An Airtight Home, What Do the Blower Door Numbers Mean?

I recently had the opportunity to attend a BS and Beer meeting in Kansas City, MO where the meeting took place in a home under construction.  The home was being built by Aarow Building (Jake Bruton) of Columbia, MO (they recently opened an office in Kansas City).  The home is a single level, slab on grade with around 3,250 square feet.  At the time of the meeting, the home was just finishing the mechanical, electrical, and plumbing systems and about to move to the insulation phase.  A perfect time for a mid-build blower door test.

The purpose of a mid-build blower door test is to confirm that the home is on track to meet the air tightness metric (Jake informed me that all his new homes are contractual bound to achieve 1 ACH50 or less) and to find any missed opportunities in the air sealing of the home.  The mid-build testing can be simple, get the home to negative or positive 50 Pascals of pressure and record the CFM rate, this type of testing is called “single point”.  If you feel the CFM rate (or the calculated air changes per hour at 50 Pascals number) is too high, set the fan on “cruise control” and go find the air leak locations.  There’s no need to perform multi-point testing this early in the build, (a type of blower door testing where CFM rates are measured at progressively lower pressure points, usually starting at 60 Pascals), save that type of testing for the final blower door test.

This home measured 3,250 square feet, 35,600 cubic feet and had 9,250 square feet of surface area (including the floor, walls and ceiling).  The home tested at a very impressive 180 cubic feet per minute of air flow through the fan at the test pressure of -50 Pascals (180 CFM50).  From this number, we can calculate the air changes per hour and CFM/square foot of surface area metrics.

ACH50 = (CFM50 x 60 minute per hour) / volume.  This home achieved .303 ACH50. 

CFM/square feet of surface area.   180 cfm/9,250 square feet = .019.

Let’s start with these two metrics.  1 cubic foot is equivalent to approximately 1 basketball.  So, this home had around 180 basketballs moving through the blower door fan every minute, or 10,800 basketballs every hour, while the blower door was operating.  When we divide all these basketballs moving across the fan by the home’s volume, we find that the entire volume of air inside the home is exchanged with outside air every .303 hours.  Another way to say this is all the air inside the home is exchanged with outside air once about every three hours.  This is under the artificial pressure of about a 20 mile per hour wind blowing at all sides of the home at the same time (50 Pascals).

The ACH50 metric compares the actual leakage of the home to its volume.  The issue is that air leaks happen on surfaces, not in volume.  Homes with large volumes tend to have an easier time passing code required leakage rates and smaller volume homes have a disadvantage.  A better way to look at air leakage is through the surface area of the home.  We calculate that with the CFM/square feet of surface area formula above.  The Aarow Building home achieved a .019 metric.  This number is a little harder to put in context, especially seeing as we have been trained by the codes to relate air leakage by air changes per hour.  Gary Nelson of the Energy Conservatory (Minneapolis Blower Door) suggested to me that most homes should have a goal of around .075 CFM/square foot of surface area or lower.  As it turns out, .075 is roughly 1 ACH50.  The .019 score is very impressive!

How else can we manipulate these numbers to give us useful information?  The website, REDcalc has a free online calculator to help with a few calculations.  I was able to input the home’s measurements and blower door CFM result and the software calculates several other metrics.

The two calculations we already performed are in the report, the ACH50 and CFM50/surface area.  Another thing that can be useful, especially when trying to convey air leakage metrics to homeowners are by comparing the equivalent size of the cumulative leakage areas, of which there are two.  The ELA and EqLA metrics.

Effective leakage area (ELA) is the cumulative size of the holes (technically a smooth, round hole similar to the holes(s) used with the blower door fan) of all the gaps and cracks in the building’s shell if there were a 4 Pascal pressure on the structure.  4 Pascals would be a light wind, a moderate temperature differential between inside and out, or an exhaust fan similar to a bath fan operating.  This home’s ELA “hole” was about the size of a credit card, 9.892 square inches.

The equivalent leakage area (EqLA) is the cumulative hole size (technically a sharp-edged hole) measured at an indoor/outdoor pressure difference of 10 Pascals.  The EqLA metric showed a total leakage area of around two credit cards, 18.916 square inches (less than a 4-inch by 5-inch hole).  By either metric, we are looking at a very small hole over the entire surface area (9,250 square feet) of the structure.

Another metric that can be calculated that is not included in the REDcalc calculator is natural air changes per hour (ACHnat).  The ACH50 test is, of course, under a pressure that the home will typically not see.  ACHnat is a rough estimate of the leakage rate when the home is under normal operating conditions.  I’m not going to get into how to calculate ACHnat in this blog post, but you can read the how to at Building Science-Natural Air Leakage – Northern Built.

A home that simply satisfies the blower door requirements of the code, (which is 3 ACH50 in my market) will have a natural leakage rate of roughly .2 air changes per hour, or 1 complete air change of a home’s entire volume of air every 5 hours, or slightly less than 5 complete air changes per day.  The home in Kansas City computed at .018 ACHnat, or 1 complete air change every 2.3 days.  Unfortunately, these are rough estimates for natural air leakage.  There are so many real-world variables (wind, temperature differences between inside and out, mechanical ventilation use, etc…) that the actual leakage rate could be several times the calculated rate.

The final calculation I would like to discuss is the cost of the air leaks for both heating and cooling.  These calculations require the heating and cooling degree days for the location of the home, the cost of fuel or electricity, the efficiency of the heating and cooling equipment, the CFM50 number from the blower door test, and something called the n-factor or LBL factor (this number is found on a map and chart, it is based on the number of stories in the home, how well shielded the home is from wind, and the zone location given on a map).

There is also a constant in the formula (26 for heating and .026 for cooling), I haven’t been able to find a satisfactory answer to what the constant number is, but I believe it relates to the capacity of heat in air (if anyone can inform the reason for the 26 and .026 number, please leave a comment).  More information on how to calculate the cost of an air leak can be found at Energy Audit-Calculating the Cost of a Home’s Air Leaks – Northern Built.

The cost of the air leaks at the Kansas City home are calculated at $19 per heating season.  The number is based on heating with natural gas at a rate of $1.50 per therm and a heating system efficiency of 95%.  The cost to cool the home is calculated at $3.50 based on $.13 per kWh cost for electricity and a cooling system efficiency of 20 SEER.  Heating degree days for KC were 4300 and cooling degree days, 1918.

Blower doors are capable of providing much more information than just a leakage rate at an artificially high-test pressure.  When you understand what the numbers mean, you can better convey to your customers the importance of building tight.

Jake Bruton is a regular contributor on the Build Show Network.  He also records a podcast called The UnBuildIt Podcast with Steve Baczek and Peter Yost.  And you can find him on Instagram @Jake.Bruton.  Big thanks to Jake for the hospitality!

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