The Blower Door test (infiltrometry test) measures the air-tightness of a house envelope. Mandatory for any EPC-A construction in Wallonia since 2026, it should be systematic before signing the provisional reception minutes. Yet I still see, in one file out of three, reception minutes signed without a Blower Door report attached — which makes the EPC-A performance not legally enforceable against the builder. Here is the detailed protocol, the legal thresholds, the recurring weak points and the good practices for requiring this test at the right time.
The technical principle and thresholds
The Blower Door test consists of installing a calibrated fan on the front door, putting the envelope under over-pressure then under-pressure at 50 Pascals. The airflow required to maintain this pressure gives the n50 indicator (number of air changes per hour at 50 Pa).
The regulatory thresholds in Wallonia 2026:
- ≤ 6 vol/h for standard EPC-B/C new builds
- ≤ 3 vol/h for EPC-A (standard since 2026)
- ≤ 1 vol/h for EPC-A+ and passive
- ≤ 0.6 vol/h for Passivhaus certification
My tightest case, out of 312 Blower Door tests I have attended: 0.42 vol/h (certified passive). My worst: 11.8 vol/h on a house claiming to be EPC-A — the annual energy performance gap represents about €2,100 of overspend per year.
Common weak points
Across 312 Blower Door tests where I was present, air-tightness defects concentrate 80% on five identifiable zones:
- Window frame perimeter joint (35% of detected leaks)
- Passage of ventilation ducts through intermediate slabs (18%)
- Attic access hatch poorly insulated (12%)
- Technical sheaths (electrical, plumbing) crossing the vapour barrier (9%)
- Bottom wall-to-floor connection (6%)
The remaining 20% are point defects: wall sockets without sleeves, non-tight lighting points, ventilation inspection hatches. These defects are generally fixed at moderate cost (€50-300 per point) if detected before painting.
The cost of the test and its return on investment
The price of a Blower Door test in Wallonia 2026:
- Apartment / small house: €400 to €500 incl. VAT
- Standard 150-200 m² house: €500 to €650
- Villa > 250 m²: €650 to €850
- Combined Blower Door + thermography test: +€180 to €250
To compare with €800 to €1,500 of annual overspend of a house failing the EPC-A threshold. The investment pays back in two years, and the report serves as legal proof enforceable in case of dispute on energy performance.
When to schedule the test ideally
The timing of the test is as important as its execution. On the projects I support, I recommend:
- Pre-reception test: 4-6 weeks before the minutes (allows correction of leaks)
- Final test: 1 week before the minutes (compliance validation)
- Adversarial test: at the minutes themselves if EPC class is disputed
A test scheduled too early (before paint, before partitions) does not give the final measure of the envelope. A test scheduled too late (after move-in) prevents corrections by the contractor.
Pitfalls to avoid
- Do not accept an EPC-A minutes without a Blower Door report attached
- Check the accreditation of the tester (official SPW list)
- Beware of “partial” tests that do not include ventilation
- Require the infrared photos of detected leaks
- Keep the report with your owner’s file (10 years)
For the official list of accredited testers in Wallonia, see energie.wallonie.be.
What next?
If your reception is coming up and the Blower Door test has not yet been scheduled, request it in writing now. My firm offers an insulation expertise with Blower Door coordination, or a broader provisional reception expert service including this check.