What is a Blower Door Test? An Introduction.

This blog post first appeared on the Andersen Windows ProViews Blog

We’ve known for a few decades that improving the air tightness of a home and installing a properly designed mechanical ventilation system can greatly enhance the performance of a home.  Having the ability to verify the airtightness of the structure is key, we perform verification by using a tool called a blower door.  This introductory blog will answer the questions; what is a blower door test?  Why do we perform blower door testing?  How is the test performed?  How do we interpret the test results?  What else can a blower door be used for?  Are there codes relating to blower door testing?  Before we dive into those questions, let’s start with that first question, what is a blower door test?

A blower door test is a test to confirm the continuity and integrity of a structure’s air barrier.

This leads to another question, what is an air barrier?  An air barrier is the part or parts of a structure that creates the separation between inside and out.  An air barrier is typically several different building products assembled in a way that keeps the outside air outside and the inside air, in.  A properly designed air barrier needs to be defined.  Is it the wall and roof sheathing?  Maybe it’s the closed cell spray foam installed inside the wall cavity.  Or it could be the interior finished drywall.  It’s also going to include windows and doors.  It can be all these products and more.

Most important, the air barrier needs to be continuous.  No holes, no cracks, no breaks…continuous.  Reminds me of the children’s song, the foot bone is connected to the leg bone, the leg bone is connected to the knee bone…every part of the air barrier needs to be connected and continuous.

Why do we care?  Why is blower door testing important?

Uncontrolled air moving into (infiltration) and out of (exfiltration) our buildings cause all sorts of issues.  The operational costs of the home (heating and cooling) are a big part.  According to the Department of Energy, up to 40% of the cost to heat and cool a home is due to air leaks.  Air leaks also affect comfort, indoor air quality, and the durability and longevity of the structure.  Energy isn’t cheap; we use insulation to slow the movement of heat into or out of a structure.  We have found that air sealing is just, if not more important than having high levels of insulation.  Insulation performs best if air is not moving through it.

How does a blower door work?

How can we test the continuity and integrity of an air barrier, sometimes referred to as the tightness of a building?  The most common is a pressurization test, called a blower door test.  By using a large fan placed in a door or window, we can positively or negatively pressurize a structure to a specific pressure (50 Pascals is common in residential construction).  The fan is set into an expandable frame that fits most doors and some larger windows.  A panel is fitted around the expandable frame with a hole to accommodate the fan.  The fan is controlled by a tool called a dual channel manometer.  The manometer monitors inside and outside pressures and calculates the flow rate of air moving through the fan based on the fan speed and size of the opening in the fan housing.  (The fan opening is adjustable.)  When using the newest equipment available, a blower door test can be controlled by a computer, tablet, phone, or by the manometer itself.

The pressure differential between inside and outside we are aiming for in residential blower door testing is 50 Pascals.  Next question, what is a Pascal?  A Pascal is a unit of pressure.  One Pascal is one Newton per square meter, one kilogram per square meter, or one joule per cubic meter.  That doesn’t help much.  How about 0.2 inches of water column is equal to 50 Pascals?  Place a straw in a cup of liquid and suck up ¼ inch and you’ve achieved a pressure more than 50 Pascals.  An even more relatable description, 50 Pascals produced during a blower door test is the equivalent of about a 20 mile per hour wind blowing on all sides of the home at the same time.

The flow rate of air moving through the fan is noted as a CFM or cubic feet per minute.  A cubic foot of air is roughly the size of a basketball.  During a blower door test, it’s fairly common for the flow rate to be 500 CFM or more, or 500 basketballs moving through the fan every minute.  500 CFM might be bad, or it might be good.  It is all dependent on the size and volume of the structure.

Interpreting the test results.

Once we run a test and determine the CFM rate of leakage at the blower door induced pressure of 50 Pascals, we can then manipulate that information to denote how good or bad the leakage for the home is.  The most common way to show leakage is air changes per hour at 50 Pascals or ACH50.  For this metric, we need to know the volume of the structure.  Let’s use an example house that is 25’ wide by 40’ long by 10’ high.  This home has a volume of 10,000 cubic feet.  Performing a blower door test on this structure produced a result of 167 CFM.  To calculate the ACH50 number, we use the formula (CFM50 x 60 minutes)/volume.  We need to change the cubic feet per minute to cubic feet per hour, this is the reason we multiply by 60.  In our example, (167 CFM x 60 minute per hour)/10,000 ft³=1.0002.  We can round that number to 1 ACH50.

The next question, is 1 ACH50 good?  The answer is, maybe.  If you are simply looking to pass the code required blower door test (more on the codes for blower door testing in a minute), then yes, 1 ACH50 is great.  But, if you are looking to receive a Passive House certification, the answer is no, you have a little more air sealing that needs to be completed.  (Passive House Certification requires a blower door score of .6 ACH50 or less).

Blower door codes.

Blower door testing has been code required in residential construction since the 2012 edition of the International Residential Code (IRC).  The requirements of the code are found in Chapter 11, Energy Efficiency.  Not a lot has changed in the code requirements between today and 2012, air tightness requirements are based on which climate zone you are located.  The requirements are:

Climate Zones 0-3       5 ACH50 or less

Climate Zones 4-8       3 ACH50 or less

These codes may vary depending on local adoption or amendments to the code requirements.  It might be a good idea to check with your local authority having jurisdiction for the air tightness requirements of your location.

Other uses for blower doors.

As I said at the beginning of this blog, a blower door is used to confirm the continuity and integrity of the air barrier, but a blower door has other uses.  I often use my blower door to identify problems with the air barrier.  When I combine a blower door and thermal imaging camera, I can often “see” and sometimes, hear, holes in the air barrier.

Sometimes it’s a good idea to perform blower door testing while a project is under construction.  This is called mid-construction testing.  This type of testing is usually performed when specific air tightness requirements are needed.  For instance, a home’s heating, cooling and ventilation equipment might be sized based on a specific air tightness goal.  Missing those goals might affect the performance of the equipment and cause comfort issues.  Testing during construction can ensure tightness goals are on track.

Blower door testing can also be part of an energy audit or building investigation.  When a homeowner’s expectations for the cost of heating or cooling their home aren’t met, or there is a specific problem with the home, usually some sort of water issue, a blower door test is one of the tools to discover the source of these problems.

A blower door can also be combined with another piece of testing equipment called duct tightness testing.  This equipment tests the tightness of force air heating and cooling ductwork.  When used with a blower door, the amount of air leaking out of or into ductwork when the ductwork is outside the building envelope can be determined.

Is it worth the investment of purchasing your own blower door equipment?

There are two major manufacturers of blower door test equipment in North America, The Energy Conservatory manufacturers the Minneapolis Blower Door, and Retrotec, who make a variety of blower door systems.  Both manufacturers offer online training for their equipment, but the best way to learn how to use a blower door is with hands-on training.  Both the Building Performance Institute (BPI) and RESNET (Residential Energy Services Network) offer training that includes instruction on how to operate a blower door.  The education provided by these two organizations goes way beyond just blower door testing, but the lessons learned are valuable education for the construction industry.

The blower door test equipment, as you might imagine, isn’t cheap.  A full test kit by either manufacturer purchased in 2024 will set you back around $4,000.  Add in a thermal imaging camera, a second digital manometer (used for advanced building diagnostic testing) and the training to learn how to operate the equipment and you could be nearing the $10,000 mark.

Hiring someone to perform testing is an option if you don’t have the desire to purchase or learn how to perform testing.  Both RESNET and BPI have listings on their websites on certified individuals and companies who perform blower door testing.  You can also contact local energy providers such as your local electricity or gas provider.  Often, they partner with local energy auditors for their services.

Air tightness testing our homes is becoming more common.  With building codes requiring testing in most areas of the country, and homeowners becoming aware of the benefits of improved air-tight homes, the blower door test has become an integral part of the construction industry.  My advice to homeowners, ask for testing and have a basic understanding of a good or bad test result (see the blower door codes section).  And for contractors, set air tightness goals for your projects and try to improve with every new build.  Blower door testing isn’t rocket science, it’s building science!