Airtightness beyond compliance

Airtightness - going beyond compliance

Airtightness has become one of the defining indicators of quality in UK construction. Within the framework of the Building Regulations - particularly Approved Document L - air permeability targets are now embedded in everyday practice. A low Q50 or ACH50 result is often taken as evidence that a building has been carefully designed and well delivered.

But there is an important question worth reflecting on: are we building envelopes that remain airtight, or are we primarily demonstrating compliance at a specific moment in time?

A number achieved on test day tells us something, but not everything. It reflects how a building performs under very specific conditions, at a very specific moment in time. What it doesn’t necessarily tell us is how that building will perform once it is occupied, adapted, and exposed to the realities of use. And yet, airtightness is still too often treated as a compliance milestone rather than a long-term performance question.

That distinction matters. Airtightness is often framed as a milestone at handover, but it is a performance characteristic that needs to be maintained throughout the building’s life.

Airtightness: the gap between passing and performing

In many projects, airtightness is delivered through a combination of design intent and on-site coordination, with details continuing to be refined as construction progresses and in the lead-up to testing.

While this approach can successfully meet test requirements, it does not always guarantee how the building will perform once it is occupied, adapted, and exposed to real conditions over time.

A building can perform well during a pressure test and still contain vulnerabilities at interfaces, service penetrations, and junctions - areas that are inherently complex and often evolve through the construction process. These conditions tend to change over time, responding to movement, environmental factors, and ongoing use, which can gradually affect airtightness performance if not robustly addressed.

Airtightness begins in design

It is tempting to think of airtightness as something that can be resolved on site. In practice, it is largely determined much earlier.

The continuity of the air barrier depends on how junctions are conceived, how materials interact, and how well those ideas translate into construction.

Because airtightness relies on continuity across multiple systems, it benefits from clear definition and coordination from early design through to construction. This shared approach helps support consistent delivery on site.

UK guidance, including CIBSE TM23 and the moisture-related principles within Approved Document C, already position airtightness as part of a wider performance system. It is closely linked to condensation risk, thermal comfort, and indoor air quality. European research reinforces this understanding, suggesting that airtightness performance can change after handover, particularly as buildings begin to be used and adapted.

In this context, airtightness is not just something to be achieved - it is something to be sustained.

From measured performance to built performance

There is a subtle but important distinction between what is measured and what is built. Closing that gap requires a shift in focus.

Rather than concentrating primarily on large, visible surfaces, greater attention is needed at junctions and interfaces, where continuity is most vulnerable. Airtightness is not defined by individual materials alone, but by how effectively they connect and how resilient those connections are over time.

The Passivhaus approach demonstrates this clearly. Airtightness is treated as a clearly defined and continuous layer, coordinated across disciplines and considered from the earliest stages of design. The pressure test becomes a way of verifying that system, rather than compensating for its absence.

Airtightness and ventilation as one system

As buildings become more airtight, the importance of ventilation increases. This is not a separate consideration, but part of the same system.

Reducing uncontrolled air leakage only delivers value when fresh air is introduced in a controlled and reliable way. In many low-energy buildings, this is achieved through mechanical ventilation with heat recovery (MVHR). The relationship is direct: consistent airtightness supports ventilation performance, while effective ventilation ensures good indoor air quality within tighter envelopes. Considering both together leads to more balanced and reliable outcomes.

From craft to control

Traditional airtightness strategies - membranes, tapes, and sealants - remain fundamental to delivery on site. Their success, however, can be influenced by factors such as access, sequencing, and the complexity of installation conditions.

Emerging approaches, including aerosolised sealing systems, offer an additional layer of support. By pressurising the building and introducing a fine sealant mist, these systems can help identify and seal leakage paths that may be difficult to reach manually. Research and case studies indicate their potential to improve consistency, support quality assurance, and provide clearer validation of outcomes.

The significance lies not only in the technology itself, but in the broader shift it represents: from reliance on isolated interventions toward more integrated and measurable delivery processes.

Designing for durability

Airtightness is largely invisible. It exists within layers, interfaces, and construction sequences that are not always apparent once the building is complete.

This is precisely why it benefits from clarity at design stage and coordination during construction. If the air barrier is clearly defined, understood across the project team, and protected throughout the build process, it is far more likely to perform reliably over time.

Durability, in this sense, is not an added feature - it is the outcome of consistent decisions made from concept through to completion.

Redefining success

The UK industry does not need fewer airtightness tests. Testing remains essential. But its role is best understood as verification rather than validation of last-minute improvements.

If airtightness is approached primarily as a target at the end of the programme, there is a risk that short-term optimisation takes precedence over long-term performance. Reframing it as a durability issue helps shift the focus toward more consistent and resilient outcomes.

Ultimately, a building that performs well only at the point of testing is not necessarily a high-performing building. One that maintains its airtightness over time - through use, adaptation, and environmental exposure - is.

Shifting the conversation in this way brings airtightness back to where it belongs: not as a final checkpoint, but as a continuous design and construction priority.

The more meaningful question is not only whether a building was airtight at handover, but how effectively it has been designed and delivered to maintain that performance over time.

Airtightness support for your project

At Henriksen Studio, we offer airtightness support, as part of our Passivhaus design service.

Through the development of airtightness and quality control systems for large scale Passivhaus delivery, our airtightness consultancy can ensure that any building envelope complies with the rigorous standards set by the Passivhaus Institute.

Our expertise in airtightness testing and our ability to identify weaknesses in the building envelope, enables our clients to achieve the desired airtightness targets set by a project - not just for the short term, but for the buildings entire lifetime.

Find out more about our airtightness support and broader Passivhaus design services on our website here.

References

UK Government (2021) The Building Regulations 2010: Approved Document L – Conservation of fuel and power. London: HM Government.

UK Government (2021) Approved Document F: Ventilation. London: HM Government.

CIBSE (2000) TM23: Testing buildings for air leakage. London: CIBSE.

CIBSE (2015) Guide A: Environmental design. London: CIBSE.

BSI Group (2015) BS EN ISO 9972: Thermal performance of buildings – Determination of air permeability of buildings. London: BSI.

ATTMA (2021) Technical Standard L1: Measuring air permeability of building envelopes (dwellings). Northampton: ATTMA.

Passivhaus Institute (n.d.) Passivhaus planning package (PHPP) documentation. Darmstadt: Passivhaus Institute.

Lstiburek, J. (2006) Air flow control in buildings. Westford, MA: Building Science Corporation.

Rose, W.B. (2005) Water in buildings: An architect’s guide to moisture and mold. Hoboken, NJ: Wiley.