Models need real leakage

Energy codes can be satisfied through prescriptive, trade-off or performance pathways, but all eventually rely on assumptions about how the enclosure behaves. Thermal transmittance is commonly modelled in detail, while air leakage is sometimes left as a generic default. Saba argues that this weakens the connection between design and measured performance because uncontrolled infiltration can add heating and cooling loads that insulation values alone cannot explain.

Continuity crosses interfaces

Airtightness is not created by the centre of a membrane. It is created at transitions between wall types, windows, roofs, foundations and penetrations. Drawings therefore need a continuous air-control layer that can be traced around the entire building, with details showing how materials connect and how movement is accommodated.

Testing creates evidence

Whole-building airtightness testing quantifies the leakage rate under a controlled pressure difference. The result can be compared with the design target and used to identify whether further investigation is required. Diagnostic tools such as smoke, infrared imaging or local pressure testing can then help locate significant paths.

Leakage can outweigh insulation

Saba uses energy modelling to compare two familiar improvement paths: adding exterior insulation and reducing air leakage. The case study shows diminishing returns as insulation thickness increases, while a realistic improvement in airtightness can produce a larger change in annual energy use. The exact relationship depends on climate, building size and mechanical systems, but the exercise makes a broader point. Air leakage should not remain an invisible default in the energy model when it can be designed and measured. Company test data reinforces the same message. Buildings using similar air-barrier strategies still show a range of leakage rates, demonstrating that material selection alone does not determine the result. Detailing, trade coordination and installation quality remain decisive. An ambitious airtightness target therefore has to be paired with a construction plan capable of delivering it.

Drawings become site conditions

The familiar exercise of tracing the air barrier with a pen is useful because it forces every transition to be acknowledged. Roof-to-wall joints, foundations, intermediate floors, windows, doors and penetrations are the places where the line changes material or passes between trades. Those are also the places where site review should concentrate before they disappear behind finishes. Whole-building testing at the end provides a quantitative result, but late discovery is expensive. Local chamber tests, mock-ups, smoke and infrared investigation can be used earlier to find discontinuities while they are repairable. Saba's message is that airtightness becomes predictable when the target, details, inspections and final test are planned as one sequence. The number in the energy model then represents an engineered property rather than an optimistic assumption.