Testing for Airtightness with a Blower Door.
The Blower Door is an amazing tool. A home energy auditor or home energy rater can use it to do performance testing by quantifying the infiltration rate of a house. They can also do diagnostic testing with it, by running the Blower Door and walking around the house to find leaks.
Basic Operation. The basic principle of its operation is simple. A powerful fan set temporarily in a doorway creates a pressure difference between the house and the outdoors, usually by depressurizing the house. All the air that the fan blows out of the house is replaced by air coming in through all the leaks. For every cubic foot of air that blows out through the fan, a cubic foot leaks in.
So, all we need to do is measure the amount of air moving through the fan, and, voila, we know how much the house leaks at that pressure. Airflow is measured in cubic feet per minute, and the pressure normally used for this test is 50 Pascals. Hence, the number that everyone who does this testing in the US uses is cfm50.
Another way to explain how a blower door test can measure air leaks:
To measure the amount of leakage in a home we use a tool called a blower door, which is comprised of a calibrated fan, a mounting system to attach the fan to an exterior door, and a manometer which measures pressure.
To understand the principle behind the blower door test imagine a large parade balloon like Kermit here. If the balloon is completely air tight we can pressurize it, shut off the valve, and the balloon will remain inflated indefinitely.
Now imagine the balloon has some small leaks at the seams. To keep it inflated we need to continuously blow in air to replace the air leaking through the seams. The larger the leaks are, the more air is required. Thus, if we can measure the amount of air we are blowing into the balloon to keep it fully inflated, we can infer how leaky the balloon is.
That’s exactly what a blower door test does: it measures the amount of air needed to keep a house at an elevated pressure of 50 Pascal (i.e. “inflated”), and we use that measurement to infer how many leaks are present.
Blower Door Test Metrics
The blower door results can be expressed in a few different metrics. The most common one is air changes per hour (ACH), or how many times a house’s air completely replaced in a given hour. Since we take our blower door measurement at 50 Pascal most codes and standards reference the air changes at that elevated pressure (ACH50), but we can also calculate the air changes under natural conditions (ACHn).
For example, a code-built new home with decent air sealing might have 7 air changes per hour at 50 Pascal (ACH50), meaning if we kept the blower door running for an hour it would pump in enough air to completely replace the home’s air 7 times. This would translate to about 0.35 natural air changes per hour (ACHn), or about one complete air replacement every 3 hours.
What’s A Good Blower Door Test Number?
The metrics and math can get a little technical so let’s put them in context. Here’s a rough scale to compare your blower door test number to other standards:
10-20 ACH50 – Older homes, like living in a “barn”
7-10 ACH50 – Average new home with some air sealing but no verification and little attention to detail
7 ACH50 – OK infiltration level and the 2009 IECC energy code requirement
3-5 ACH50 – Good and achievable target for most new homes. The ENERGY STAR reference home is 5 ACH50 for climate zone 4 which covers DC, MD, VA and part of PA. The majority of PA is 4 ACH50 for the ENERGY STAR reference home.
3 ACH50 and lower – Tight home with great air sealing, and required by the 2012 energy code adopted in MD and coming to other jurisdictions soon.
.6 ACH50 – Super tight home and the Passive House standard.