Blower Door Test: Airtightness for Self-Build Houses

Your house can have 30 cm of insulation, triple-glazed windows and an MVHR unit — if airtightness is not carefully managed, you will lose up to 25% of your thermal performance through invisible leaks around sockets, loft hatches and roof penetrations. Since RE2020, the blower door test (or air permeability test) is mandatory at completion of every new build in France. But the stakes go beyond compliance: an airtight house uses less energy, lasts longer and breathes better. This article explains how a blower door test works, which targets to aim for, where typical leaks hide, and how to fix them before the certified tester arrives.

What is airtightness

The airtightness of a building is its ability to prevent outdoor air from entering — and conversely, to prevent heated indoor air from escaping — outside the controlled ventilation circuit. Be careful not to confuse:

• Ventilation (controlled): fresh air brought in by MEV or MVHR, extracted through outlets. This is necessary for health and moisture removal.
• Infiltrations (uncontrolled): air passing through cracks, membrane defects, unsealed cable entries. This is harmful for energy efficiency, comfort and air quality.

A good house is airtight but mechanically ventilated. The two go hand in hand: if you seal a house without fitting a high-performance ventilation system, you create a confined, humid environment with a risk of condensation.

Four consequences of air leaks

1. Heat loss: infiltrations account for 15 to 25% of the heating energy consumption of a poorly sealed house, and up to 40% in older unrenovated buildings
2. Comfort: sensation of “draught” near skirting boards, windows and loft hatches — cold surfaces near defects
3. Insulation degradation: humid air passing through the insulation condenses against the cold outer wall. Result: mineral wool compressed and blackened within 5–10 years, rotting timber frame
4. Acoustic discomfort: airtightness defects are also acoustic bridges — you hear the wind, neighbours, traffic through the same gaps that let air in

The blower door test: principle and standard

The air permeability test — also called blower door test, air tightness test or infiltrometry test — measures the quantity of air leaking through the building envelope under an imposed pressure difference.

Physical principle

A calibrated fan is mounted on an airtight panel fitted in the frame of an external door. It depressurises (or pressurises) the building to 50 Pa (pascals), roughly equivalent to a 35 km/h wind blowing uniformly on all facades. At this stabilised pressure, the air flow rate the fan must supply to maintain 50 Pa is measured. That flow rate = the total of all leaks.

Standard and protocol

The test follows standard NF EN ISO 9972 (method 3, in France) and its application is specified by GA P50-784. It must be carried out by a Qualibat 8711 certified tester (or equivalent — e.g. ATTMA-certified in the UK), using calibrated equipment. The report is legally valid for RE2020 and for dommages-ouvrage insurance (DO — the mandatory French 10-year builders’ liability policy).

Test conditions

On the day of the test, the tester:

1. Prepares the building: closes all external windows and doors, opens all internal doors, seals MVHR inlets/outlets, traps and any intentionally ventilated openings
2. Installs the panel in the main entrance door (or the most suitable one), with the calibrated fan
3. Measures the reference pressure without ventilation (natural pressure due to wind and temperature)
4. Progressively depressurises the building in 10 Pa steps up to 60–80 Pa
5. Records flow rates at each step — 8 to 10 measurement points minimum
6. Repeats under pressurisation to average the two (standard protocol for RE2020)
7. Calculates the indicators and writes the report

The indicators: Q4Pa-surf, n50 and leak flow rate

Three quantities describe the airtightness of a building. Understanding them is essential to interpret the thresholds.

Q4Pa-surf (France, RE2020)

This is the French regulatory indicator since RT2012 and carried forward in RE2020. It expresses the leak flow rate under a pressure difference of 4 Pa, divided by the area of the cold envelope (walls + roof + ground floor in contact with the exterior, excluding ground-bearing slabs).

Unit: m³/(h·m²). Lower is better.

n50 (Europe, passive house)

Indicator used in the German passive house standard (Passivhaus) and in most European countries. It expresses the number of times the interior air volume is replaced by leaks in one hour, at 50 Pa.

Unit: air changes per hour (ACH) or h⁻¹.

Indicative conversion

For a standard house (ground + first floor, 120 m², volume 300 m³, cold envelope 250 m²):

• Q4Pa-surf = 0.6 m³/(h·m²) ≈ n50 = 1.2 ACH
• Q4Pa-surf = 0.3 m³/(h·m²) ≈ n50 = 0.6 ACH

Regulatory thresholds and targets

When to test during the build

There are two key moments for a blower door test, and they have different objectives.

Intermediate test — “watertight and windtight” stage

At this stage:

• Roof fitted and watertight
• External joinery installed
• Vapour control layer (VCL) / airtightness membrane fitted, taped and visible (not yet covered with plasterboard)
• Electrical and plumbing runs completed, but not connected

Objective: detect leaks before they are hidden. This is the most technically useful test, because at this stage corrections are easy and inexpensive. The tester can walk through the building with a thermal camera or smoke generator and pinpoint exactly where air enters.

This test is not mandatory but is strongly recommended in self-build.

Final test — building completion

At this stage:

• Building finished, partitions fitted, finishes in place
• MVHR installed and operational
• Internal doors hung

Objective: validate RE2020 compliance. This test is mandatory to obtain the RE2020 certificate to be filed with the completion declaration (DAACT).

Where leaks hide — the 10 weak points

Studies by the Agence Qualité Construction (AQC) and CSTB show that 90% of leaks are concentrated in 10 types of defects (numbered on the diagram above). Knowing them allows you to address them at source.

1. Wall / ground floor junction

At the base of facade walls, the joint between the concrete slab and the internal lining often leaves a gap of a few millimetres filled with plain mortar. Air passes through the micro-cracks.

Correction: fit a compressible foam strip between the slab and the first course, or apply acrylic sealant at the base of each wall before fitting the plasterboard.

2. Wall / roof junction (wall plate)

At the junction between the facade wall and the roof structure (wall plate, purlin), a construction gap of 10–20 mm is typical. Air circulates freely between the heated volume and the loft space (even an unheated one).

Correction: airtightness membrane folded back at a right angle against the wall plate, taped with a specific adhesive tape (Siga Fentrim, Ampacoll XT, Proclima Contega).

3. Membrane penetrations (conduits, cables)

Every electrical conduit or pipe passing through the VCL pierces a hole. When penetrations are numerous (home automation, CCTV, alarm), the cumulative leakage becomes substantial.

Correction: specific pre-formed grommets for each penetration (Siga Rissan, Proclima Kaflex), taped to the membrane.

4. Recessed electrical boxes

A single socket on an external wall is an air funnel: air from inside the partition passes through the unsealed box, then through the cable entries. Multiplied across 40 boxes in the house, the impact is significant.

Correction: airtight back boxes (Legrand Ecobatibox, Schneider Multifix Air, Simon 500 airtight) or acrylic sealant around each standard box.

5. External joinery — installation

Around windows and doors, the installation gap between the frame and the structure must be treated:

• PU expanding foam alone: insufficient (ages poorly, shrinks)
• Compriband or resilient strip: adequate if correctly sized
• EPDM adhesive strip + sealant: reference solution

6. Roller shutter box

On windows with integrated roller shutters, the facade housing is a classic disaster: aluminium guide rails penetrating the wall, handle slot, access hatch. Under depressurisation, smoke jets out through an untreated shutter box.

Correction: airtight roller shutter boxes specified at order, or a dedicated airtightness kit (not always reliable in renovation).

7. Loft hatch

The loft inspection hatch is often a plain timber board simply rested in place. Hot air shoots through it like a fountain.

Correction: specific hatch with perimeter EPDM seal + compression lock (Stumpf, Fakro). Insulate the hatch on its upper face.

8. MVHR — roof penetration and ductwork

The MVHR collector passing through the roof, the flexible ducts crossing the ceiling, poorly fitting extract valves: all concentrated defects.

Correction: specific roof sleeve (Siga Fentrim, Isocell), extract valves with gaskets. Reminder: the MVHR itself is not a leak (it is within the controlled circuit).

9. Stove or fireplace — flue and combustion air inlet

The flue of a wood-burning stove passes through the roof and floor: every penetration must be sealed. The combustion air inlet, if unsealed when not in use, is a gaping hole.

Correction: airtightness at roof penetration with high-temperature sealant, combustion air inlet with damper or closure flap.

10. Membrane / membrane joints (between rolls)

Every joint between two rolls of VCL or airtightness membrane is a potential line of failure. In a 120 m² house, there are easily 200 linear metres of joints.

Correction: minimum 150 mm overlap + specific adhesive tape over the full length, not just spot-fixed. Roll down with a rubber roller.

Correction strategy before the final test

If your intermediate test reveals a value that is too high, proceed methodically.

Method: chasing defects with smoke

During the intermediate test, ask the tester to use a smoke generator (or a portable smoke pencil):

1. The building is under depressurisation at 50 Pa
2. Outside, bring the smoke source close to suspected points
3. Inside, smoke jets through each defect — it is immediate and unambiguous
4. Mark each defect with chalk or a sticky note

In parallel, an infrared thermal camera (under depressurisation, in cold weather) reveals colder zones corresponding to incoming air currents. The two methods are complementary.

Correction materials

Cost of a blower door test

Saving compared to a failed final test: post-completion corrections (opening up plasterboard, reworking joinery) cost £2,500–8,500, not counting the delay. The intermediate test is always cost-effective.

Self-build: the habits that gain you points

Beyond the test, it is on site every day that airtightness is won or lost. A few reflexes to ingrain:

During membrane installation

• Unroll before bonding: never any tension in the membrane, always leave 2–3 cm extra at joints
• Rubber roller mandatory on all adhesive tapes — pressing by hand is not enough
• Temperature > 5°C for most specialist adhesives — check the technical data sheet
• Primers on bare concrete, blockwork, unpainted plasterboard — otherwise the adhesive detaches within 1–2 years

Trade coordination

• The VCL must be installed after electrical conduits and plumbing have been run — otherwise the electrician pierces your membrane
• Conduits built into cold walls must be foam-filled + sealed before the plasterboard goes up. Every conduit is an air duct.
• The dryliner must not fit recessed downlights in a ceiling below the loft without an airtight recessed box (Aric, Spittler type)

Site documentation

Photograph systematically:

• Every membrane joint before the plasterboard goes on
• Every penetration grommet
• Electrical back boxes with their seal

This file is required for the DO insurance and the next test — essential if a defect appears later.

Airtightness and ventilation: inseparable

A very airtight building without effective ventilation quickly becomes unhealthy: moisture stagnates (condensation on windows), CO₂ accumulates, VOCs from decoration do not dissipate. Mechanical controlled ventilation is the sine qua non of good airtightness.

• Humidity-controlled MEV (hygro B): minimum acceptable, ventilation modulated by humidity
• MVHR with heat exchanger: optimal for a house with Q4Pa-surf ≤ 0.4 — recovers 70–90% of the heat from extracted air
• Natural ventilation (permanent grilles): incompatible with RE2020 in new build

Regulations and references

• RE2020 (decree 2021-1004) — Q4Pa-surf ≤ 0.60 mandatory for new detached houses in France
• NF EN ISO 9972 — method of measuring air permeability
• GA P50-784 — French application guide
• Qualibat 8711 — mandatory certification for French testers
• ATTMA TS1 — UK standard for air permeability testing
• Effinergie+ — enhanced BBC reference (Q4 ≤ 0.40)
• NF DTU 31.2 — timber frame / airtightness junction
• NF DTU 25.41 — plasterboard + VCL
• AQC guide « Airtightness: the most common defects »