A blower door test measures envelope air leakage under an imposed pressure difference. An interim test helps locate defects while they remain accessible.
What is measured
A fan creates a pressure difference while instruments measure the airflow needed to maintain it. The report must identify the procedure and building preparation.
The n50 metric
n50 is airflow at 50 Pa divided by the defined internal volume, expressed in h⁻¹. It is not the natural air-change rate under ordinary weather conditions.
Interim and final testing
An interim test follows completion of the air barrier while joints are still exposed. A final test records the completed condition using the agreed procedure.
Preparation
Agree the test boundary and treatment of doors, windows and intentional openings with the tester. Address combustion appliances and other pressure-related hazards before starting.
Finding leaks
During diagnosis, inspect junctions and penetrations with suitable tools. A single final number without leak locations gives the contractor little guidance.
Reporting and interpretation
Record the geometric basis, conditions, equipment, preparation and measured results. Compare them with the target in the selected standard and agreed edition.
Limits and remediation
The test does not measure wall U-value or prove ventilation quality. Retest significant repairs rather than estimate their benefit visually.
| Item | Parameter | Check / result |
|---|---|---|
| Before lining | Locate defects | Repair register |
| After repairs | Verify improvement | Retest |
| Completion | Record condition | Full report |
What this means for your project
Include testing in the schedule and budget before finishes begin. Request a report rather than a verbal assurance that airtightness is good.