12.4 Tools of the Trade: The Blower Door
12.4 Tools of the Trade: The Blower DoorHow Auditors Measure Your Home's "Leakiness"
What Is a Blower Door?
A blower door isn't a door at all, it's a calibrated fan system temporarily mounted in an exterior doorway that deliberately sucks air out of your house to create controlled negative pressure inside.
Text description of the Blower Door image.
The image shows a doorway that leads into a room. An open door is positioned on the left side. The entrance is obscured by a large red plastic barrier that acts as an air seal test setup. The large circular object at the base of the barrier is a blower, which is attached to the barrier for measuring air pressure. There is also a black control panel mounted next to the door, connected by wires, featuring buttons and a digital display.
The physics in action:
When the fan pulls air out, indoor pressure drops below outdoor pressure. Nature hates imbalance, so outdoor air immediately rushes in through every unsealed crack, gap, and hole in the building envelope. The leakier the house, the harder the fan must work to maintain the pressure difference.
Note: A blower door doesn't create leaks—it reveals them by exaggerating natural air movement 20–50x beyond normal conditions.
Why 50 Pascals? Understanding the Standard Test Pressure
Auditors don't just run the fan at random speed. They depressurize the house to a precise 50 Pascals (Pa)—roughly equivalent to a 20 mph wind blowing against all sides of the house simultaneously.
Why this number?
- Strong enough to overcome natural wind/stack effects during testing
- Standardized globally—allows comparison between homes
- Safe for the building structure (won't rip shingles off your roof!)
Blower Door Test (5:34)
Transcript: Blower Door Test (5:34)
It's test day. We're verifying the air tightness of this new home.
We're talking air tightness today and how to verify if the house you just completed actually met the air tightness goals. You know, this test is called a blower door test, and it's become a lot more important in recent years as the codes have changed and required this test. I'm here with Sean from Positive Energy. Sean, you're here to test and find out how well this house did. Walk us through the basics of what you're doing today.
Thanks, Matt. So, this is a blower door. It's basically a fan that has these calibrated plates on it, so I know how much airflow is moving through the fan. I then depressurize the house. You can pressurize or depressurize, but I depressurize the house to 50 pascals.
And a pascal is a very, very small pressure difference. Think like a PSI on a car tire, but it's a fraction of that.
Gotcha. So that basically simulates 20 mph winds on all sides of the house at once. So, in other words, we're depressurizing the house. We're going to be blowing out of this door. This is actually the door to the carport on this house. And so we're going to blow out and depressurize the house.
Correct.
Correct. And then tell me about how you can find out how leaky the house is based on this fan's measurements.
Once I get to the 50 pascal pressure measurement, I know that because I've got a hose that's going to the outside that's measuring the pressure outside versus the pressure inside. And once that is stabilized, I can then look at my reading here, which I enter into this manometer with the ring size, and it tells me how much airflow is moving through the fan. And that airflow number, I can then use that and the volume of the home to determine how many air changes per hour happen.
Got it. So this fan is going to tell you a CFM number at 50 pascals of difference between the inside and out. And then how do you translate that CFM number? So we actually ran the test a minute ago and we ended up about 2,100 CFM at 50 pascals. How does that translate back to a number that we can use based on code for ACH50?
Yeah. So, I take the volume of the home, which in this case was about 63,000 cubic feet of air in this home, and I then look at the number of CFM that actually is coming out of the home. This is CFM, so it's cubic feet of air per minute. And the ACH number is an hourly number. So, I need to multiply the CFM by 60 to get the airflow per hour that goes out of the house.
Gotcha.
Once I get that number, I then divide that into the volume of the home to determine the air changes per hour at the 50 Pascal test pressure.
Got it. So code nationally right now, based on the 2012 International Energy Conservation Code, is three air changes per hour at 50 pascals. How did we do on this one, Sean? What was my test?
Matt, you actually scored a 1.9.
1.9. That's good. So, just to remind you, code nationally is 3 ACH50 on the current 2012 codes. And here in Austin, I need to make a 5 ACH50. So I'm about 30% better than national code, over 60% better than the local codes. You know, I had to make a couple of compromises on this house, Sean, based on the architecture and the location. We're in a spot facing the lake. We have some giant, gorgeous views. We've got a couple of windows and doors that are a little more leaky than I would typically like to use, but I think a 1.9 is a really good score considering some of the compromises we made. And that's, of course, much better than is required in this area.
Let's change gears a little bit and say if we would have had a higher score than code. What could that tell us about this house, and how could we use this as a diagnostic tool?
The number one thing that I'm looking for is making sure that the trades did what you wanted them to do. One of the things I see that's often missed are the top plates where all the penetrations go through your top plate. Those things are not sealed and that can, you know, be death by a thousand cuts. It can lead to a lot of issues later on.
Tell me what you've seen in terms of bigger mistakes in houses that you've tested.
Yeah, Matt. Sometimes I see where some trades needed to get into a part of an attic, and so they just go ahead and cut an access hole into the insulation and don't tell anybody. And now there's this four-foot by four-foot hole in your air barrier, your insulation barrier, everything.
Yeah, that's a really good point, Sean. And I think that this test can help you find not only the big mistakes, but as you run this test for many years like I have, it's allowed me to really hone it down. I'm really looking to get as close to 1 ACH50 as I can on my houses. And I really appreciate you testing this house and all my houses.
Sean, for more information, visit Positive Energy's website. They've got a great podcast where they talk about air sealing and HVAC systems, all kinds of things when it comes to testing. You can also visit my blog at MattRisinger.com. And, of course, I'd love to have you follow me on Instagram or Twitter. We'll see you next time on The Build Show.
Heat. Heat. Heat.
The Testing Procedure: Step by Step
Pre-Test Prep
Before the fan even turns on, the auditor must:
- Close all windows and exterior doors
- Open all interior doors (bedrooms, closets, bathrooms)
- Turn off HVAC systems, fireplaces, and exhaust fans (bath/kitchen)
- Close fireplace dampers and woodstove doors
- Cover dryer vents and kitchen range hoods (temporarily)
During the Test
- Mount the blower door frame in an exterior doorway (usually the main entry)
- Seal the frame edges with nylon panels to prevent bypass air
- Start the fan and gradually ramp up to 50 Pa depressurization
- Walk the house with smoke pencil/incense to visually locate leaks as air rushes in
- Optional but powerful: Pair with IR camera to photograph thermal patterns while depressurized (see connection below)
Post-Test
- Record CFM₅₀ and calculate ACH₅₀
- Generate leakage report with prioritized repair recommendations
- Retest after air sealing to verify improvement (typical goal: 20–40% reduction)
The Power Combo: Blower Door + Thermography
Remember our lesson on IR cameras? Alone, thermography shows where surfaces are cold—but can't distinguish between missing insulation, thermal bridging, or actual air leaks.
Enter the blower door:
When you depressurize the house while scanning with an IR camera, air leaks become dramatically visible as cold streaks being sucked through gaps:
| Scenario | IR Alone | IR + Blower Door |
|---|---|---|
| Electrical outlet on exterior wall | Slightly cool surface | Jet-black streak around outlet plate as cold air rushes in |
| Rim joist (foundation-to-wall joint) | Uniform cool band | Pinpoint cold spots revealing exact gap locations |
| Recessed can light | Warm spot on ceiling (heat escaping) | Cold air visibly streaming upward into attic |
Why This Matters to Homeowners (and Future You)
| Problem | Blower Door Reveals | Typical Savings After Fixing1 |
|---|---|---|
| Drafty windows/doors | Quantifies exact leakage contribution | $100–$300/year on heating bills |
| Unsealed attic hatches | Shows massive bypass around insulation | 10–15% reduction in heating load |
| Leaky recessed lights | Visualizes air streaming into attic | Prevents ice dams + saves energy |
| Missing sill plate sealant | Pinpoints foundation-to-wall gaps | Eliminates cold floors in winter |
1Energy Trust of Oregon has a nice page with information and tips to save money and energy.
Bigger picture: The U.S. DOE estimates that air sealing alone can reduce heating/cooling energy use by 15–30% in typical homes—making it one of the highest-return energy upgrades available. Air leakage tests are also done to confirm any sealing or insulation has been properly done. This may be a requirement for some tax credits or incentives.