What Does a Duct Air Leakage Test Actually Tell You About HVAC System Performance?
Duct air leakage testing is increasingly specified on Saudi commercial building projects, required by the Saudi Building Code, and referenced in green building certification programmes including LEED and Mostadam. But the number required to pass a test — the leakage percentage, the CFM value, the m³/h·m² figure — is rarely explained to the building owners, facility managers, and even consultants who receive the test report. A figure like ‘4.2% of design airflow at 25 Pa’ is meaningful to a commissioning engineer. To a building owner or asset manager, it is an abstract number without context.
This article explains what a duct air leakage test actually measures, what the result tells you about your HVAC system’s performance, how to interpret the number you receive, and what actions different results should trigger.
What the Test Measures
A duct air leakage test — also called duct pressure testing or duct blaster testing — measures the total volume of air that escapes from a duct system through unintended gaps, unsealed joints, holes, and connection failures when the system is pressurised to a standard test pressure.
The test is performed by sealing all supply and return air terminals throughout the system, connecting a calibrated fan device (a duct blaster) to the system at the air handling unit, pressurising the sealed ductwork to the standard test pressure (typically 25 Pa for low-pressure systems or 250 Pa for medium-pressure systems), and measuring the airflow required to maintain that pressure. Since the only way air can leave the pressurised system is through leakage pathways — all intentional openings have been sealed — this airflow measurement is the system’s total leakage rate.
The result is expressed as a percentage of the system’s design supply airflow, or alternatively as a leakage flow rate in cubic metres per hour, cubic feet per minute, or litres per second. A result of 4% means that when the system is operating at design conditions, 4% of the total supply airflow produced by the AHU leaks out of the duct system before reaching the supply grilles. A result of 20% means one fifth of the conditioned air produced by the system is lost in transit.
What the Result Tells You About System Performance
The leakage percentage is a direct measure of the efficiency of the duct system as an air delivery mechanism. Every percentage point of leakage represents conditioned air that was produced at cost — cooled by the refrigeration system, dehumidified if necessary, filtered, and pressurised by the supply fan — and then delivered nowhere useful. It escaped into ceiling voids, plant areas, and unconditioned spaces rather than reaching an occupied zone.
A system with 4% duct leakage delivers 96% of its AHU output to occupied spaces. A system with 20% duct leakage delivers 80%. The difference in effective cooling capacity is the same as if the 20%-leakage system had a chiller and AHU 25% smaller than the 4%-leakage system — because 25% more capacity is required to deliver the same useful output.
The energy implication is direct: the AHU fan must supply more air than intended to compensate for what is lost in transit. Fan power scales approximately with the cube of airflow rate, so a fan running 20% faster than designed consumes approximately 73% more power than a correctly sized fan at design speed. The refrigeration system must condition additional airflow. And the system must run for longer or at more aggressive set points to maintain occupied space conditions — all of which increases energy consumption above the design estimate.
How to Interpret the Number
The Saudi Building Code specifies a maximum total duct leakage of 4% of design supply airflow for commercial HVAC systems, tested at 25 Pa for low-pressure systems. This is the compliance threshold — the line between a passing and a failing system.
However, the 4% threshold is a minimum standard, not a performance target. It represents the maximum acceptable leakage, not the leakage level that optimises system performance. High-performance buildings targeting LEED Gold or Platinum certification, or buildings designed to net-zero energy standards, typically specify 2% or less. Some critical facilities — cleanrooms, pharmaceutical manufacturing, and laboratory environments — may specify 1% or less for the systems serving their most sensitive spaces.
When you receive a duct leakage test report, compare the result to three benchmarks: first, the SBC compliance threshold of 4%; second, any project-specific leakage target specified by your MEP consultant; and third, your own operational experience — if the system has been commissioning well and occupant complaints are minimal, a result at or below 4% confirms the system is performing as expected. If the system has been struggling to maintain setpoints despite apparently functioning equipment, even a technically passing result at 3.8% warrants investigation of other contributing factors.
What Different Results Should Trigger
Result of 0–4%: Pass
The system meets the SBC threshold. No mandatory remediation is required. If the building is experiencing performance problems despite a passing result, the cause is elsewhere — investigate the building envelope, terminal distribution, or commissioning baseline.
Result of 4–10%: Moderate Leakage
The system fails the SBC threshold. Leakage at this level typically produces noticeable performance impacts: some zones running warmer than designed, HVAC equipment operating at elevated loads, energy consumption above budget. Aeroseal internal duct sealing can reliably bring systems in this range to below 2% in a single treatment session. Remediation is strongly recommended and in many cases contractually required for SBC compliance.
Result of 10–25%: Significant Leakage
Significant leakage that substantially impairs system performance. At this level, the system cannot maintain design airflows to all served zones simultaneously, equipment is running at sustained elevated load, and energy consumption is materially above design. The HVAC equipment may appear to be undersized or underperforming — a common misdiagnosis that leads to unnecessary equipment replacement. Aeroseal internal duct sealing is highly effective at this leakage level and produces dramatic performance improvements. Before any equipment replacement is contemplated, this level of duct leakage must be addressed.
Result above 25%: Severe Leakage
Severe leakage that fundamentally compromises the duct system’s function as an air delivery mechanism. At this level, it is unlikely that the system has ever performed as designed. Investigation should include robotic CCTV inspection to identify structural failures — collapsed sections, disconnected joints, missing access panel covers — in addition to Aeroseal sealing for the distributed leakage. Severe leakage can also indicate incorrect system design (duct system operating at pressures above its construction class rating) or systematic construction failures requiring targeted manual repairs before internal sealing is applied.
What the Test Does Not Tell You
A duct leakage test tells you the total leakage of the system — it does not tell you where the leakage is located. For commissioning and remediation planning, this location information is valuable. Aeroseal Arabia combines duct leakage testing with robotic CCTV inspection to produce both the quantified leakage result and a visual record of the duct system’s condition, including visible leakage points, structural damage, and contamination — giving building owners and facility managers the complete picture they need to make informed remediation decisions.
Conclusion
A duct air leakage test result is not just a compliance number — it is a direct indicator of how efficiently your HVAC system is delivering the conditioned air it produces, how hard your equipment is working to compensate for distribution losses, and how much energy is being spent conditioning air that never reaches the spaces it was intended to serve. Aeroseal Arabia provides RetroTec-certified duct leakage testing and, where results indicate remediation is needed, Aeroseal internal duct sealing that reliably achieves SBC-compliant leakage levels in a single treatment session. Contact our team to arrange testing and performance assessment for your building.
