Blower Door Testing to Measure Air Leaks
Every home has air leaks, but the cumulative amount of leaks can vary widely based on the air sealing efforts. Infiltration and air sealing basics are covered in part 1 of this post.
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.
Using a Blower Door Test to Reveal Defects
In addition to quantifying air sealing effectiveness, a blower door test can also help find defects, especially in conjunction with an infrared camera. The blower door will exacerbate the natural infiltration occurring in a house making air leaks easier to find because the air outside forcing its way in shows up as a different color on the IR camera. For example the image below shows a bathroom soffit built below an attic without a proper air barrier.
The photos below were taken in the summer during an existing home energy audit. The infrared photo on the right shows warmer colors in yellow and is the hot summer air coming in through the can lights and walls next to the soffit.