Blower Door Test (ACH50) Calculator

About the Blower Door Test (ACH50) Calculator

A blower door test is a standard way to measure a building's air tightness. During the test, a powerful fan mounts in an exterior door and depressurizes the house to 50 Pascals. The airflow required to maintain that pressure (CFM50) reveals how leaky the building envelope is.

The key metric derived from the test is ACH50 — Air Changes per Hour at 50 Pascals. This tells you how many times per hour the entire volume of air in your home is replaced through leaks when the blower door is running. Lower ACH50 means a tighter, more energy-efficient home.

Typical existing homes measure 5–15 ACH50, while many modern new-construction programs target roughly 3–5 ACH50. Passive House certification requires 0.6 ACH50 or less. This calculator converts your blower door test CFM50 reading into ACH50 using the standard formula.

When This Page Helps

ACH50 is the standard benchmark for building air tightness used in energy codes, green building programs, and weatherization standards. Converting raw CFM50 to ACH50 lets you compare your home against improvement targets and document progress after air-sealing work.

How to Use the Inputs

1. Enter the CFM50 reading from your blower door test.
2. Enter your home's conditioned floor area in square feet.
3. Enter the average ceiling height (typically 8–9 feet).
4. The calculator computes the house volume and ACH50.
5. Compare your result to building tightness benchmarks.
6. Use the result to estimate natural air infiltration.

Example Calculation

Tips & Best Practices

• ACH50 below 3 is considered tight; 3–7 is moderate; above 7 is leaky.
• The N-factor for estimating natural infiltration varies by climate (14–26).
• Always test with all exterior doors and windows closed, interior doors open.
• Seal combustion appliance flues during the test for accuracy.
• Test before and after air sealing to quantify improvement.
• Include conditioned basement or attic space in your volume calculation.

Understanding Blower Door Results

The blower door test creates a standardized pressure difference (50 Pa) so results are comparable between homes regardless of weather conditions. ACH50 normalizes the CFM50 reading by house volume, allowing fair comparison between large and small homes.

Air Sealing Priorities

Once you know your ACH50, a thermal imaging scan during the blower door test reveals exactly where air is leaking. Common priority areas include the attic floor (top of the thermal envelope), rim joists, recessed lights, and plumbing/electrical penetrations. Sealing the top and bottom of the building envelope is typically most effective.

Code Compliance

Recent IECC-based code targets often land in the 3–5 ACH50 range for new construction depending on climate zone. Many jurisdictions adopt those benchmarks or similar local requirements. Renovation projects may need to demonstrate improvement even if they do not reach new-construction targets.

Frequently Asked Questions

• What is a good ACH50 score?

  For new construction, many modern energy codes and program targets land in the 3–5 ACH50 range. ENERGY STAR programs often use climate-specific tightness targets. Passive House requires 0.6 ACH50. Existing homes typically measure 5–15 ACH50.

• What does CFM50 mean?

  CFM50 is cubic feet per minute of airflow at 50 Pascals of pressure. It's the raw measurement from the blower door fan — how much air must flow through the fan to maintain 50 Pa depressurization. Higher CFM50 means more leakage.

• How do I convert ACH50 to natural air changes?

  Divide ACH50 by the N-factor for your climate zone. The N-factor ranges from about 14 in windy, cold climates to 26 in mild, sheltered areas. For example, ACH50 of 10 divided by 20 gives roughly 0.5 natural ACH.

• How much does a blower door test cost?

  A standalone blower door test costs $150–$300. It's often included as part of a comprehensive energy audit ($300–$500). Some utilities and weatherization programs offer free testing.

• Can a house be too tight?

  Yes, but mechanical ventilation solves this. Very tight homes (below 3 ACH50) should have an HRV or ERV to provide fresh air while recovering heat. The saying is "build tight, ventilate right." Proper ventilation prevents moisture and indoor air quality issues.

• What causes high ACH50 readings?

  Common leakage sources include recessed lights, plumbing penetrations, electrical outlets, attic hatches, ductwork connections, window and door frames, and sill plates. A thorough air sealing effort targets all these areas.