Air Sealing Testing and Verification — Air Permeability and Smoke Control Standards

A building can look complete while still leaking quietly through its hidden edges.

That is the awkward truth behind air sealing. Drawings may be neat, the finishes may be sharp, and the services may appear resolved, yet the building’s real performance is decided at the joints: around penetrations, at slab edges, above ceilings, behind risers and where one construction package hands over to another. Air escapes there. Heat is lost there. Smoke will often look for the same weaknesses. This is why testing and verification matter. Not as a bureaucratic flourish at the end of the job, but as proof that the hidden line of control in the building has actually been built.

Approved Document F defines air permeability as the measure of airtightness of the building fabric. For buildings other than dwellings, it is expressed as the air leakage rate per square metre of envelope area at a reference pressure difference of 50 pascals. Approved Document L then treats airtightness as part of the route to reducing unwanted heat loss. Together, they make it plain that air leakage is not a vague comfort issue. It is a measurable performance matter. 

Air Permeability Testing Is About Evidence, Not Optimism

Air sealing only becomes meaningful when the building is tested.

That is the useful severity of air permeability testing. It replaces assumption with measurement. The building either performs at the required level or it does not. Approved Document L for buildings other than dwellings sets out airtightness as part of energy performance compliance, while Approved Document F provides the underlying definition of air permeability used in this context. The point is simple enough: uncontrolled air leakage undermines energy efficiency, and the only credible way to verify the envelope is to test it. 

This is where many defects reveal themselves. The weakness is rarely in the middle of a well-formed wall. It is at interruptions: service penetrations, perimeter interfaces, access points, void edges and abutments. Those are the places where the line of airtightness becomes dependent on workmanship rather than specification language. A building may therefore fail its air permeability target not because the design principle was unsound, but because the junctions were treated casually.

Testing is useful precisely because it is unsentimental. It tells you whether the building is holding its air where it should.

Smoke Control Demands a Different Discipline

Airtightness and smoke control are not the same thing, but they occupy related territory.

Approved Document B requires concealed spaces to be designed so that unseen spread of fire and smoke is inhibited, and it requires joints, imperfect fits and service openings in fire-separating elements to be properly fire-stopped so fire resistance is not impaired. That is a fire safety requirement, not an energy one. But it points to the same physical reality: gaps and weak junctions create routes of movement through the fabric. 

Where a building includes dedicated smoke and heat control systems, the relevant standards move beyond general airtightness and into system performance. The BS EN 12101 series covers smoke and heat control systems, including smoke barriers, natural and powered smoke and heat exhaust ventilators, smoke duct sections and pressure differential systems. BSI’s current listings also show BS EN 12101-13:2022 as the standard covering pressure differential systems, including design and calculation methods, installation, acceptance testing, routine testing and maintenance. 

That matters because smoke control is not simply a matter of making a building “tight.” Some systems rely on controlled air movement, pressure relationships and defined leakage characteristics. The objective is not blanket sealing at all costs. The objective is controlled behaviour under fire conditions.

The Overlap Happens at the Weak Points

The overlap between air sealing verification and smoke control standards appears where buildings are most vulnerable: penetrations, shafts, ceiling voids, wall heads and compartment junctions.

A poorly resolved service opening can increase air leakage under test and also weaken the fire strategy by creating a smoke route or compromising a fire-separating element. A badly formed riser line can undermine energy performance on an ordinary day and smoke control performance on a very bad one. This does not mean an air permeability test is a substitute for smoke control commissioning, or that smoke control testing proves envelope airtightness. It means the same defects can disturb both systems for different reasons.

That distinction needs protecting. General airtightness testing checks whole-building leakage performance against energy-related targets. Smoke control verification checks whether smoke control components and systems perform in the way the fire strategy requires. The standards are related through building physics, but they are not interchangeable. Approved Documents B, F and L point to this from different angles, while the BS EN 12101 family deals with the smoke-control-specific side of the matter. 

The practical lesson is straightforward. If a building is leaking carelessly, there is a fair chance its hidden junctions deserve closer scrutiny for more than one reason.

Verification Must Be Planned, Not Improvised

Good verification is not a final-minute ritual performed after the building has already disappeared behind finishes.

It should begin with identifying the control lines that matter: the airtightness line, the compartment lines, the smoke-control-related enclosures and the penetrations that interrupt them. Those details then need inspecting before they vanish behind ceiling grids, plasterboard, casings and cladding. By the time a failed test arrives at the end of the programme, the building is already charging interest on earlier neglect.

The sensible approach is layered. Test the building envelope for air permeability. Commission smoke control systems against the relevant standards and the project fire strategy. Check that fire-stopping and cavity barrier details are continuous where they must be. Treat abnormal leakage results, failed smoke control behaviour or unexplained pressure issues as signs that the hidden construction may not be as coherent as the drawings assumed.

That is the real value of testing and verification. It restores honesty to the building.

Air sealing is not just about warmth and energy bills. Smoke control is not just about fans, shafts and automatic openings. Both depend on the discipline of hidden construction. Both fail at the gaps. And both require proof, not confidence, before anyone should call the building complete.

Air Sealing Manual