What the Blower Door Measures
The fan creates a pressure difference between inside and outside, and the leakier the house, the harder the fan must work to hold 50 pascals.
The instrument reports CFM50 — the cubic feet of air per minute moving through the fan to maintain the 50-pascal pressure difference. A house leaking 2,400 CFM50 is leaking twice as much as one at 1,200 CFM50. Because CFM50 scales with house size, it is normalized to ACH50 (air changes per hour at 50 Pa) by factoring in the conditioned volume, which lets a small ranch and a large colonial be compared on equal footing.
How to Read Your ACH50
The number locates your house on a spectrum from leaky to tight.
| ACH50 | What it means |
|---|---|
| 10–15+ | Leaky — typical pre-1950 MA home, major savings available |
| 7–10 | Average existing MA home |
| 3–5 | Well air-sealed retrofit |
| ≤3.0 | Massachusetts Stretch Code new-construction target |
| ≤0.6 | Passive House standard |
Lower is tighter. A tighter house holds conditioned air longer, which is why air sealing usually returns more comfort per dollar than adding insulation in a leaky home.
Why It Is Run Before and After
The blower door is both diagnosis and proof.
Run at the start, it locates leaks — paired with a smoke pencil or infrared camera, the depressurized house pulls outside air through every gap so the auditor can find rim joists, top plates, recessed lights, and chases that leak. Run again after air sealing, it quantifies the improvement: a drop from 11 to 6 ACH50 is a 45% leakage reduction the homeowner can see in a number, not just feel.
Can a House Be Too Tight?
Tightening a house changes its ventilation needs.
Below roughly 3 ACH50, a home no longer leaks enough air to ventilate itself, and mechanical ventilation — a bath fan on a timer, or a heat-recovery ventilator (HRV/ERV) — becomes necessary to manage moisture and indoor air quality. This is why aggressive air sealing and a ventilation plan go together. Tightening without a ventilation strategy can trap humidity and pollutants; the goal is "build tight, ventilate right."
Where the Blower Door Fits in a Mass Save Job
The test anchors the rebate and the work scope.
A Mass Save Home Energy Assessment establishes the baseline; air-sealing work targets the leaks the blower door and infrared scan reveal; a post-work test documents the result. Because the program ties incentives to measured improvement, the number is not academic — it determines scope and savings.
Frequently Asked Questions
What is a good ACH50 for a Massachusetts house?
What is the difference between CFM50 and ACH50?
Does a blower door test hurt anything?
Can a house be too airtight?
How much can air sealing reduce my ACH50?
Is a blower door test required for Mass Save?
References & Sources
- Mass Save air sealing. https://www.masssave.com/residential/rebates-and-incentives/insulation-and-air-sealing
- Massachusetts energy codes. https://www.mass.gov/info-details/massachusetts-energy-codes
