Air can pass through openings in a building that are too small to notice during an ordinary inspection. Gaps around windows, incomplete air-barrier transitions, service penetrations and poorly sealed joints can collectively allow a substantial amount of uncontrolled airflow.
That leakage may increase heating and cooling demand, create drafts and make it harder for the mechanical system to maintain stable indoor conditions. The challenge is that airtightness cannot be judged reliably by looking at the building envelope. It must be measured.
ASTM E779 provides a standardized method for doing that. It allows a testing team to create controlled pressure differences across the building envelope and measure how much air must be moved to maintain each pressure.
What Is ASTM E779?
ASTM E779 is formally titled Standard Test Method for Determining Air Leakage Rate by Fan Pressurization. The current active edition is ASTM E779-19.
The test is intended to quantify the airtightness of a completed building envelope. One or more calibrated fans create a pressure difference between the interior and exterior. The airflow needed to maintain that pressure is then measured.
A tighter building requires less fan airflow to maintain the selected pressure. A leakier building requires more.
ASTM E779 is primarily written for buildings that can be tested as a single pressure zone. Some buildings containing several rooms or areas may still be treated as one zone when interior doors are opened or adjacent spaces are maintained at similar pressures.
For commercial, multi-family and other complex projects, professional Large Building Airtightness Testing helps project teams determine the appropriate test boundary, fan capacity, building preparation procedure and reporting method before the test begins.
It is important to understand that ASTM E779 describes how to measure air leakage. It does not establish one universal airtightness target that every building must meet. The acceptable result normally comes from the project specification, building code, energy model or applicable performance standard.
How Does an ASTM E779 Test Work?
An ASTM E779 test is more involved than installing a fan and recording a single airflow number. The testing team must first define what part of the building is being evaluated and establish consistent conditions across the test boundary.
Preparing the building
Exterior windows and doors are generally closed. Intentional openings may need to be sealed, left open or placed in a defined operating condition, depending on the project requirements.
The testing team also reviews mechanical systems, exhaust fans, dampers, combustion equipment and temporary construction openings. Interior doors may be opened so the test area behaves as a single pressure zone.
Building preparation matters because an open damper or temporary unsealed penetration can alter the result. The final report should therefore document how the building was configured during testing.
Weather must also be considered. Strong wind can create unstable pressure across different elevations and sides of a building. A large difference between indoor and outdoor temperatures may produce stack-effect pressures that interfere with the fan-induced pressure. ASTM E779 is intended for relatively low-wind conditions and small indoor-to-outdoor temperature differences.
Installing calibrated fans
Calibrated fans are temporarily installed in an exterior doorway, window opening or another suitable location.
A small building may require one blower-door fan. A larger building may need several fans operating together. The equipment must have enough capacity to reach the required test pressures while accurately measuring the airflow passing through the fans.
Pressure gauges and reference tubes measure the difference between indoor and outdoor pressure. In taller buildings, pressure may need to be monitored at several locations because wind and stack effect do not influence every floor equally.
Taking readings at several pressures
The fans either remove air from the building or supply air to it.
Removing indoor air creates a depressurization test. Supplying outdoor air creates a pressurization test. Depending on the testing plan, the building may be tested in one direction or both.
Instead of relying on one measurement, the technician records airflow at several pressure differences. These readings show how leakage airflow changes as pressure increases. The data can then be fitted to an airflow-pressure relationship and used to calculate the building’s leakage rate at the required reference pressure.
A Simple Example of the Test
Imagine a newly constructed apartment building with hundreds of exterior-wall joints, windows, balcony connections and mechanical penetrations.
From the ground, the façade may appear complete. Individual gaps may be too small to see, yet air can still pass through many of them at once.
During the test, several calibrated fans are installed at exterior openings. The building is first depressurized. As indoor pressure falls below outdoor pressure, air begins entering through leakage paths across the envelope.
The fans must continuously remove this incoming air to maintain the required pressure difference. The testing equipment measures the airflow passing through the fans.
Suppose the first test requires much more airflow than the project’s specified airtightness limit allows. That result does not automatically identify the faulty window or wall transition. It shows that the total leakage across the test boundary is too high.
The project team can then use diagnostic methods such as smoke, thermal imaging or targeted pressure testing to investigate likely leakage locations. After repairs are completed, another whole-building test can determine whether the overall leakage rate has been reduced.
The fans do not simulate ordinary weather perfectly. Their purpose is to create a stable, measurable pressure condition so the building envelope can be assessed consistently.
What Do ASTM E779 Results Mean?
An airtightness result must include both an airflow rate and a reference pressure.
A report might state the airflow needed to maintain 50 Pa or 75 Pa. Reporting airflow without the pressure would be incomplete because the same building will leak more air at a higher pressure difference.
Common reporting formats include:
- Cubic feet per minute at a stated pressure, such as CFM50
- Litres per second at a stated pressure
- Air changes per hour at 50 Pa, commonly written as ACH50
- Airflow normalized by the area of the building envelope
The appropriate metric depends on the project. ACH50 is common in residential testing because it relates airflow to the building’s internal volume. Large-building specifications often use leakage airflow divided by envelope area at a reference pressure.
Normalization is important when comparing buildings of different sizes. A large building may have a higher total airflow than a small building while still having a better leakage rate per square metre of enclosure.
Does ASTM E779 define a passing result?
No. ASTM E779 provides the measurement procedure rather than a universal pass/fail limit.
A project may have an airtightness requirement established by:
- A building code or municipal policy
- An energy-performance program
- The owner’s project requirements
- A consultant’s specification
- A building-envelope commissioning plan
Before testing, the project team should confirm the required metric, reference pressure, envelope-area calculation and allowable leakage rate. Without those details, a technically valid test result may still be difficult to compare with the project target.
What ASTM E779 Does Not Measure
ASTM E779 measures leakage under an imposed pressure difference. It does not directly measure the amount of outdoor air entering the building during normal operation.
Actual infiltration changes with wind, temperature, mechanical-system operation and the location of leakage openings. ASTM identifies tracer-gas testing under ASTM E741 as a method for directly measuring air-change rates.
An ASTM E779 result also does not automatically provide:
- The exact location of every leak
- A ventilation-system airflow measurement
- A duct-leakage result
- A universal energy-savings estimate
- Proof that every potentially harmful leakage path has been sealed
Whole-building testing measures the combined performance of the test boundary. Separate diagnostic procedures may be needed to locate individual leakage sites.
ASTM E779 and Related Airtightness Standards
Several ASTM standards address building air leakage, but they do not all serve the same purpose.
ASTM E779 and ASTM E1827
ASTM E1827 covers airtightness testing using an orifice blower-door system. ASTM E779 provides a fan-pressurization method for determining the leakage rate of a building envelope.
The project specification, equipment configuration and building type may determine which standard is referenced.
ASTM E779 and ASTM E3158
ASTM E3158 is specifically intended for large or multizone buildings and portions of those buildings. Its current active edition is ASTM E3158-24.
It includes procedures for defining test boundaries, preparing building-envelope or operational-envelope conditions and testing large buildings using fan-induced pressures. It also addresses multipoint, repeated single-point and repeated two-point procedures.
ASTM E779 may remain relevant when a building can be evaluated as a single zone or when it is explicitly required by the project documents. For a large or genuinely multizone building, however, the testing consultant should review whether ASTM E3158 is the more appropriate primary method.
ASTM E779 and ASTM E1186
ASTM E779 quantifies total leakage across the test envelope. ASTM E1186 addresses practices for locating air-leakage sites in building envelopes and air-barrier systems.
The two can support different phases of the same investigation: one measures the overall leakage rate, while the other helps diagnose where air may be passing through the enclosure.
When Is ASTM E779 Testing Used?
ASTM E779 testing may be used during new construction, before occupancy or after an enclosure retrofit.
For a new project, the test can verify whether the constructed envelope meets the specified airtightness requirement. When testing occurs early enough, the team may have an opportunity to correct leakage before finishes, ceilings or other components restrict access.
For an existing building, testing may help evaluate current envelope performance or measure improvement after air-sealing work. It can also provide inputs for energy analysis when the limitations of the test are properly understood.
Typical applications include:
- Building-envelope commissioning
- Airtightness verification required by project documents
- Performance testing before substantial completion
- Investigation of unexpected drafts or heating demand
- Comparison of pre-retrofit and post-retrofit leakage
- Quality-control testing for high-performance buildings
Testing should be planned rather than treated as a last-minute inspection. Large buildings may require substantial fan capacity, temporary access arrangements, coordination with mechanical contractors and control of doors or elevators during the test.
What Should Be Confirmed Before the Test?
Project teams should resolve several questions before scheduling airtightness testing:
- Which ASTM standard and edition does the specification require?
- What area of the building forms the test boundary?
- Which intentional openings must be sealed?
- What is the required reference pressure?
- How is the enclosure area calculated?
- What leakage metric will be reported?
- What is the maximum allowable leakage rate?
- Will the building be pressurized, depressurized or tested in both directions?
- Are diagnostic leak-detection services included?
- What happens if the building does not meet the target?
Answering these questions early allows the testing team to select appropriate equipment and avoids disagreements about the result after the test has been completed.
Plan the Test with a Building-Science Specialist
ASTM E779 provides a consistent way to measure air leakage, but successful testing depends on decisions made before the fans are turned on.
The building boundary must be understood. Temporary openings must be addressed correctly. The selected method must suit the size and zoning of the building. Most importantly, the result must be compared with a clearly defined project requirement.
For guidance on an upcoming airtightness test, project teams can consult Shahin Arvandi, Founder of Monolith Housing Solution and a Senior Energy and Building Science specialist with more than 15 years of experience in energy-efficient construction, commissioning and high-performance building design.
Early consultation can help establish the correct testing standard, required fan capacity, site preparation responsibilities and reporting expectations before they affect the construction schedule.
Frequently Asked Questions
Is ASTM E779 the same as a blower-door test?
ASTM E779 uses calibrated fan-pressurization equipment, commonly associated with blower-door testing. On a large building, the setup may involve several fans installed in multiple openings rather than one residential blower door.
Does ASTM E779 locate individual air leaks?
Not by itself. The test quantifies total air leakage across the defined envelope. Smoke, thermal imaging and other diagnostic methods may be used while the building is pressurized or depressurized to investigate specific leakage paths.
What pressure is used for ASTM E779 testing?
Readings are taken across a range of pressure differences. The final result is then calculated or reported at the reference pressure required by the project. The specification must identify the pressure used for compliance.
Can ASTM E779 be used for a large building?
It may be used when the building can reasonably be treated as a single pressure zone or when the project documents specifically require it. ASTM E3158 is designed specifically for large or multizone buildings, so the appropriate method should be confirmed before testing.
Can wind affect the test result?
Yes. Wind can cause pressure fluctuations across the envelope and make stable readings more difficult. Testing should be scheduled under acceptable weather conditions, with pressure references positioned according to the test plan.
What is a good ASTM E779 result?
There is no single result that is appropriate for every building. Performance must be evaluated against the leakage rate and reporting metric established for the specific project.
Does a low leakage result guarantee that the building has no problem areas?
No. A building can meet an overall leakage target while still containing a concentrated leak at a location that presents a comfort or moisture risk. Overall performance testing and leakage-site investigation answer different questions.
When should airtightness testing be scheduled?
The final compliance test usually occurs when the envelope is complete and the building can be prepared consistently. Earlier diagnostic testing can be valuable while air-barrier transitions remain accessible for repair.
