Building Science-A Visual for Blower Door Testing

I recently tested the Code Minimum House for air tightness at the rough framing stage.  We ended up where I was hoping at this first test, .55 ACH50, 140 CFM.  Given the volume of the home, the leakage area is equal to approximately 15 square inches.  We were at a good point in the construction of the home to do a little blower door experimenting.  I wanted to know how many holes of a specific size it would take to get the home to the code minimum 3 air changes per hour at 50 pascals.  I decided to start drilling 2.5 inch holes in one of the yet to be cut out window openings.

A little more information on the home.  The code minimum house is more than 1500 square feet of living area and a volume of around 14,800 cubic feet.  To achieve the code required blower door result of 3 air changes per hour at 50 Pascals, there has to be less than 740 cubic feet per minute of air flow moving past the blower door fan.  (740 CFM x 60 minutes in an hour = 44,400 cubic feet per hour / 3 ACH50 = 14,800 cubic feet, or the volume of the home.)  The test result was 140 CFM.  I needed to add the equivalent of 600 CFM of air leaks to the home to get to the code minimum, so I drilled a hole in the OSB where a window would eventually be place.

The 2.5 inch hole, which has an area of 4.909 cubic inches, increased the CFM of air moving across the fan to less than 180 CFM.  An increase of less than 40 CFM.  I drilled a second hole.

The photo above is a screen shot of the Tectite software from Minneapolis Blower Door I use to control the blower door and provide the test results.  As you can see, the second hole brought the CFM rate to 235.  I needed to drill a bunch more hole to get to the 740 CFM point.

Five holes got us to around 300 CFM.  Still need more.

It took 15 holes to get us to around 725 CFM, just under 3 ACH50.  The 16th hole would have pushed the home past 3 ACH50.

The takeaway, 3 ACH50 or 90 square inches of leakage area for a 1500+ square foot home is considered “tight”.  The goal for this home is 1 ACH50 or less of total air leakage.  The investment to get there is well under 1% the total construction budget.  What are we gaining?  A savings of around 6% in energy costs.  Not a lot of money, but extend that savings over the life of the building and we are looking at nearly $10,000 savings in today’s money.  If the cost of energy increases, the savings will be greater.  Comfort, the home will have less air leaks bringing in cold, dry winter air or warm, moist summertime air.  Durability, less air leaks, especially during the winter months, will limit the moisture moving through the building shell finding a surface to condense on.  Less water in the buildings assemblies equals less moisture related rot and mold.  Better indoor air quality.  The heat recovery ventilator (HRV) will be responsible for fresh air into the home, not the air leakage around the window spray foam or across dust, mold or other nasties.  Places where leaky homes can get their “fresh air”.

In my opinion, sealing air leaks is worth the investment for a structure that will hopefully be around for well over 100 years.  All the holes needed to achieve the code minimum is an eye opener.

6 Replies to “Building Science-A Visual for Blower Door Testing”

  1. Fun experiment! Nicely written as well. I like the acknowledgment that cost savings aren’t the big win. Durability -looking long term, and health are much more convincing motivators for clients.
    Thanks for writing this!

    1. Thanks for the comment Michael, and I agree 100%. Both of us being from Minnesota, hopefully we will have the opportunity to meet at some point.

  2. Thanks for sharing. I am amazed it took that many holes.
    Do you think the leakage increased in a linear proportional way with each hole?
    How well does the hole area correlate with ELA reported by the blower door software?

    1. Hi Ian, thanks for the comment. I didn’t run a test during the hole drilling experiment, just had the fan set on cruse control while watching the data. I’m not sure how the ELA relates to the actual area of the holes. That is a good question and I which I had run an actual test. The leakage did appear, for the most part, linear. As far as the C ring, it’s the only ring I’ve needed for “tight” construction. Haven’t had a test result below 80 CFM yet, which is the limit of C. I do own a D ring, just haven’t needed it. Hoping to someday soon purchase a duct blaster setup, haven’t had a demand for one yet. I’ll purchase a new panel for the smaller fan to test air tight homes.

  3. Is the C ring on the model 3 fan good enough to do this work or do you need the smaller ones? I have a blower door but use it mainly for energy audits on older houses.

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