What is a Blower Door airtightness test and how can it improve my building?
A Blower Door test is used to measure the airtightness of a building by quantifying its air leakage. The test involves using a large fan, temporarily mounted in an exterior doorway or other such opening, to either pressurize or depressurize the building.
What is a Blower Door airtightness test and how can it improve my building?
What is a whole building Blower Door airtightness test?
A Blower Door test is used to measure the airtightness of a building by quantifying its air leakage. The test involves using a large fan, temporarily mounted in an exterior doorway or other such opening, to either pressurize or depressurize the building. Sensors then measure how much air leaks through the cracks, gaps, and joints in the building envelope, giving an accurate picture of how well the structure prevents uncontrolled air infiltration or exfiltration.
Additionally, various tools can be used to identify exactly where air leaks are occurring so they can be corrected and sealed: these include handheld (generally hot wire) anemometers, smoke generators, thermographic camaras, and (our favourite and most high-tech sensor) a large feather, taped to an extendable fibre-glass fishing rod for reaching inaccessible spots where leaks are otherwise difficult to detect.
ISO 9972:2015 “Thermal performance of buildings – Determination of air permeability of buildings – Fan pressurization method” is the international norm that establishes the procedure for going about doing a Blower Door test and taking the measurements, although national buildings codes and specific building certification systems (such as Passivhaus) sometimes include variations on the norm, or require a specific methodology for calculating the internal air volume.
The test results are typically expressed according to one of the two following metrics, that can be used to assess the airtightness of buildings consistently across different types and sizes:
n50: the number of air changes per hour of air leakage at a pressure difference of 50 Pascals (ACH50), referenced to the internal air volume of the building. This is probably the most common airtightness metric used. How the internal air volume is calculated is of course fundamental to the n50 result and must be clearly stated in the test report. ISO 9972 requires that the gross internal dimensions should be used and that “In general, the interior dimensions should be used to calculate this volume […] The volume of the interior floors or walls should not be subtracted. The volume of the interior cavities of the building envelope must not be subtracted”. Passivhaus, in contrast, requires that suspended ceilings and service voids are deducted from the calculation.
qe50: with this metric the air leakage is expressed according to the thermal envelope area, in units of m3/h·m2 (the m2 being the envelope area that encloses the heated and/or cooled space of the building). Again, the envelope area calculation is fundamental to the qe50 result and how it’s been calculated needs to be stated in the test report. ISO 9972 states it should be calculated according to internal dimensions, whereas Passivhaus typically uses external envelope dimensions.
Test methods according to ISO 9972
The norm states that there 3 methods for testing the airtightness of a building. Here too, the method fundamentally influences the end result and needs to be clarified before the test and in the final test report.
The 3 methods and their differences can be found here:
| Method 1 | Method 2 | Method 3 | |
| Classification of openings in the building | Building in use | Building envelope | Specific purpose |
| Natural ventilation openings | Closed | Sealed | Closed, sealed, or open, as directed |
| Mechanical ventilation system openings (continuous use) | Sealed | Sealed | Closed, sealed, or open, as directed |
| Mechanical ventilation system openings (intermittent use) | Closed | Sealed | Closed, sealed, or open, as directed |
| Exterior windows, doors & hatches | Closed | Closed | Closed, sealed, or open, as directed |
| Openings not designed for ventilation | Closed | Sealed | Closed, sealed, or open, as directed |
Feeling the pressure? The process of a Blower Door Test!
1. Preparation: All external doors, windows, and vents are closed, except for the doorway or window where the fan is mounted. All internal doors must be opened to allow for a consistent pressure throughout the building, that mustn’t exceed +/- 10 % in any part of the building. For larger and particularly tall buildings, fans are often required at two or more points in the building to make sure pressure is equalised, and to counteract wind and internal air movement due to buoyancy. It’s generally preferable to test early in the morning, when there tends to be less wind, and to locate the testing equipment in the lee side of the building, where it is more protected from wind.
2. Pressurisation/Depressurisation: The fan blows air into or out of the building, creating a controlled pressure difference (in “cruise mode” this will usually be 50 Pa) between the interior and exterior. This difference replicates typical outdoor conditions such as wind or temperature fluctuations. 50 Pa of pressure difference is roughly equivalent to a 30 km/h wind. ISO 9972 states you can test either by pressurise or depressurise for a valid test. Passivhaus certification requires that you do both, with the final result being the average of the two. This is because there may, in practice, be both infiltration and exfiltration occuring in a building, often at the same time: the buoyancy of relatively warm air makes it rise, causing exfiltration on the upper floors, and drawing air into the lower floors (infiltration).
3. Measurement: As the fan operates, sensors measure the airflow required to maintain the pressure difference, which directly correlates with the volume of air escaping through leaks in the building envelope. For the final test, ISO 9972 requires that measurements are taken at a minimum of 5 different pressure intervals, with no more than 10 Pa at each interval, going from a minimum of 10 Pa (or at least 5 times the baseline pressure), to a maximum pressure of over 50 Pa for residential buildings and 25 Pa for non-residential.
4. Data Analysis: the data collected during the test is analysed to calculate the building’s n50 or qe50 value, indicating the air changes per hour or flow rate per metre squared of envelope. A lower n50 or qe50 score indicates a more airtight building. Results can also be expressed in terms of ELA, or Effective Leakage Area, which is the area of a theoretical hole which would exhibit same leakage as the building’s actual holes at a 4 Pa pressure difference.
Advantages of conducting a Blower Door Test
1. Improved Energy Efficiency: A Blower Door test identifies the specific areas where air is escaping, allowing builders and homeowners to address leaks. By sealing these leaks, the building’s overall energy consumption can be significantly reduced, lowering heating and cooling costs and contributing to long-term energy savings. A high level of airtightness must always be accompanied by a controlled mechanical ventilation system.
2. Enhanced Indoor Air Quality (IAQ): Air leaks can allow pollutants, allergens, and even moisture to enter the building from the outside. These unwanted infiltrants can compromise indoor air quality, especially in urban or industrial areas. An airtight building ensures that ventilation systems can filter and supply fresh air as needed, rather than relying on uncontrolled air infiltration.
3. Increased thermal and acoustic comfort: Drafts from air leaks can create cold spots and temperature fluctuations, impacting the comfort of occupants. By reducing leaks, a Blower Door test helps create a more consistent indoor environment that remains comfortably warm in winter and cool in summer. Airtight buildings also tend to be quieter, as they prevent sound from traveling through cracks in the envelope.
4. Compliance with building standards and certifications: Many building standards and green certifications, such as Passivhaus, LEED, and BREEAM, require a Blower Door test as part of the building performance verification process. Achieving a high level of airtightness is essential for meeting these standards and can also contribute to increased property value and recognition in the market.
5. Moisture damage and durability of the building structure: Uncontrolled air leaks can allow moisture to enter, which can lead to condensation within walls or roofs. Over time, this moisture can cause mold growth, wood rot, and structural damage. A Blower Door test helps prevent these issues by ensuring that the building envelope is properly sealed against unwanted air and moisture.
Conclusion
A Blower Door test is more than just a diagnostic tool; it’s an investment in the efficiency, comfort, and durability of a building. By identifying and sealing air leaks, building owners can enjoy lower energy bills, healthier indoor environments, and increased structural longevity. In addition, this test is an essential step for projects aiming to meet strict building standards and achieve sustainable certifications.
For anyone involved in the design, construction, or maintenance of high-performance buildings, a Blower Door test is an invaluable step in quality assurance. Whether it’s for new construction or retrofits, the benefits of knowing—and controlling—air movement in a building make this test a valuable asset in sustainable building practices.