How to achieve blower-door airtightness below 1.0 ACH in an Adelaide build

The airtight layer is a continuous plane through the building fabric that prevents air from moving from inside to outside. It is not one product; it is a design strategy that assembles multiple products into a continuous barrier.

Three common airtight layer strategies work in Adelaide. First, an internal airtight membrane, typically a vapour-permeable membrane behind the plasterboard on all external walls and ceilings. This is the most common approach and gives good access for later services penetrations.

Second, an external airtight layer using a wrap membrane with taped joints, sitting outside the structural frame but inside the cladding cavity. This works well with lightweight framed construction and is often the choice for prefab or panellised builds.

Third, a concrete airtight layer on masonry construction, where the concrete or masonry itself becomes the airtight plane, with detailed sealing at all penetrations and joints.

Choose the strategy at concept stage. Retrofitting airtightness later is extremely difficult. The strategy must be reflected in every architectural and structural drawing.

Penetrations through the airtight layer are where air leakage happens. Services penetrations (electrical cables, plumbing pipes, HVAC ducts), window frames, door frames, skylight frames, and any other item that crosses the airtight layer must be detailed at design stage.

For services penetrations, use dedicated airtight service grommets or specific airtight sealants approved for the membrane in use. Standard silicone or expanding foam is not sufficient; both fail over time.

For window and door frames, the airtight seal is between the frame and the wall structure, not on the frame itself. A high-performance airtight tape (typically an acrylic tape rated for construction sealing) is used to bridge the frame to the airtight membrane. The detail must show the tape width, the substrate preparation, and the overlap sequence.

For skylights, the airtight detail extends around the entire perimeter of the shaft, including the interface with the ceiling and roof airtight layers. Skylights are the highest-risk penetration for airtightness; some Passivhaus designs deliberately eliminate skylights to reduce this risk.

The single most important quality control step in a Passivhaus build is the mid-construction blower door test conducted at the framing lockup stage, before internal linings are installed.

At this stage, the airtight layer is complete and visible. Any leakage points can be located precisely (using a smoke pencil or thermal imaging) and remediated before they are covered. Fixing a leak at framing lockup takes minutes; fixing the same leak after linings are installed can take a day.

The mid-construction test should be conducted by an independent Passivhaus-accredited technician, not the builder. Expect fees in the low thousands for the test. The test report becomes an interim quality record and evidence of specification compliance.

If the mid-construction test fails to hit the target (typically 0.5 to 0.7 ACH at this stage, targeting 0.6 or better at final test), do not proceed to lining installation until the leaks are remediated and a follow-up test confirms improvement.

The final blower door test is conducted at practical completion, before handover. It confirms the achieved airtightness of the completed home.

The final test should be a contract completion condition. If the airtightness target is not met, practical completion is not certified. This is the single most important contract clause on a Passive House project.

The clause should specify the target ACH at 50 pascals, the reference volume calculation method, the accredited test provider, and the remediation approach if the test fails. Discuss this clause with a building contract lawyer before signing.

Some builders resist tying practical completion to airtightness. This resistance is a strong indicator that the builder is not confident in delivering the number. A qualified Passivhaus builder will welcome the clause because it aligns incentives with quality.

Airtightness is achieved through hundreds of small decisions during construction. No single decision is critical; the aggregate discipline is what determines the final number.

Weekly site meetings should include an airtightness review. Which penetrations are complete? Which are outstanding? Are the tapes correctly applied? Are the membrane laps sufficient? A five-minute review per week catches most issues.

Include airtightness in the builder subcontractor briefing. Every trade that penetrates the airtight layer (electrician, plumber, HVAC installer, cabinet-maker) must understand the airtight discipline. A single unbriefed trade can undo weeks of careful membrane work.

Photograph every penetration before and after airtight sealing. This creates a documentation trail and creates accountability during construction. It also gives the certifier a record for the certification package.

The final principle is patience. Airtightness cannot be rushed. If the target ACH is critical, schedule buffer weeks into the program for remediation. Most successful Passivhaus builds in Adelaide use this buffer at least once.