Air Sealing and Blower Door Testing

Air Leakage in the 2021 WSEC-R

Air leakage — uncontrolled airflow through gaps, cracks, and holes in the building envelope —is one of the largest sources of energy loss in residential buildings. The 2021 WSEC-R sets a maximum air leakage rate of 4.0 ACH50 (air changes per hour at 50 pascals of pressure) for Climate Zone 4 and 4.0 ACH50 for Climate Zone 5. Meeting this target requires deliberate air sealing at every stage of construction, from framing through finish work.

Blower Door Testing

A blower door test measures how airtight your home is. A calibrated fan is mounted in an exterior door frame and depressurizes the house to 50 pascals. The fan speed required to maintain that pressure tells you the air leakage rate in cubic feet per minute (CFM50), which is then converted to ACH50 based on your home’s volume.

Testing happens after the building is substantially complete — drywall installed, windows and doors in place, all penetrations sealed — but before the final inspection. Some builders do a mid-construction test after air sealing but before drywall to catch problems early.

Exemptions

• Additions under 750 square feet are exempt from blower door testing requirements.
• Existing duct systems with documented prior sealing are exempt from duct leakage testing.
• Ducts with less than 40 linear feet in unconditioned spaces are exempt from duct leakage testing.

R406.3 Credits

Under the Option 2 (R406.3) energy credits path, you can earn credits by achieving air leakage rates below the code maximum. Tighter envelopes earn more credits — achieving 1.5 ACH50 or lower earns the maximum credit in this category. This is one of the most cost-effective ways to pick up credits toward your total requirement.

Common Leakage Points

During a blower door test, the most common leakage sources include:

• Rim joists — where the floor framing meets the foundation or between stories
• Electrical penetrations — outlet boxes, switch boxes, and panel entries on exterior walls
• Plumbing penetrations — drain, vent, and supply lines passing through top and bottom plates
• Recessed lights — non-IC-rated fixtures in insulated ceilings are major leak sources
• Attic hatches and pull-down stairs — often overlooked, these need gaskets and weatherstripping

Air Sealing and Ventilation

A tighter house needs mechanical ventilation. When you seal the envelope to 4.0 ACH50 or below, the building can no longer rely on natural air leakage for fresh air. The WSEC-R requires a mechanical ventilation system — typically a heat recovery ventilator (HRV) or energy recovery ventilator (ERV) — sized per ASHRAE 62.2. This is not optional: mechanical ventilation is a code requirement for tight construction.

Frequently Asked Questions

When does the blower door test happen?

The test occurs after the building is substantially complete — drywall is installed, all windows and exterior doors are in place, and penetrations are sealed. It must be done before the final inspection. Some jurisdictions allow or encourage a pre-drywall test as well.

What if I fail?

If the test result exceeds 4.0 ACH50, you’ll need to find and seal the leaks, then retest. Common fixes include caulking gaps at framing intersections, applying spray foam at penetrations, and adding weatherstripping to attic hatches and doors. Most failures can be resolved in a day with targeted sealing.

How much does a blower door test cost?

In Washington State, a single-family blower door test typically costs $300–$500, depending on the size of the home and the testing company. Many HVAC contractors and energy auditors offer the service.