What Does a Blower Door Test Actually Tell You?
A blower door test is one of the most useful diagnostic tools in building science — but it's often misunderstood. Here's what the test actually measures, what it doesn't measure, and how it fits into a professional home evaluation.
What a Blower Door Test Measures
A blower door test measures the total air leakage of a building. The test uses a calibrated fan mounted in an exterior door to depressurize the home to a standard pressure (typically 50 Pascals). The fan measures how much air is required to maintain that pressure — which tells you how much air is leaking through the building envelope. The result is expressed as CFM50 (cubic feet per minute at 50 Pascals) or as ACH50 (air changes per hour at 50 Pascals).
What the Number Means
A lower number means a tighter building — less air leakage. Energy codes for new construction in New York State currently require homes to achieve 3 ACH50 or better. Older homes often test at 10–20 ACH50 or higher. The test result tells you how leaky the building is overall — but it doesn't tell you where the leaks are.
What a Blower Door Test Doesn't Tell You
The blower door number is a total — it doesn't identify the location of individual leaks. A home with a high ACH50 could have one large leak or dozens of small ones. Finding where the air is actually moving requires additional investigation: walking the building while it's under pressure, using smoke pencils or theatrical fog to visualize air movement, or combining the blower door with thermal imaging to identify temperature differences at leakage points.
How It's Used in a Diagnostic Evaluation
In a Home Infiltration Assessment, the blower door test is one tool among several. The test establishes the overall airtightness of the building. Pressure diagnostics can help identify which zones or areas are contributing most to the total leakage. Thermal imaging, when environmental conditions allow, can help visualize where air and heat are moving. Together, these tools provide a much more complete picture than the blower door number alone.
Blower Door Testing for New Construction
For new construction, a blower door test is required by energy code in most jurisdictions in New York State. The test is typically performed after the building is substantially complete and all penetrations are sealed. The result is documented and submitted as part of the code-compliance package. Saratoga Cellulose provides professional residential blower door testing for new construction at $350 for a standard residential test.
The Bottom Line
A blower door test is a valuable diagnostic tool — but the number alone is just a starting point. Understanding where the air is leaking, and what to do about it, requires professional interpretation and additional investigation.