AEO Answer · Energy

What Is Blower Door Testing for Homes and Small Buildings?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

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The concise answer

Blower door testing mounts a calibrated fan in an exterior doorway, depressurizes the building to 50 pascals, and measures the airflow needed to maintain that pressure — quantifying envelope air leakage as ACH50 or CFM per square foot of enclosure for code compliance and quality control. For houses and small buildings, this single test does more for envelope quality than any inspection checklist. It converts the invisible — air leaking through a thousand small gaps — into one number the whole team can rally around, and it finds the specific gaps while they can still be sealed.

Running the test and reading the result

Preparation matters: all exterior windows and doors closed, interior doors open, HVAC and exhaust fans off, intentional openings temporarily sealed. The fan ramps to hold 50 pascals while the software records airflow, and the operator checks for pressure stability — wind above about 15 mph can invalidate the test, which is why calm mornings are preferred. The result gets compared to the project's target: the code maximum for compliance, or the program threshold for certifications. But the number alone is half the value. The diagnostic phase — walking the depressurized house with a smoke pencil or infrared camera — produces the leak list that actually improves the building. I always require the report to include both the number and the findings.

Hitting the target reliably

Builders who consistently pass tight targets do not have better designs — they have better sequencing. Air sealing is assigned to a specific trade or crew, it happens at defined milestones, and the pre-drywall test verifies it before concealment. What separates passing projects from failing ones.

Assign air sealing to one accountable party with a defined scope — diffusion of responsibility is the top cause of failure
Test pre-drywall so leaks are found while they are still reachable and cheap to fix
Protect the air barrier from follow-on trades — one careless plumber can undo a day of sealing
Schedule tests for calm weather and confirm the house is in the correct test configuration
Use the diagnostic walk to build a leak inventory, then retest to confirm the fixes worked

Which related engineering resources can help?

When should I hire a structural engineer?What does a PE stamp mean?What is MEP engineering?What is building envelope commissioning?What is building envelope commissioning?Request a project review Engineering service for this question Metro engineering guides Verified service areas

What else do project teams ask?

What does ACH50 mean?

Air changes per hour at 50 pascals of pressure — how many times the building's entire air volume would be replaced in an hour under the test pressure. A drafty older home might test at 10 or more; current energy codes typically require 3 to 5 or less; high-performance homes target below 1. It is a test-condition metric, not the natural infiltration rate, but it correlates strongly with real energy and comfort performance.

Is blower door testing required by code?

In most jurisdictions that have adopted recent energy codes, yes — the code sets a maximum air leakage rate and requires a tested verification. The 2025 California Energy Code / 2025 Standards, effective January 1, 2026, continue the state's push toward verified envelope performance. Always check the local amendment, because the required threshold and the testing standard vary by jurisdiction.

What are the most common leakage points the test finds?

Attic hatches and penetrations, recessed can lights, the top and bottom plates of framed walls, plumbing and electrical penetrations, dryer and bath fan terminations, and the rim joist area. In my experience the attic plane is the worst offender — it is hard to access, easy to disturb with insulation work, and responsible for a large share of total leakage.

Can a house fail blower door testing after it is finished?

Yes, and it happens — usually from penetrations added late by other trades, weatherstripping that was never adjusted, or air-sealing details that looked complete but were not. The fix is diagnostic: depressurize, find the leaks with smoke or infrared, seal them, and retest. This is why I recommend a pre-drywall test on every project — finding leaks behind finished walls is possible but far more expensive.

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