Calculate air changes per hour at 50 Pascals (ACH50) from blower door test results. Free ACH50 calculator for home energy auditors and inspectors.
A blower door test is the gold standard for measuring 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 new construction targets 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.
Precise measurement of this value supports sustainable energy planning and helps organizations reduce their environmental impact while maintaining operational performance and comfort levels.
ACH50 is the universal benchmark for building air tightness quoted in energy codes, green building programs, and weatherization standards. Converting raw CFM50 to ACH50 lets you compare your home against code requirements and improvement targets. Consistent measurement creates a reliable baseline for tracking energy efficiency improvements and validating the impact of conservation measures and equipment upgrades over time.
House Volume = Floor Area × Ceiling Height ACH50 = (CFM50 × 60) / House Volume Estimated Natural ACH ≈ ACH50 / N-factor (typically 14–26)
Result: 9.4 ACH50
A home with 2,000 sq ft floor area and 8 ft ceilings has a volume of 16,000 cubic feet. With a blower door reading of 2,500 CFM50: ACH50 = (2,500 × 60) / 16,000 = 9.4. This indicates a moderately leaky home that would benefit from air sealing.
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.
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.
The 2021 IECC requires 3–5 ACH50 for new construction depending on climate zone. Many jurisdictions are adopting this standard or tighter requirements. Renovation projects may need to demonstrate improvement even if they don't reach new construction targets.
For new construction, most energy codes require 3–5 ACH50. ENERGY STAR homes target 4–7 depending on climate zone. Passive House standard requires 0.6 ACH50. Existing homes typically measure 5–15 ACH50.
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.
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.
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.
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.
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.