What Is a Blower Door Test? MA Guide

Published: May 2026 | Blower Door TestMassachusettsAir Leakage

If you’re building or renovating in Massachusetts, you’ll need a blower door test. But what exactly is it, how does it work, and what does the result mean? Here’s everything you need to know.

What Is a Blower Door Test?

A blower door test is a diagnostic procedure used to measure a building’s air tightness — specifically, how much air leaks through the building envelope (walls, windows, doors, ceiling, and floor). It gets its name from the large fan (blower door) that is temporarily mounted in an exterior doorway to pressurize or depressurize the building.

How Does a Blower Door Test Work?

Step 1: Setup

The technician mounts a calibrated fan in an exterior door frame, sealing the gap around it. All other exterior doors, windows, and intentional openings (fireplace dampers, exhaust fans) are closed. Interior doors are left open.

Step 2: Pressurization or Depressurization

The fan either blows air into the house (pressurization) or pulls air out (depressurization) to create a 50-pascal pressure difference. The amount of airflow required to maintain that pressure difference tells us exactly how much air is leaking through the building envelope.

Step 3: Measurement & Calculation

The fan speed and pressure readings are used to calculate CFM50 (cubic feet per minute at 50 pascals) and ACH50 (air changes per hour at 50 pascals). The lower the number, the tighter — and more energy-efficient — the building.

What Does the Blower Door Test Measure?

• ACH50: Air changes per hour at 50 pascals. The MA Stretch Code maximum is 3.0 ACH50 for new residential construction.
• CFM50: Cubic feet per minute at 50 pascals. Used to compare across different home sizes.
• Leakage locations: Technicians can use smoke pencils or thermal imaging to identify where air is escaping.

Why Is It Required in Massachusetts?

The blower door test is required in Massachusetts for several reasons:

• MA Stretch Code: Any municipality that has adopted the Stretch Code requires a blower door test proving ≤ 3.0 ACH50 for new residential construction.
• HERS Rating: A blower door test is a required component of all HERS Ratings.
• Mass Save rebates: Mass Save air sealing rebate programs require a blower door test to document air leakage before and after air sealing work.

When Should the Blower Door Test Be Performed?

For new construction, the blower door test is typically performed after drywall installation is complete but before final finishes and interior trim. This timing allows any air sealing deficiencies to be corrected before the walls are closed up.

What If the Test Fails?

If a home fails to achieve 3.0 ACH50 or less, the builder must identify and seal air leakage points and then schedule a re-test. Common failure points include attic bypasses, top plates, plumbing penetrations, electrical boxes, and recessed lighting. Most failures can be corrected with targeted air sealing in a few hours.