Blower Door Test Explained: What BC Builders Need to Know

Every new Part 9 residential building in BC must pass a blower door test before occupancy. It’s the one mandatory airtightness measurement under Step Code compliance, and it’s the test that most often catches builders off guard. Failing a blower door test after drywall is up means expensive remediation, occupancy delays, and potentially lost FortisBC rebate money.

This guide explains exactly what a blower door test is, how it works, what the numbers mean, how to prepare, and what to do if you fail.

What Is a Blower Door Test?

A blower door test is a measurement of how much air leaks out of a building when pressurized. A calibrated fan is mounted in an exterior door frame. The fan runs backwards to depressurize the house (pulling air out), and the amount of air the fan has to move to maintain a specific pressure difference tells you how leaky the house is.

The standard test pressure is 50 pascals - equivalent to a moderate wind blowing on the building. At that pressure, the test measures how much air the house leaks and calculates two key numbers:

CFM50 - cubic feet per minute of air leaking at 50 pascals
ACH50 - air changes per hour at 50 pascals, derived from CFM50 and the house volume

ACH50 is the number that matters for BC Step Code compliance.

How the Test Works

A typical blower door test on a Part 9 residential building runs like this:

Setup. The technician mounts the calibrated fan assembly in an exterior door - usually the front door. The fan has a flexible fabric panel that seals the remaining door opening.
House prep. All exterior doors and windows are closed. Interior doors are open. HVAC systems are off. Fireplace dampers are closed. Any exhaust fans (range hood, bathroom fans, dryer) are sealed.
Baseline measurement. The fan is started at a low speed. A pressure gauge measures the difference between interior and exterior pressure.
Depressurization run. The fan speed is increased until the interior is 50 pascals lower than exterior. The fan has to move a certain amount of air to maintain that pressure difference. That volume of air equals the total leakage through the building envelope.
Calculation. The fan’s airflow at 50 Pa is recorded as CFM50. That number is divided by the house volume and multiplied by 60 (minutes per hour) to produce ACH50.
Documentation. The test result is recorded and compared against the Step Code target (2.5 ACH50 for Step 3, 1.5 for Step 4, 1.0 for Step 5).

The whole test takes 30-60 minutes for a typical home once the technician is set up.

What ACH50 and CFM50 Mean

These two numbers can be confusing because they measure the same thing in different ways.

CFM50 (cubic feet per minute at 50 Pa) is a raw measurement of air leakage volume. A home with CFM50 = 500 leaks 500 cubic feet of air per minute when depressurized to 50 Pa.

ACH50 (air changes per hour at 50 Pa) normalizes that leakage to the house volume. If a 2,000 square foot home with 8-foot ceilings leaks at 500 CFM50, its volume is 16,000 cubic feet, and it leaks enough air to replace the entire interior volume every 32 minutes at test pressure. That’s 60/32 = 1.875 ACH50.

Why both numbers? Because CFM50 gives you an absolute leakage rate (useful for comparing similar-sized homes), while ACH50 normalizes across home sizes (useful for comparing a small cottage to a large custom home).

BC Step Code targets are always specified in ACH50. FortisBC rebate programs also use ACH50. Your energy advisor will record both numbers in the final compliance report.

Pre-Drywall vs Final Testing

Builders doing Step Code work should understand two testing windows:

Pre-Drywall Blower Door Test

Performed after framing, insulation, windows, and air barrier work are complete, but before drywall goes up. At this stage, any leaks that show up can be addressed cheaply because the wall cavities are still accessible.

Pre-drywall testing is not mandatory in BC but is strongly recommended for any project targeting Step 4 or 5. The City of Kelowna offers a $325 rebate for mid-construction tests (maximum 3 per builder per year) specifically to incentivize this practice.

A pre-drywall test is also the logical time to apply AeroBarrier, since the home is already pressurized for the test.

Final (As-Built) Blower Door Test

The legally required test for occupancy. Performed when the home is substantially complete - drywall, trim, windows, doors, and mechanical systems all installed. The house has to pass this test for the energy advisor to issue the final compliance report.

Failing the final test means the home cannot receive its occupancy permit until remediation is complete. Remediation at this stage is expensive because most air barrier elements are hidden behind finished surfaces.

How to Prepare for a Blower Door Test

Regardless of whether you’re doing a pre-drywall or final test, preparation matters. Common steps:

Close all exterior doors and windows. Sounds obvious but checking every one takes time.
Seal combustion appliance vents. Gas fireplace and furnace flues should be sealed or closed.
Turn off HVAC. Furnace, heat pump, ERV, HRV all powered down.
Seal exhaust fans. Kitchen range hood, bathroom fans, dryer duct - covered with tape or inflatable bags.
Open interior doors. The test measures the whole house, not individual rooms, so interior doors should be open.
Close fireplace dampers where applicable.
Clear clutter near test door. The technician needs 3-4 feet of clear space to set up the fan assembly.

Some testers bring their own blockers for common appliance vents; others ask the builder to handle it. Confirm with your energy advisor before test day.

What Happens If You Fail?

The short version: no occupancy permit until the home passes. The long version depends on how far over the target you are.

Just Over the Target (e.g., 2.7 ACH50 when targeting 2.5)

Often fixable with targeted remediation. Common culprits at this level:

Unsealed mechanical penetrations at plumbing stacks, furnace flues, or range hood ducts
Attic hatch gaskets or weatherstripping
Minor gaps at window and door trim
Electrical box air leaks on exterior walls

These can sometimes be addressed from the interior with targeted caulking and gasket upgrades. Budget a day or two and a few hundred dollars.

Moderately Over (e.g., 3.5 ACH50 when targeting 2.5)

Harder to fix without cutting into walls. You may need to:

Open up strategic drywall sections to expose air leakage areas and seal them
Add an AeroBarrier treatment - aerosol sealing can often seal hidden leaks without opening walls
Repair or replace failed gaskets on penetrations

Budget $2,000-$5,000 and a week of delay.

Significantly Over (e.g., 5.0+ ACH50 when targeting 2.5)

You have a fundamental air barrier problem, and fixing it will be expensive. Options include:

Extensive drywall removal to find and seal leaks - labor-intensive and messy
AeroBarrier rescue - this is where AeroBarrier often shines, because the aerosol sealant can reach leaks behind finished drywall that would otherwise require demolition
Full envelope retrofit - worst case, you’re redoing significant portions of the air barrier

See our failed blower door test guide for a more detailed response plan.

The Best Way to Avoid Failing

The cheapest way to fix a failed blower door test is to never fail one in the first place. That means:

Have a documented air barrier strategy at design stage
Train your trades on what they need to do to preserve airtightness
Inspect during framing and rough-in - walk the site before drywall
Do a pre-drywall blower door test before closing up walls
Apply AeroBarrier as insurance on any project targeting 2.0 ACH50 or lower

Our Step Code checklist walks through the phase-by-phase details.

Frequently Asked Questions

How much does a blower door test cost in BC?

A standalone blower door test typically runs $250-$450 depending on the tester, the home size, and travel distance. Pre-drywall tests are usually billed at the same rate. Energy advisor packages that include both the test and the compliance report run higher - usually $800-$1,500 all-in.

Who is qualified to perform a blower door test?

BC Step Code compliance requires the test be performed by a registered BC energy advisor with current certification. Your builder can perform informal tests at any stage for troubleshooting, but the official compliance test must be done by a registered energy advisor.

Can I do my own informal blower door test?

Yes, some builders own their own blower door equipment for in-house QA. These informal tests don’t satisfy code requirements but they’re invaluable for catching problems before the official test.

Does the weather affect the test?

Yes, somewhat. Wind can affect readings, and extreme temperature differences between inside and outside can make calibration more challenging. Testers adjust for these factors. Most tests are scheduled on days with moderate conditions.

What happens to the house during the test?

Nothing permanent. The house is depressurized for 30-60 minutes while the fan runs, and air is pulled out faster than normal. You can feel the air movement near leaks - it’s actually a useful diagnostic tool for finding gaps you can then fix.

Is there a way to simulate a blower door test without one?

Not really. Infrared cameras can spot temperature differences that hint at leaks, and smoke pencils can trace air currents through small openings, but neither replaces the quantitative measurement a blower door provides.

Next Steps

If you’re preparing for your first blower door test:

Read the Step Code checklist to make sure your air barrier work is on track
Budget for a pre-drywall test as insurance against a failed final test
Consider AeroBarrier for any project targeting below 2.0 ACH50
Work closely with your energy advisor throughout the project

Call 250-864-8727 or get a free consultation if you want help planning an air sealing strategy that will pass the blower door test on the first try. Okanagan AeroBarrier Inc. has completed 100+ verified residential seals and we have achieved results as low as 0.24 ACH50.