at this scale, because the volume-to-surface relationship in a large building bears no resemblance to a house. Commercial and Part 3 testing normalizes leakage against enclosure surface area instead, and the measured value is typically fed into the energy model to demonstrate compliance rather than compared against a standalone pass or fail threshold. Testing follows ASTM E779 or ASTM E3158, with CAN/CGSB-149.10 applying to residential work.
What to Do If Your Results Are Not Good Enough
A disappointing number is a starting point, not a verdict. The following sequence is ordered by return on effort, and following it in order usually gets a building most of the way to target before the expensive options come up.
- Find the leaks before sealing anything. The single biggest waste of money in air sealing is caulking on instinct. Run the fan and use infrared thermal imaging under depressurization to see exactly where air is entering.
- Seal the top of the building first. Attic hatches, plumbing stacks, recessed lights, bath fan housings and any chase running from the basement to the attic. Stack effect makes these the highest-value targets in a heating climate.
- Address the base of the walls and the foundation junction. The rim joist and the sill plate connection are consistently among the leakiest details in wood frame construction and are usually accessible.
- Move to windows, doors and penetrations. Worth doing, but genuinely lower yield than the two steps above, which is the opposite of most homeowners’ intuition.
- Retest. Sealing without a follow-up measurement leaves you guessing about whether the work paid for itself.
Why Timing Changes Everything
On new construction, when you test matters as much as what you measure. Once drywall, insulation and finishes are in place, the air barrier is buried and every correction becomes destructive. This is also why the checklist exists in three versions: the pre-construction target is a commitment, and the mid-construction test is the last opportunity to keep that commitment affordably.
A 3,850 square foot custom home in Vancouver illustrates the difference. It initially tested at 4.7 ACH50, well short of its target. Because the test happened during construction while the enclosure was still accessible, targeted sealing brought the final result down to 1.1 ACH50, a reduction of more than 75 percent, and the project achieved Step Code compliance. The same corrections after occupancy would have cost several times as much and delivered less. This is the entire argument for mid-construction blower door testing.
Getting Your Results Interpreted Properly
Numbers on a report are only useful once someone connects them to your compliance path, your building type and the specific details that produced them. If you are holding a result you are unsure about, or planning a build where the target is not yet locked in, our team works across residential, multi-family and commercial enclosures throughout North Vancouver and the Lower Mainland. You can review the full range of building science and energy performance services we provide.
Frequently Asked Questions
What is a good ACH50 for a new home in BC?
2.5 ACH50 is the practical benchmark, because it is the Step 3 target and Step 3 is the current province-wide minimum for new Part 9 residential buildings. Results at or below 1.5 ACH50 correspond to Step 4 and are considered strong. Below 1.0 ACH50 reaches Step 5 performance.
Is 3 ACH50 good?
It depends entirely on the building. For an existing home it is a good result and better than most of the housing stock in the Lower Mainland. For a new build in British Columbia it falls short, because 3.0 ACH50 corresponds to Step 2, which no longer satisfies the provincial minimum for new Part 9 construction.
What does CFM50 mean on my report?
CFM50 is the cubic feet per minute of air the blower door fan must move to maintain a 50 pascal pressure difference between inside and outside. It is the raw measurement from which every other figure is calculated. Because it does not account for building size, it cannot be compared between buildings on its own.
How do I convert CFM50 to ACH50?
Multiply CFM50 by 60 to get cubic feet per hour, then divide by the conditioned volume of the building in cubic feet. A home with 21,600 cubic feet of conditioned volume testing at 900 CFM50 works out to 2.5 ACH50.
Why does my report pass on ACH50 but fail on NLR50?
Because the two divide the same leakage by different denominators. ACH50 divides by interior volume while NLR50 divides by enclosure surface area, so building shape alone can move one across a threshold and not the other. The BC Energy Step Code allows compliance through one of the metrics rather than all three.
What blower door result does the BC Energy Step Code require?
For Part 9 residential buildings the targets are 3.0 ACH50 at Step 2, 2.5 at Step 3, 1.5 at Step 4 and 1.0 at Step 5. The code also defines airtightness levels AL-1 through AL-4 with Normalized Leakage Rate and Normalized Leakage Area values, so confirm with your Energy Advisor which metric governs your compliance path.
Can a blower door test fail?
A test cannot fail, but a building can miss its target. If that happens on new construction, the usual response is to locate the leaks, seal them and retest. Catching it during construction rather than at final inspection is what keeps the remediation affordable.
