What Is a Blower Door Test, and Why Is It Performed?

The blower door test set up.

A comfortable, energy-efficient building depends on more than insulation and efficient heating or cooling equipment. It also depends on how well the building controls air movement. Small gaps around windows, doors, plumbing lines, electrical penetrations, attics, and other areas can allow conditioned air to escape and outdoor air to enter.

A blower door test is used to measure this uncontrolled air leakage and determine how airtight a home or building is.

How Does a Blower Door Test Work?

During the test, a trained technician installs a temporary frame and a calibrated fan in an exterior doorway. Exterior doors and windows are closed, and the fan is used to create a controlled pressure difference between the inside and outside of the building.

As the fan operates, specialized equipment measures how much air is needed to maintain the test pressure. This provides an accurate measurement of the building’s overall air leakage.

While the building is under pressure, the technician may also use tools such as smoke pencils, infrared cameras, or physical inspection to help locate specific leaks.

Why Is a Blower Door Test Performed?

Improve Energy Efficiency

Air leaks can force heating and cooling systems to work harder to maintain a comfortable indoor temperature. Identifying and sealing these leaks can reduce energy waste and may help lower utility costs.

Increase Indoor Comfort

Uncontrolled air leakage can cause drafts, cold floors, hot or cold rooms, and uneven temperatures throughout a building. A blower door test helps identify the areas contributing to these comfort problems.

Find Hidden Air Leaks

Many significant leaks are located in places that are difficult to see, including:

• Attic access openings

• Recessed lighting fixtures

• Plumbing and electrical penetrations

• Basement rim joists

• Crawl spaces

• Window and door frames

• Ductwork and mechanical penetrations

The test helps determine where air-sealing work will have the greatest benefit.

Help Control Moisture

Air leakage can carry moisture into walls, ceilings, attics, and other building assemblies. Over time, this may contribute to condensation, mold growth, damaged insulation, or deterioration of building materials.

Improving airtightness helps reduce uncontrolled moisture movement and supports better long-term building performance.

Support Better Indoor Air Quality

A building should receive fresh air through intentional ventilation, not through random cracks and gaps. Reducing uncontrolled leakage allows ventilation systems to operate more effectively and makes indoor air quality easier to manage.

A blower door test does not measure indoor air quality directly, but it provides important information about how air moves through the building.

Verify Construction and Air-Sealing Work

For new construction, a blower door test can confirm whether the building envelope was sealed properly. For existing buildings, testing can be performed before and after air-sealing improvements to document the results.

This gives homeowners, contractors, builders, and energy professionals a reliable way to verify that the work made a measurable difference.

Meet Building Codes or Program Requirements

Many building codes, energy-efficiency programs, and certification standards include airtightness requirements. A blower door test provides a consistent and repeatable way to determine whether a building meets the applicable standard.

Why Are Testing Standards Important?

Blower door tests must be performed using standardized procedures so the results are accurate and comparable. Standards define how the building should be prepared, how the equipment should be installed, what pressure should be used, and how the final results should be calculated and reported.

Without a consistent procedure, two technicians could test the same building and produce different results. Following recognized standards helps ensure that the measurements are dependable and can be used for code compliance, energy assessments, quality control, and improvement planning.

What Do the Results Mean?

Blower door results are reported as CFM50 and ACH50.

CFM50 measures the number of cubic feet of air moving through the building each minute while it is tested at a pressure difference of 50 pascals.

ACH50 means air changes per hour at 50 pascals. It estimates how many times the total volume of air inside the building would be replaced in one hour under test conditions.

In general, a lower number indicates a more airtight building. However, the appropriate result depends on the building’s size, design, age, climate, ventilation system, and applicable code or program requirements.

Schedule a Blower Door Test

The equitment used to perform a blower door test.

A blower door test provides valuable information about how your building performs. Whether you are constructing a new home, investigating high energy bills, improving indoor comfort, planning air-sealing work, or verifying code compliance, the test can help identify problems and guide effective solutions.

At Rockbridge Homes and Barns, we have our own equipment that came from The Energy Conservatory. Contact us to learn more about blower door testing or to schedule an assessment for your home or building.