What Is a Blower Door Test? A Simple Guide for Homeowners

You know your house is drafty. You can feel it on a cold winter day—that phantom chill in the living room or the constant cold floor in the bedroom. You suspect you’re losing heat and wasting money, but the problem feels invisible. You can’t see the drafts, so how are you supposed to fix them?

If you’re asking these questions, you’re ready to learn what is a blower door test. It might sound intimidating, but it’s actually a simple, safe, and incredibly revealing diagnostic procedure. Think of it as an X-ray for your home’s air leaks. It’s the single most important tool an energy auditor uses to make the invisible visible, turning a vague feeling of draftiness into a clear action plan for a more comfortable and energy-efficient home.

What Is a Blower Door Test?: A Look at the Equipment and Process

The equipment itself looks more complex than it is. It consists of three main parts:

A Flexible Frame: A sturdy, adjustable frame that is temporarily fitted into the opening of an exterior door, creating a seal.
A Nylon Tarp: A flexible panel that fits within the frame, sealing off the doorway from top to bottom.
A Powerful, Calibrated Fan: This is the heart of the system. It’s a large, variable-speed fan mounted into the nylon tarp.

The process is straightforward and non-destructive. On the day of your energy audit, the auditor will ask you to prepare the house by closing all exterior windows and doors, opening all interior doors, and turning off any combustion appliances like furnaces or water heaters for safety.

Here’s what the test itself looks like:

Setup: The auditor will choose one exterior door and install the blower door frame and fan. This usually takes just a few minutes.
Depressurization: The auditor will turn on the fan so that it blows air out of the house. This creates a slight negative pressure inside—typically 50 Pascals (Pa), which is roughly equivalent to a 20-mph wind blowing on all sides of your house at once. This sounds dramatic, but it’s a gentle, standardized pressure that won’t harm your home.
Measurement: As the fan runs, it has to work harder to pull air out of a tight house and less hard to pull air out of a leaky one. A sensitive pressure gauge called a manometer, connected to the fan by small tubes, measures both the pressure difference and the amount of air the fan is moving. This gives the auditor a precise, scientific measurement of your home’s total air leakage rate.
Leak Detection: This is the most valuable part of the process for you as a homeowner. While the house is depressurized, all the tiny, hidden cracks and gaps in your home’s shell are forced to reveal themselves. The 20-mph “wind” outside is now trying to get in. An auditor will walk through the house with you, using tools like a non-toxic smoke pencil or an infrared camera to pinpoint the exact locations where cold air is streaming in. You’ll be able to see and feel the leaks for yourself, often in surprising places like around electrical outlets, recessed lights, or the trim at the base of your walls.

The entire on-site blower door test is usually completed in about 30 to 60 minutes. It’s a safe, standard procedure that provides a wealth of information that simply can’t be gathered any other way.

Decoding the Results: Understanding CFM50 and ACH50

After the test, your auditor will give you a report with some numbers and acronyms that can look confusing at first. The two most important metrics are CFM50 and ACH50. Let’s break down what they mean.

CFM50 (Cubic Feet per Minute at 50 Pascals): This is the primary measurement from the test. It tells you the total volume of air, in cubic feet per minute, that was moving through the fan to maintain the 50 Pascal pressure difference.

Simple Analogy: Think of the CFM50 number as the total size of all the leaks in your house combined into one single hole. A high CFM50 number means you have a very large “hole” in your house, while a low number means the hole is small.

ACH50 (Air Changes per Hour at 50 Pascals): This number puts the CFM50 result into a more practical context. The auditor calculates it by taking the CFM50 value and dividing it by the total volume of your house. It tells you how many times the entire volume of air inside your home is replaced with outside air in one hour when the house is under that 50 Pascal pressure.

Simple Analogy: An ACH50 score of 10 means that during the test, the equivalent of all the air in your house was swapped out for new air ten times in an hour. This is a much easier way to compare the “leakiness” of a small house to a large house.

These are the key metrics in having your blower door test results explained. They transform the abstract concept of “draftiness” into hard data.

What’s a “Good” Score for an Old House?

So, you have your ACH50 number—but what does it actually mean? Is it good or bad? A good blower door test score is relative; it depends heavily on the age of your home and your local climate. A brand-new home built to modern energy codes will have a very different score than a charming but un-retrofitted home from the 1950s.

However, we can use a general guide to understand where your older home stands and what the result implies. The following table provides a reference for interpreting your ACH50 score.

ACH50 Score Range	Performance Level	What It Means for an Older Home
> 10.0	Very Leaky	Significant energy loss. Air sealing is a top priority with a very high potential for savings and comfort improvement.
7.0 – 10.0	Leaky (Typical)	Typical for an un-retrofitted older home. A great opportunity for cost-effective air sealing improvements.
4.0 – 7.0	Moderately Tight	Good performance for an older home, likely has had some air sealing work done. Targeted sealing can still yield savings.
< 4.0	Tight / High-Performance	Excellent performance, similar to a modern energy-efficient home. Mechanical ventilation may be required to ensure good air quality.

For most owners of pre-1980s homes, seeing a score above 7.0 is common and shouldn’t be discouraging. Instead, it should be seen as a massive opportunity. A high score means that simple, cost-effective air leakage testing and sealing can have a dramatic impact on your energy bills and comfort.

Bringing the Numbers to Life: A Real-World Scenario

Let’s make these numbers tangible. Imagine the Miller family lives in a 1960s ranch-style house. They love the home’s character, but they’re constantly fighting drafts in the winter and high cooling bills in the summer. Their energy auditor performs a blower door test and gives them a score of 11.5 ACH50. According to the chart, their home is “Very Leaky.”

But what does that mean for their daily life? It means the constant chill they feel by the large picture window isn’t just from the glass; it’s from massive, unsealed gaps under the window trim and floorboards. It means the furnace has to run almost constantly on cold nights because it’s not just heating the house—it’s trying to heat the entire neighborhood as warm air pours out through dozens of hidden holes in the attic. The 11.5 ACH50 score isn’t just a number; it’s the scientific explanation for their high utility bills and their need to wear sweaters indoors.

For the Millers, the result of their blower door test is empowering. It confirms their suspicions and, more importantly, it tells them that focusing on air sealing will yield huge returns. By investing in sealing the attic and basement, they could potentially cut their score in half, translating directly into a quieter, more comfortable home and hundreds of dollars in real, measurable energy savings each year.

The Real Value: What You Learn From a Blower Door Test

The true power of a blower door test lies in what it enables. It’s not just about getting a number; it’s about getting a plan. There are three key benefits:

It Quantifies the Problem: Before the test, “drafty” is just a feeling. After the test, it’s a number. This objective data allows an auditor to calculate potential energy savings and estimate the return on investment for air-sealing work. It helps you make a financial decision, not just an emotional one.
It Localizes Hidden Leaks: This is the “Aha!” moment for most homeowners. You might blame your old windows for drafts, but the test might reveal the biggest leaks are actually coming from the top of your foundation walls in the basement or from unsealed attic access hatches. As one homeowner discovered, “The blower door test was the biggest value for me to see where the doors and window were leaking… Discovered my closet door casings were leaking like sieves.” The test guides the auditor (and you) to the problems that have the biggest impact, preventing you from wasting money on the wrong fixes.
It Provides a Baseline for Improvement: The initial blower door test gives you a starting score (e.g., 10.0 ACH50). After a contractor performs air-sealing work, they can run the blower door test again. The goal is to see a significantly lower score (e.g., 6.0 ACH50). This provides powerful, objective proof that the work was done correctly and that you got what you paid for. It’s the ultimate form of quality control.

Why This Test is a Non-Negotiable Safety Check

Beyond saving money, a blower door test is a critical health and safety procedure, especially in older homes. The issue is a dangerous phenomenon called “backdrafting.”

Many older homes have fuel-burning appliances—like a furnace, boiler, or water heater—that draw the air they need for combustion from inside the house and vent exhaust gases up a chimney or flue. In a leaky house, this isn’t a problem. But as you begin to seal air leaks, you make the house more airtight.

A tight house can be easily depressurized by exhaust fans, like your kitchen range hood or bathroom fan. If these fans are running, they can create enough negative pressure to overpower the natural draft of your furnace or water heater’s chimney. When this happens, instead of venting safely outside, the toxic exhaust gases—including deadly carbon monoxide—can be pulled back down the chimney and into your living space. A professional energy auditor uses the blower door test in conjunction with other tools to perform mandatory combustion safety testing. They will intentionally depressurize your house and then test your appliances to ensure they continue to vent properly even under a worst-case scenario. This is why a professional audit is essential before undertaking major air-sealing work; it ensures that making your home more energy-efficient doesn’t accidentally make it unsafe.

Conclusion: From a Vague Feeling to a Clear Action Plan

A blower door test is far more than just a piece of technical equipment; it’s a truth detector for your home. It transforms the vague, frustrating feeling of draftiness into a clear, quantifiable, and solvable problem.

It answers the big questions:

How leaky is my house? (CFM50 & ACH50)
Where are the biggest leaks? (Smoke pencil & infrared diagnostics)
Is my home safe to air seal? (Combustion safety testing)

By making the invisible visible, the blower door test gives you a reliable, data-driven roadmap. It empowers you to invest your money wisely, targeting the fixes that will deliver the biggest improvements in comfort and the greatest savings on your energy bills. It’s the essential first step in taking control of your older home’s performance.

Now that you understand the test, see how it fits into the overall Home Energy Audit Cost and process.