A builder’s net zero envelope course by Xerox Builders · October 2026
The final blower door test gives the airtightness number used for code compliance and the energy model. This Detail explains the multi-point test, ACH50, NLR50 and NLA10, the test standards and BC airtightness levels, how to set up the house, and how to troubleshoot a failed result.
The result that counts
The final airtightness test is done at completion by an energy advisor. Its result is used to show code compliance and goes into the energy model of the house. By then the air barrier is hidden behind drywall and trim, so the final test is a check, not a chance to fix things cheaply. A mid-construction test with a good margin (Detail 18), and an air barrier protected since, are what make the final test routine.
How the test works
A calibrated fan in an exterior door opening holds the house at several pressure differences in turn, recording the airflow at each. A curve is fitted through the readings, and the airflow at 50 Pa is read from it. In BC, airtightness values used in the energy model come from a multi-point depressurization test; a single reading at one pressure is not enough.
Three ways to report the same test
| Metric | What it means | Units |
|---|---|---|
| ACH50 | Airflow at 50 Pa divided by the air volume of the house, per hour | air changes per hour |
| NLR50 (normalized leakage rate) | Airflow at 50 Pa divided by the area of the whole envelope | L/(s·m²) |
| NLA10 (normalized leakage area) | Equivalent leakage area at 10 Pa divided by the envelope area | cm²/m² |
ACH divides by volume, so a small house with the same leaks as a large one gets a worse number, because it has less air to share the leaks. Small homes and townhouses are penalized. NLR and NLA divide by envelope area instead, which is fairer. When BC updated the Step Code in May 2023, it added airtightness testing options suited to smaller homes.
Test standards and pressure
The BC Step Code change documents list these test methods for Part 9 buildings: CAN/CGSB-149.10 (fan depressurization), ASTM E779, the US Army Corps of Engineers Version 3 protocol and the EnerGuide Rating System protocol. The test pressure for Part 9 is at least 50 Pa. Vancouver’s by-law lists ASTM E779, the USACE protocol and the NRCan EnerGuide protocol, and requires compliance at final inspection; the City’s code change for the 2025 by-law (CCR 24-0021) also adds ASTM E3158. Confirm the adopted wording with the City. Your energy advisor picks the standard; confirm with your city that it is accepted.
BC airtightness levels
| Level | ACH50 | NLR50 L/(s·m²) | NLA10 cm²/m² |
|---|---|---|---|
| AL-1 | 2.5 | 0.89 | 1.20 |
| AL-3 | 1.5 | 0.53 | 0.72 |
| AL-4 | 1.0 | 0.35 | 0.48 |
Higher Step Code steps call for tighter levels. In Vancouver, houses have a building by-law maximum of 2.5 ACH50 (or 0.89 L/(s·m²) NLR50, or 1.20 cm²/m² NLA10, the same values as AL-1), and the Passive House limit is 0.6 ACH50. These targets change over time and differ between cities, so confirm the current requirement with your city and energy advisor.
Setting up the house on test day
- Close and lock. Exterior doors and windows closed and latched, as they would be in normal use.
- One pressure zone. Interior doors open, including closets and the door to a basement, so the whole conditioned space is tested together.
- Water in every trap. Fill sinks, tubs, showers and floor drains so sewer air can’t leak in through a dry trap.
- Vents as the standard says. Dampers closed or openings sealed as the chosen standard requires, and combustion appliances turned off so they can’t backdraft. Where mechanical openings are left unsealed, the energy model has to account for that leakage.
Wind and big indoor to outdoor temperature differences make readings less steady. A calm day gives the most reliable result.
Why final tests fail
Most failures come from work done after the mid-construction test: large holes cut for tub and shower drains, fan ducts without working dampers, pot lights and ceiling fixtures, exterior door weatherstripping and sills that don’t seal, an attic hatch that isn’t gasketed and latched, and late holes for cable, security, solar and service lines.
Troubleshooting a failed test
- Check the set-up. Doors and windows latched, traps filled, vents treated as the standard requires, interior doors open, and not too windy.
- Hunt the leaks. Depressurize and use smoke and a thermal camera, starting at the attic, rim joists and service penetrations.
- Fix the biggest first. Seal from inside where finishes allow: hatch gaskets, fixture and penetration seals, door weatherstripping.
- Re-test and record. Keep the report, photos and leak list in the project file.
Site checklist
- Target level and metric confirmed with the city and energy advisor
- Mid-construction leak list and sealed-openings list on hand
- Late penetrations (drains, ducts, cable, solar) sealed
- Fan dampers installed and closing
- Attic hatch gasketed and latched
- Exterior door weatherstripping and sills adjusted
- Traps filled, windows latched, interior doors open on test day
- Vents set as the test standard requires
- Result, report and photos filed
Test yourself
Try to answer each question, then click it to check.
What does NLR50 divide the measured leakage by?
The area of the whole building envelope, at 50 Pa.
Why can a small house struggle to meet an ACH50 target?
It has less air volume to share the same leaks, so the same leaks give a higher ACH50.
What should you check first after a failed final test?
The test set-up: doors and windows latched, traps full, vents set as the standard requires, and the wind.
Next: Detail 20, Lower-Carbon Envelope Choices. See also our Net Zero Homes page.
Planning a high-performance home in Vancouver, Burnaby or Surrey? Talk to our team about getting the envelope right from day one.