A magic 0.6 – as airtight as it gets

Reaching a major milestone is a reason to celebrate. But what do you do when you exceed all expectations and it takes you by surprise?

Old buildings like ours are notoriously leaky. Even a lot of newer buildings are. That’s something you will notice when you try to escape the cold drafts in your own home on a windy winter day. We know how leaky our building was before we started with the deep energy retrofit, because we conducted a blower door test on the 2nd floor unit. It leaked a whopping 4,763 cfm @ 50 pascal.

The blower door basically depressurizes the building or unit by 50 pascal. At that point you take a reading of the airflow. In our case, the 4,763 cfm @ 50 pascal is the air volume that is leaking through the building or unit. To get to a metric that has a common denominator, the airflow is converted into air exchanges per hour or ACH @ 50 pascal. We ended up with 13.9 ACH @ 50 pascal.

Our results were not that unusual. Blower door test results often range from 7 – 15 ACH @ 50 pascal, depending on the building.

What are the air leakage standards?

Air leakage standards in the U.S. are a moving target. There is no national standard; instead, a builder or homeowner can pick from a list of requirements. Here are some examples:

The PHIUS standard discontinued to use the metric of air exchanges per hour (ACH), and instead replaced it with cubic feet per minute (cfm) of air infiltration per square foot of gross building envelope area, measured at 50 and 75 pascal. The rationale behind this metric is that it can be scaled, unlike ACH. Because ACH is based on the building volume, it allows larger buildings to have a greater rate of air leakage compared to smaller buildings. And where do air leaks occur? At the surface of the building enclosure. Thus it appears logical to measure air leakage based on the gross building envelope area.

How did we do?

We ran a blower door test on the 2nd floor unit after we finished with the spray foam insulation and major air sealing. The results were 720 cfm @ 50 pascal or 2.1 ACH @ 50 pascal. That was a remarkable improvement over the 13.9 ACH @ 50 pascal and caused some excitement.

About a year later we conducted a blower door test on the finished 1st floor unit during a project tour organized by the Chicago Community Loan Fund.That test gave us a reading of 630 cfm @ 50 pascal which also equated 2.1 ACH @ 50 pascal.


I have learned over the past years through reading and talking with green builders that once the major air sealing is done, shaving the last few 1/10th off an ACH @ 50 pascal is painstaking work. It involves a lot of chasing, tracking and sealing small air leaks.

All our major air sealing work was completed. My hope was that I would be able to get a final result of just under 2 ACH @ 50 pascal by plugging the last few leaks I could find. That would have made me very happy!

In for a major surprise…

This past week, I hosted another tour of our deep energy retrofit. It was organized by Eco Achievers, our LEED rater and also a PHIUS+ Rater. Part of the tour was another blower door test demonstration. This time we tested the 1st and 2nd floor unit together.


The reading of 385 cfm @ 50 pascal seemed unreal. So much so that I asked to start the test over again including another recalibration of the blower door equipment. Yet we got the same reading for a second time.

385 cfm @ 50 pascal for the 1st and 2nd floor unit equates 0.62 ACH @ 50 pascal! If I use the PHIUS metric, I come up with 0.05 cfm50 per square foot of gross envelope area.

Do I need to say that it took a while for this to sink in? Never in my wildest dreams would I have thought a less than or equal to 1 ACH @ 50 pascals would be possible on our deep energy retrofit. I guess it’s time to celebrate! And I have to find out how I managed to drop from 2.1 to 0.62 ACH @ 50 pascal.

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About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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