Measuring Airtightness of Buildings with a Blower Door Test

Gregory Duncan performing a blower door test to determine the airtightness of a Passive House project under construction in Pennsylvania.

Use a blower door to test the airtightness of a building. For very small buildings or spaces, use a duct blaster.


Pressurization test with air flows due to leaks with negative pressure. Measuring the pressure difference in the building.

source: Passipedia

Building owners benefit from an airtight building envelope because it reduces:

  • water problems
    • a small air leak will allow a lot of moist air to get inside a wall and condense, risking structural damage and mold
  • heat transfer (heat loss in winter and heat gain in summer)
  • environmental tobacco smoke (ETS) nuisance between apartment units
    • involuntary exposure to second-hand smoke is likely to become even more of a liability for landlords in the future
  • transfer of smoke and heated gases from a fire
  • noise transmission
    • if there are air gaps, sound will find a way through the wall or ceiling assembly
    • to minimize noise between occupancies, use the airtight drywall approach (ADA) even for interior partitions

There are two main ways to document the airtightness of a building. One is to take the leakage in cubic meters per hour or cubic feet per minute and divide it by the area of the building envelope. The other method is to determine the number of air changes per hour (ACH) by measuring the leakage and dividing by the air volume. For example a 1000 m3 building with a measured air flow leakage rate of 1000 m3/h would have an airtightness rating of 1 h-1 since 1000 m3/h/1000 m3 = 1 h-1 . Using non-metric units: 583 CFM * (60 min/hr)/35,000 CF = 1 ACH. The Passive House standard requires testing at 50 Pascal while USACE requires 75 Pa. Testing at a higher pressure yields more accurate results, especially for larger buildings. (See the US Army Corps of Engineers Air Leakage Test Protocol PDF.) In order to distinguish between the natural leakage rate and the tested leakage rate at 50 Pa pressure, the airtightness number is noted as n50 or ACH50.

“Build tight and ventilate right” is a popular saying among building science consultants. Proper ventilation is essential for healthy indoor air quality, especially for airtight buildings. Building codes require mechanical ventilation if natural ventilation is not sufficient. Beyond code requirements, relying on open windows for natural ventilation can be problematic due to street noise, dust, rain, and extreme outdoor temperatures. Does a building need to breathe? That’s a confusing metaphor because there are two distinct issues. One is uncontrolled air leakage (bad) and the other is vapor permeability (usually good).

Use a blower door to find air leaks, not just to determine ACH50. Before the drywall goes up, pressurize the building with a blower door and use theatrical smoke, fingers (surprisingly effective), or an infrared camera to find leaks. The first test can be at a higher pressure to make it easier to find leaks. At this point, it might be interesting to determine the ACH50 using the Passive House methodology, but it isn’t really necessary until all the leaks have been fixed. In the US and the UK, count on three or four iterations of testing and sealing to get below the limit of 0.6 ACH50. In Austria and Germany, where Passive House construction methods are more standardized, they often just do one blower door test towards the end.

A guide to Volume Calculations for Passivhaus Air Tightness Testing and the Difference with the UK Method

Blower Door Basics

Email greg@duncanarchitectpllc.com to schedule a blower door test and air tightness field report.