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. I’ll explain what you need to know about performing the test, interpreting the results, and how it relates to code. 

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 importantly, the air barrier needs to be continuous. No holes, no cracks, no breaks … continuous. Think 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.

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 is for the home. The most common way to show leakage is air changes per hour at 50 Pascals, abbreviated as 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 since Passive House Certification requires a blower door score of .6 ACH50 or less.