A blower door turns invisible air leakage into a hard number, so you can prove how tight your house is instead of guessing.
You cannot manage what you cannot measure, and air leakage is invisible. A blower door test fixes that by putting a calibrated fan in an exterior doorway and measuring exactly how much air your house leaks. The result is a single number that tells you whether your envelope is loose, average, or tight, and it is the best way to confirm that spray foam air sealing actually worked. This guide explains how the test works, what the ACH50 number means, what Texas code requires, and what typical results look like before and after foam. It also connects to related pieces so you can see how leakage ties into comfort, moisture, and energy bills.
A blower door is a powerful, calibrated fan mounted in an adjustable frame that seals into an open exterior doorway. The tester turns the fan on to either pressurize or, more commonly, depressurize the house, pulling indoor air out so that outdoor air is forced in through every crack and gap in the envelope. Gauges measure both the pressure difference between inside and outside and the volume of air the fan must move to maintain it.
The standard reference pressure is 50 pascals, roughly the pressure a 20 mile per hour wind puts on a house. By measuring how much air the fan moves to hold the house at 50 pascals below outdoor pressure, the tester learns how leaky the envelope is. A loose house needs the fan to move a lot of air to hit that pressure; a tight house needs very little.
The point is to make leakage visible and comparable. Instead of vague impressions like the house feels drafty, you get a repeatable number that can be measured before work, after work, and compared to code targets and to other homes.
Two numbers come out of the test. The first is CFM50, the cubic feet of air per minute the fan moves to hold the house at 50 pascals. A higher CFM50 means a leakier house. The second, and the one most people quote, is ACH50, or air changes per hour at 50 pascals.
ACH50 takes the CFM50, multiplies it out to an hourly volume, and divides by the volume of the house. In plain terms, it tells you how many times per hour the entire volume of air in your home would be replaced under that 50 pascal test pressure. An ACH50 of 10 means the house's full air volume leaks in and out ten times an hour during the test; an ACH50 of 3 means it happens three times.
Because ACH50 accounts for house size, it lets you compare a small cottage to a large two-story fairly. Lower is tighter. The number is measured at 50 pascals, an artificial test pressure, so it does not mean your house literally changes its air ten times an hour in normal weather; it is a standardized yardstick for comparing envelopes.
The tighter the house gets, the more important it becomes to supply fresh air on purpose. A home pushed well below 3 ACH50 should be paired with a ventilation strategy, which we cover in fresh air ventilation and ERV vs HRV. Tightness and controlled ventilation go together.
Every crack in your envelope is a two-way street. In a DFW summer, conditioned air leaks out and hot, humid outdoor air leaks in, driven by wind and by the stack effect. Your air conditioner then has to cool and dehumidify all that incoming air, which shows up as high bills and rooms that never quite get comfortable.
Air leakage is often a bigger real-world factor than raw insulation R-value, because moving air carries far more heat and moisture than conduction through a still wall. A house can have thick insulation and still feel hot and sticky if it leaks badly, which is why we put R-value in context in the R-value guide and stress air sealing as the foundation of real-world comfort.
Leakage also imports humidity, and humidity is a large part of the Texas comfort and health problem. Moist air riding in through gaps raises indoor humidity, feeds the conditions mold prevention tries to avoid, and makes the house feel warmer than the thermostat reading. Sealing the envelope is the most direct way to cut all of that off.
The energy code that governs most new construction, based on the International Energy Conservation Code, sets a maximum allowable air leakage rate that must be verified by a blower door test. In climate zones covering Texas, that target has generally been in the range of 3 to 5 ACH50 depending on the code edition a jurisdiction has adopted. New homes must pass the test to demonstrate compliance.
This is a meaningful shift. Not long ago, air tightness was assumed rather than measured, and many homes fell far short of what they were supposed to achieve. Requiring a measured test means builders have to actually seal the envelope and prove it, which is one reason careful air sealing during new construction has become standard practice.
For existing homes, testing is not usually required by code, but it is one of the most valuable diagnostic tools available. A blower door test is a core part of a home energy audit, pinpointing where a house leaks so that sealing effort goes where it matters most.
The number is useful, but the diagnostic power comes from what a tester does while the fan is running. With the house depressurized, outdoor air rushes in through every gap, so leaks that are normally undetectable become easy to find. A tester can feel the drafts by hand, use a smoke pencil to make the air currents visible, or scan with an infrared camera.
Combining a blower door with thermal imaging is especially powerful. The depressurization pulls outdoor air across the leaks, which shows up as temperature streaks on the camera, revealing exactly where the envelope is failing. Common culprits include top plates, recessed lights, plumbing and wiring penetrations, rim joists, attic hatches, and the junction between the house and the attic or garage.
This is why a blower door test is not just a pass or fail exercise. It is a map. It tells you not only how much your house leaks but where, so the sealing work can be targeted and verified rather than guessed at.
Spray foam is uniquely effective at improving blower door results because it seals and insulates in one step. As it expands, it fills the cracks, gaps, and irregular voids that other insulation simply covers, so it stops air movement rather than just slowing conductive heat. That is why homes sealed with foam so often post dramatic before-and-after improvements.
It is common to see an older DFW home start around 8 to 12 ACH50 and drop to the 2 to 4 range after a thorough spray foam retrofit, especially when the attic roof deck and other major leakage paths are addressed. The exact result depends on the home, but the direction is consistent: foam makes houses substantially tighter.
A post-work blower door test is the proof. It confirms that the air sealing achieved what was intended, verifies code compliance on new builds, and gives the homeowner a documented number rather than a promise. It also flags whether the home is now tight enough that controlled ventilation should be added. If you want to see the range of what tightening a home can do for comfort and bills, the air leakage guide goes deeper.
One myth is that a low ACH50 means a house is unhealthy or suffocating. Tightness itself is not the problem; the problem is a tight house with no ventilation plan. The correct response to a low number is to add controlled fresh air, not to leave the house leaky. A tight, well-ventilated home is both efficient and healthy.
Another misconception is that the test pressure reflects everyday conditions. The 50 pascal reference is an artificial, standardized pressure chosen so results are repeatable and comparable. Your house does not actually cycle its air ten times an hour in normal weather; ACH50 is a yardstick, not a description of daily airflow.
A third myth is that new homes are automatically tight. Without measured testing and deliberate sealing, many are not. That is exactly why the code now requires a blower door test on new construction, and why verifying the result matters as much as doing the work.
What is a good blower door number for a Texas home? Current energy codes target roughly 3 to 5 ACH50 for new construction, and a well-sealed home often lands at or below 3. Older existing homes frequently test around 8 to 12 ACH50, so a retrofit that gets a house into the low single digits is a large improvement.
Does a tight house from a blower door test mean it cannot breathe? No. It means the house no longer leaks randomly, which is a good thing. The right move is to supply fresh air intentionally with a ventilation system such as an ERV. Tightness plus controlled ventilation gives you efficiency and healthy air together.
Is a blower door test required in Texas? For new construction under the current energy code, yes, air leakage must be verified by a blower door test to demonstrate compliance. For existing homes it is not usually required, but it is a valuable diagnostic and a standard part of an energy audit.
How much can spray foam improve my blower door number? A lot. Because foam seals and insulates at once, older DFW homes commonly drop from the 8 to 12 range into the 2 to 4 range after a thorough foam retrofit, depending on the home. A post-work test documents the actual improvement.
Does a lower number always mean lower bills? Generally yes, because less leakage means your HVAC conditions less unwanted outdoor air. Air sealing is one of the highest-return efficiency upgrades, though the exact savings also depend on insulation, ductwork, and equipment. The mechanics are covered in HVAC savings.
Should I get a test before and after sealing work? Ideally, yes. A before test shows where and how much the house leaks so the work can be targeted, and an after test proves the result. You can start by getting a free instant estimate for the sealing itself.
A blower door test turns the invisible problem of air leakage into a hard, comparable number. It measures how much your house leaks, helps locate exactly where, and proves whether air sealing worked. In the Texas climate, where leaked-in humid air drives both bills and moisture problems, that measurement is one of the most useful things you can know about your home.
Lower ACH50 means a tighter, more efficient, more comfortable house, and spray foam is one of the most effective ways to get there because it seals and insulates in a single step. Just remember that a very tight home needs a plan for controlled fresh air. When you are ready to tighten up your home and see the numbers move, a free instant estimate is the fastest first step, and the air sealing guide explains where the biggest leaks usually hide.
Air Sealing: The Most Overlooked Upgrade in Texas Homes
Why air sealing matters more than adding insulation alone, where Texas homes leak, and how spray foam seals and insulates in one step.
Thermal Imaging: Seeing the Heat Leaks Hiding in Your Walls
How infrared thermal imaging reveals missing insulation, air leaks, and moisture in walls and attics, and how pros use it in Texas homes.
Fresh Air Ventilation: Why Tight Homes Need Controlled Fresh Air
Build tight, ventilate right. Why sealed spray foam homes need controlled fresh air and the ventilation options that work in Texas.
Air Leakage: The 24/7 Drain on Your Comfort and Your Wallet
How much conditioned air a typical Texas home leaks every hour, where it escapes, and how sealing the envelope pays you back.
Home Energy Audits: What They Measure and When They Are Worth It
What a professional home energy audit includes, blower doors, thermal imaging, duct testing, and how to act on the results in a Texas home.
Get a free instant estimate for your Dallas-Fort Worth home and see how spray foam air sealing can dramatically lower your blower door number, your bills, and your humidity.