Understanding Your Home’s Hidden Air Leaks: The Power of Blower Door Testing

Every building has unseen pathways where conditioned air escapes and outdoor air infiltrates – through gaps around windows, cracks in walls, and countless other small openings that together can dramatically impact your home’s energy efficiency and comfort. While these air leaks often seem insignificant individually, they collectively create what building scientists call your structure’s air leakage rate, which can be precisely measured using a blower door test.

The blower door creates controlled, measurable conditions that allow professionals to quantify exactly how leaky your building is and pinpoint where those energy-wasting air leaks are located. Its real power lies in providing standardized, comparable results regardless of weather conditions or natural air pressure variations.

This standardized approach is why professionals convert results to “air changes per hour at 50 Pascals (ACH₅₀) “- a metric that accounts for your home’s volume and provides meaningful context. A newer, energy-efficient home might achieve just 2-3 air changes per hour, while older homes often register 11-15, and some weatherization candidates can exceed 30.

Watch the clip below to learn about blower door fundamentals from Brynn Cooksey, instructor of the BPI Building Analyst Professional (BA-P) Certification Prep course. If you’re interested in mastering these essential building science skills, learning all the calculations, and earning your BA-P Certification, consider enrolling in Brynn’s comprehensive BA-P course.

Transcript below.

 The blower door is a fundamental tool for building science professionals. Its purpose is to simulate the air leakage of a building, and this air leakage is quantified to evaluate how leaky the structure is. This blower door also helps us to locate where the leaks are.

How it works…

There is a calibrated fan that is used to pressurize or depressurize the house with depressurization being most common.

We need to blower door because natural air leakage is far too unreliable to give us consistent measurements. So we use a blower door to exaggerate the pressure differences so we can make a repeatable test so that we can do the test at the same pressure at the same time, so that we can evaluate results.

For example, if we do work, we need to evaluate that work. We need to blower door to run at the same pressure to see how well our work was completed. Exaggerated air leakage measured with the blower door gives us an idea of the amount of air leakage that would occur under natural conditions. Air leakage measured by the blower door is proportional to the size of the holes in the house between inside and outside.

Another way to state this is that the leakier the house is, the higher my blower door number will be. 

Blower door tests can be conducted before and after ceiling work to determine how effective the work was. Blow door testing can also tell us which houses have the most potential for energy savings through air sealing.

The way the blower door works is that we’re actually going to measure differential pressure between two different spaces, typically between inside the building and outside. When we measure two different pressure measurements, we’re always going to compare them, and we use this term with reference two (WRT).

With reference to means that we are referencing one pressure to another. When using a blower door, sometimes we measure pressures under controlled or artificial conditions, and sometimes under normal operating conditions. 

 Pascal is the metric standard and the building science standard to evaluate pressure. Some HVAC technicians may use the American standard 1 inch of water column to evaluate pressure. 

In building science, we prefer Pascal. The reason being there are 249 Pascals in one inch of water column. The blower door pressure differential is quite low, and if we were to use a scale such as inches of water column, we will have a decimal followed by many zeros, and it may not be easy to understand what that value is. As a result, we have adopted the Pascal. 

To give you a reference point, the Pascal is about the weight of one post-it note. Another reference will be: it takes one inch of water column to remove a drink from a container up a straw. Pascals are by far a more reliable measurement. 

The blower door creates a pressure differential. That pressure differential creates air movement through the building’s gaps, cracks, and holes. As a result, we now have a rate of airflow, which we’re going to describe as CFM (or cubic feet per minute). Because we are dealing with the blower door and we’re using 50 Pascals as a test reference pressure, we’re going to now describe cubic feet per minute with the influence of 50 Pascals that is being generated by the blower door.

So quite simply, when we run a blower door, it will produce a CFM 50 number, which describes the building’s leakiness.