What Is a Blower Door Number and What Does It Mean

We hear this constantly: homeowners receive a blower door number from an energy assessment and have no frame of reference for what it means. A result like 12.4 ACH50 gets handed over with no explanation of whether that is bad, typical, or fixable.

If you have had a blower door test done on your home, or if you are about to, this guide explains exactly what the numbers mean, what ranges are typical for Maine homes, and what the numbers tell you about your home's energy performance and comfort.

The Two Main Numbers: CFM50 and ACH50

A blower door test produces two primary measurements. They describe the same thing - how leaky your house is - but from different angles.

CFM50 (Cubic Feet per Minute at 50 Pascals)

This is the raw measurement. When the blower door fan depressurizes your house to -50 Pascals (roughly equivalent to a 20 mph wind hitting every side of the house simultaneously), the fan measures how much air flows through the house to maintain that pressure difference.

What it means: CFM50 tells you how many cubic feet of air per minute are leaking through your building envelope under the test conditions.

Example: A CFM50 of 2,500 means 2,500 cubic feet of air per minute is flowing through gaps, cracks, and holes in your home's walls, ceiling, floor, and around windows and doors.

The limitation: CFM50 is an absolute number. A large house will naturally have a higher CFM50 than a small house, even if they are equally well-sealed, because there is more surface area for air to leak through. This makes CFM50 hard to compare directly between homes of different sizes.

ACH50 (Air Changes per Hour at 50 Pascals)

ACH50 normalizes the air leakage measurement by the size of the house. It tells you how many times per hour the entire volume of air inside your house is replaced through leaks at the 50 Pascal test pressure.

The formula: ACH50 = (CFM50 x 60) / House Volume in cubic feet

Example: A 2,000 sq ft home with 8-foot ceilings has a volume of 16,000 cubic feet. If the CFM50 measurement is 2,500: ACH50 = (2,500 x 60) / 16,000 = 9.4

That means the entire volume of air in the house turns over 9.4 times per hour at the test pressure. In other words, the house is quite leaky.

Why ACH50 is more useful: Because it accounts for house size, ACH50 allows you to compare your home to others and to benchmarks. A 1,200 sq ft bungalow and a 3,000 sq ft colonial can both have an ACH50 of 8 - meaning they are similarly leaky relative to their size - even though their CFM50 numbers will be very different.

What the Numbers Mean for Your Home

Here is a reference table for ACH50 values. These ranges are based on our testing experience over 20+ years in Greater Portland, combined with national building science data.

Most older homes in Greater Portland - the 1920's Victorians in Deering, the 1950's Capes in South Portland, the 1960's ranches in Scarborough - test somewhere between 8 and 15 ACH50. That is not unusual. It is the standard those homes were built to. But it means the building envelope is allowing a tremendous amount of heated air to escape.

Putting the Numbers in Context: Energy and Dollar Impact

The relationship between ACH50 and energy use is roughly linear within the typical range of Maine homes. Reducing your ACH50 by 1 point reduces your air-leakage-related heat loss by roughly the same percentage.

Here is a rough illustration for a 2,000 sq ft home in Greater Portland heating with oil:

These numbers are approximate and depend on fuel prices, thermostat settings, and home specifics. But the pattern is clear: tightening the building envelope translates directly to lower heating costs. And remember - air leakage is only part of total heat loss. Insulation addresses conductive heat loss, which is the other major component.

The Natural Air Change Rate: ACHnat

There is a third number you might encounter: the Natural Air Change Rate, or ACHnat. This estimates how much air actually leaks through your home under normal conditions (not the artificial 50 Pascal test pressure).

The rough conversion: ACHnat is approximately ACH50 divided by 20 (this varies by climate, wind exposure, and building height, but the factor of 20 is a reasonable approximation for Maine).

Example: A home with ACH50 of 10 has an approximate ACHnat of 0.5. That means the entire volume of air in the house is naturally replaced about once every 2 hours through air leakage alone.

Why this matters: building scientists generally consider an ACHnat of 0.35 to be the minimum for adequate indoor air quality from natural ventilation. Below that threshold, mechanical ventilation (an ERV or HRV) becomes necessary to maintain healthy indoor air.

Working backward: ACHnat of 0.35 corresponds to roughly ACH50 of 7. This means homes tightened below 7 ACH50 should have mechanical ventilation evaluated. Homes tightened below 5 ACH50 almost certainly need it.

This is why we assess ventilation needs as part of every air sealing project. Tightening the building envelope is good for energy performance and comfort, but it must be done with attention to indoor air quality. A house that is tight enough to save energy is also tight enough to trap moisture, CO2, and indoor pollutants if not properly ventilated.

CFM50 Per Square Foot: Another Useful Metric

Some building scientists prefer expressing air leakage as CFM50 per square foot of building envelope area. This metric accounts for the relationship between leakage area and the surface area available for leaks to occur.

The calculation divides CFM50 by the total surface area of the building envelope (walls + ceiling + floor). A target of 0.25 CFM50/sq ft or lower is considered good practice.

This metric is less commonly shared with homeowners but is useful for comparing buildings with unusual shapes - a two-story colonial has more wall area per square foot of floor space than a single-story ranch, so ACH50 alone can be misleading when comparing the two.

What Causes High Blower Door Numbers

When a home tests at 10+ ACH50, the air is not leaking through one or two big holes. It is leaking through hundreds or thousands of small gaps that add up to a massive total leakage area. Common sources include:

Attic Plane Leaks (Often the Biggest Source)

• Open chases around plumbing stacks, chimneys, and flues
• Unsealed recessed light housings
• Gaps around the attic hatch or pull-down stairs
• Open stud cavities in interior walls that connect to the attic
• Bathroom and kitchen exhaust fan housings

The attic plane is typically the single largest source of air leakage in Maine homes because hot air rises (stack effect) and pushes upward through every available path. In many homes, the attic plane accounts for 40-50% of total air leakage.

Basement and Crawlspace Leaks

• Gaps around the sill plate (where the wood frame meets the foundation)
• Open rim joist cavities
• Unsealed pipe and wire penetrations through the foundation
• Floor drains and sump covers

Walls and Exterior Penetrations

• Gaps around window and door frames
• Electrical outlet and switch boxes on exterior walls
• Pipe and wire penetrations through exterior walls
• Dryer vents, exhaust fans, and hose bibs

Building Connections

• Where additions meet the original structure
• Where the garage shares a wall with conditioned space
• Where porch roofs intersect with the building envelope

Before and After: What Good Air Sealing Looks Like

Here are typical results from our projects:

1950's Cape in South Portland:

• Before: 13.2 ACH50 (3,800 CFM50)
• After: 5.8 ACH50 (1,670 CFM50)
• Reduction: 56%

1970's Colonial in Scarborough:

• Before: 9.6 ACH50 (4,200 CFM50)
• After: 4.1 ACH50 (1,800 CFM50)
• Reduction: 57%

1920's Foursquare in Portland:

• Before: 14.8 ACH50 (4,100 CFM50)
• After: 7.2 ACH50 (2,000 CFM50)
• Reduction: 51%

The older Portland home started leakier and ended leakier than the newer homes - older construction has more leakage paths that are difficult to access. But a 51% reduction in air leakage is still a major improvement in comfort and energy performance.

Notice that we do not get every home to 3.0 ACH50. Existing homes have practical limitations - finished walls, inaccessible cavities, and architectural features that limit how much air sealing is possible. We target the biggest leaks first (attic plane, basement rim joist, major penetrations) and achieve the best results the building will allow.

How We Use Blower Door Numbers in Project Planning

The blower door test is not just a before-and-after measurement. We use it as a diagnostic tool throughout the project:

Pre-work test: Establishes baseline and helps us estimate the scope of air sealing work needed. A home at 8 ACH50 needs different attention than one at 15 ACH50.

Guided air sealing: With the blower door running, our crew can feel exactly where air is entering the building. Combined with an infrared camera (which shows temperature differences caused by air leakage), we can pinpoint and seal leaks that would otherwise be invisible.

Post-work test: Verifies the improvement. This number is documented for the homeowner and for Efficiency Maine rebate qualification. It is the proof that the work performed as intended.

We include blower door testing in every insulation and air sealing project at no additional charge. It is not an optional add-on - it is a core part of how we do quality work.

Getting Your Number

If you do not know your home's blower door number, our free energy assessment is the place to start. We evaluate your home's overall energy performance, including air leakage, insulation condition, and heating system efficiency. You get clear numbers, honest explanations, and specific recommendations.

If you already have a blower door number and want to understand what it means for your specific home, we are happy to discuss it. Every house is different, and context matters - building age, construction type, existing insulation, and heating system all factor into what your numbers mean and what improvements make the most sense.

Ready to find out how tight (or leaky) your home is? Schedule your free energy assessment or call (207) 221-3221. We bring the blower door, the infrared camera, and 20+ years of experience reading what the numbers tell us about your home.