What Are Common Reasons a House Fails an Airtightness Test?
The blower door fan comes out of the door frame, the tester hands over the result, and the number is higher than you needed. Now what?
The first thing to understand is that airtightness test failures are almost never random. The same leakage pathways appear on site after site — not because builders don't care, but because certain junctions are genuinely difficult to seal reliably using standard NZ construction methods, and because the airtightness layer is often an afterthought rather than a designed system. This article covers the most common failure points, why they leak, and what to do about it.
A "failed" airtightness test is only meaningful relative to a target. Most NZ homes have never been tested and would score well above 10 m³/(h·m²) at 50 Pa. Whether your result is a problem depends entirely on what you were trying to achieve.
First: What Does "Failing" Actually Mean?
There is no universal pass/fail airtightness requirement in New Zealand for standard residential construction. Whether a result represents a failure depends on the programme or specification the building is being tested against:
| Programme / Requirement | Metric | Target |
|---|---|---|
| Passive House Classic (PHI) | n₅₀ (ACH at 50 Pa) | ≤ 0.6 h⁻¹ |
| EnerPHit (retrofit) | n₅₀ | ≤ 1.0 h⁻¹ |
| Low Energy Building (PHI) | n₅₀ | ≤ 1.0 h⁻¹ |
| Green Star NZ (airtightness credit) | q₅₀ (m³/h·m²) | Varies by credit level; typically ≤ 3–5 m³/(h·m²) |
| Contract specification (private) | Varies | As specified — confirm in writing before testing |
| NZBC H1 (reference building) | n₅₀ | 5.0 h⁻¹ (reference building assumption used in energy modelling) |
For Passive House certification, 0.6 ACH₅₀ is a hard limit — the building cannot be certified without meeting it. For Green Star and private specifications, the consequences of missing the target depend on what was agreed. Either way, knowing which target you're working toward before construction starts is essential — the strategies for achieving 5 m³/(h·m²) and 0.6 ACH₅₀ are very different.
The Most Common Leakage Pathways in NZ Construction
These are listed in approximate order of how often they contribute significantly to a high result. In a typical lightweight timber frame house, several of these will be present simultaneously.
1. The ceiling-wall junction and top plate penetrations
In platform frame construction, the top plate of external walls is where electrical cables, plumbing pipes, and framing voids connect the conditioned interior to the roof cavity — which is typically outside the thermal and airtightness envelope. This junction is one of the single largest contributors to air leakage in NZ homes. Cables and pipes pass through holes drilled in the top plate; unless these are individually sealed with acoustic sealant or expanding foam, each one is a direct air path.
The fix is sealing at the top plate level — before ceiling insulation is installed and before ceiling linings go on. Once linings are up, this junction is inaccessible without destructive work.
2. Recessed downlights
Each standard recessed downlight is a circular hole cut through the ceiling lining — the primary airtightness layer in most NZ homes — into the roof void. Even with a fitting in place, there is typically a gap between the fitting body and the ceiling lining, and the fitting itself is not airtight. A house with twenty downlights may have the equivalent of a 200 mm open hole in its ceiling.
Solutions: airtight downlight covers installed above the ceiling before insulation is laid; surface-mounted LED fittings; or recessing into a bulkhead that keeps the ceiling plane intact. Retrofitting airtight covers from below is possible but not always effective.
3. Window and door frame perimeter — the lining-to-frame gap
The junction between the window or door frame and the interior lining is a consistent leakage point. Standard practice in NZ construction is to fill this gap with expanding foam or backing rod and sealant, then cover with an architrave. The problem is that foam alone is not reliably airtight, and the architrave covers the gap without sealing it.
In high-performance construction, an airtight connection is made between the window frame and the airtightness membrane (typically the interior face of the wall) using flexible tape before linings are installed. This is a construction sequencing issue as much as a materials one — once the architrave is on, the opportunity is gone.
4. The floor-wall junction in suspended timber floor construction
Where a suspended timber floor meets an external wall, there is typically a complex framing junction — bearer, joist, blocking, bottom plate — through which air can travel from the subfloor (which is outside the conditioned envelope) into the wall cavity and from there into the living space. This pathway is difficult to seal because it runs continuously around the perimeter of the building and is often inaccessible from both above and below once construction is complete.
In high-performance builds, this junction is addressed at the design stage — typically by running an airtight membrane continuously from the floor into the wall and detailing the junction explicitly. It cannot be reliably fixed as a retrofit from inside.
5. Exhaust fan bodies and duct penetrations
Bathroom and kitchen exhaust fans penetrate the ceiling or wall airtightness layer. Even with the external termination sealed for the test, the fan body itself may have gaps where it sits in the ceiling cutout. Duct penetrations through external walls or ceiling planes are also frequently left unsealed at the membrane level, even when the duct itself is present.
MVHR and HRV supply and extract terminals are sealed for testing (they are intentional openings with closeable dampers), but the collar where the duct penetrates the airtightness layer must be permanently sealed regardless.
6. Structural penetrations: posts, beams, and cantilevers
Where structural elements pass through the building envelope — a steel post through a concrete floor slab, a timber beam through an external wall, a cantilevered floor joist — there is a junction between two dissimilar materials that is difficult to seal continuously. These are also locations where thermal bridging occurs, so cold surface temperatures make them doubly problematic: they leak air and they create condensation risk.
7. Ceiling access hatches
Attic access hatches are routinely installed with no compression seal and no airtight rebate — just a flat panel sitting in a frame. Under 50 Pa of pressure, a poorly fitted hatch contributes disproportionately to the total leakage result given its area. An airtight hatch with a compression gasket is a simple and inexpensive specification change that makes a measurable difference.
8. Services penetrations — electrical and plumbing
Individual electrical cables and plumbing pipes through external walls, floor plates, and ceiling planes are low-leakage individually but add up when there are dozens of them. The most common locations are where cables exit the wall cavity to surface-mounted fittings, at switchboards on external walls, and where plumbing waste pipes exit through the floor into the subfloor. Each requires a sealed collar or sealant at the membrane crossing point.
Why These Failures Keep Happening
The root cause of most airtightness failures is not incompetence — it's that the airtightness layer is rarely designed as a continuous system. In high-performance European construction, the airtightness membrane is drawn on every drawing at every junction, and the site team works to a documented airtightness strategy. In typical NZ construction, airtightness is often assumed to be provided by the general quality of the build rather than by a specific, detailed design.
The result is that each trade seals what they know about and leaves what they don't. The electrician seals the cable entry at the switchboard but not the top plate penetrations. The plasterer fills the window reveals but doesn't connect to the membrane. The insulation installer fills the ceiling but doesn't seal around the exhaust fan body. No single trade is responsible for the whole, and the whole ends up leakier than the sum of its parts.
The solution is an airtightness design — a documented layer that is traced on drawings and assigned to specific trades — combined with a shakedown test during construction before linings cover the junctions that matter most.
What to Do After a Poor Result
If the test result is higher than your target, the next step is leakage detection — not guessing. During the test, with the fan running and the building at 50 Pa, a smoke pencil or handheld anemometer can locate the dominant leakage pathways precisely. This is far more efficient than systematic inspection, and it tells you where the effort will have the most impact.
Once leakage pathways are identified, the decision is whether to seal from inside (limited options once linings are on) or from outside (re-opening cladding or ceiling is expensive but sometimes the only option for major pathways like the floor-wall junction). In many cases, accessible pathways — downlight covers, top plate cable penetrations from the roof void, hatch seals — can recover significant performance without destructive work.
If you're facing a retest under a certification programme, BEO can advise on which interventions are likely to produce the result you need, rather than working through the building systematically and hoping for the best.
Airtightness Testing and Leakage Diagnosis Across New Zealand
BEO Buildingscience provides ATTMA-accredited blower door testing, shakedown testing during construction, and post-test leakage detection for residential and commercial projects across New Zealand. If you have a result you need to understand — or a target you need to meet — start here.
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