A builder’s net zero envelope course by Xerox Builders · October 2026
A mid-construction blower door test finds air leaks while the air barrier is still exposed and easy to fix. This Detail covers when to test, how to prepare the house, how to hunt leaks with smoke and a thermal camera, and how to fix and re-test before insulation and drywall.
Why test in the middle of the job
A final blower door test tells you whether the house passed. A mid-construction test tells you where it leaks, while you can still do something about it. At this stage the air barrier is complete but exposed: every joint, tape lap and penetration can be seen and reached. A leak that takes a few minutes to seal now can mean cutting drywall and repainting later. The test is also the best training tool on site, because trades see their own leaks with smoke.
BC Housing’s airtightness guide makes the same point: start testing as early as possible, because defects found late are harder and more expensive to fix.
Where it fits in the schedule
| Air barrier approach | When to test |
|---|---|
| Exterior membrane or taped sheathing | After windows, doors and service penetrations are in and sealed, before insulation |
| Interior sealed poly | After the poly is installed and sealed, before drywall |
| Airtight drywall | A pre-drywall test checks the exterior side and framing; the drywall layer itself can only be tested once it is up |
Book the energy advisor early and leave a day or two in the schedule to fix and re-test. Treat the test as a hold point: insulation does not start until it is done.
Getting the house ready
- Close the shell. All windows and exterior doors installed, closed and latched. A temporary door is fine if it is sealed.
- Finish the holes. Plumbing, electrical and duct sleeves through the envelope should be installed and sealed before the test. Anything drilled later is an unmeasured leak.
- Temporarily seal what isn’t built yet. Tape over open duct ends for the heat recovery ventilator, range hood and dryer, cap open drains and cover future vent holes. Keep a written list so the final test can be compared fairly.
- Clear the site. No one should be opening doors or running equipment during the test. Strong wind makes readings unsteady, so pick a calm day if you can.
How the test works
A calibrated fan is mounted in a sealed panel in an exterior door opening. It pulls air out of the house, which lowers the indoor pressure. A manometer measures the difference between inside and outside, and for leak hunting the advisor often holds the house at about 50 Pa below outside. Outside air then streams in at every gap. The fan’s airflow at that pressure gives a first estimate of the airtightness result.
Finding the leaks
- Smoke pencil: with the house depressurized, outside air rushes in at a leak. Inside, the smoke is blown away from the gap; outside, along an exterior membrane, it is drawn into the gap. At a tight joint it simply drifts.
- Thermal camera: shows cold streaks where outside air enters. It needs a good temperature difference between inside and outside. Comparing an image taken before the fan starts with one taken under depressurization helps tell air leaks apart from thermal bridges: air leaks grow, thermal bridges don’t change.
- Back of the hand: a quick way to feel draughts along plates, laps and penetrations.
Work through the house in the same order every time: base of wall and sill plate, rim joists, window and door perimeters, top plates under the attic, the attic hatch and ceiling penetrations, pipes, ducts and wires, and electrical boxes.
Fix, check and re-test
Mark each leak with tape or a flag, photograph it and note which trade owns it. Seal it with a tape, sealant or membrane that is compatible with your air barrier (see Detail 02), then check it again with smoke while the house is still under pressure. Many advisors offer blower door guided air sealing, where the house stays depressurized while the crew works and each repair is confirmed straight away. After the big fixes, re-test to prove they worked, and record the result, the date and the leak list.
Program requirements
Some programs set a number for this stage. The City of Vancouver’s Zero Emission Buildings bulletin for R1, RT and RA districts, which trades zoning relief for zero emission houses, asks Passive House projects for a mid-construction blower door test (ISO 9972 with Passive House Institute modifications) at or below 0.6 ACH50. Its CHBA Net Zero path asks for a mid-construction result before the interior walls are closed. Burnaby’s mid-construction energy verification report has a section for measured air leakage. Requirements change and vary between cities and programs, so confirm with your city and energy advisor which tests are required and when.
Site checklist
- Air barrier complete and all penetrations sealed
- Windows and exterior doors installed, closed and latched
- Unfinished openings temporarily sealed and listed
- Energy advisor booked; test set as a hold point before insulation
- Site clear of trades during the test
- Leaks marked, photographed and assigned to a trade
- Repairs checked with smoke under pressure
- Re-test done and result recorded with the date
- Result well below the final target
Test yourself
Try to answer each question, then click it to check.
When is the best time for a mid-construction blower door test?
When the air barrier is complete and sealed, but not yet covered by insulation or drywall.
Why do we depressurize the house to find leaks?
Outside air is pulled in at every gap, so smoke, a thermal camera or your hand can show each leak.
Why aim well below the final target at this test?
Later trades can still add leaks before the final test, so you need some margin.
Next: Detail 19, Final Testing & Troubleshooting. See also our Building Science Construction Guides.
Planning a high-performance home in Vancouver, Burnaby or Surrey? Talk to our team about getting the envelope right from day one.
Sources: Illustrated Guide: Achieving Airtight Buildings (BC Housing / RDH); City of Vancouver bulletin: Zero Emission Buildings in R1, RT and RA Districts; Burnaby Energy Verification Report (Mid-Construction); Better Homes BC: mid-construction blower door testing; Green Building Advisor: When to perform a blower door test.