Air Infiltration & ACH Calculator
Understanding Air Infiltration Calculation
Calculating the air infiltration rate is a critical step in assessing building envelope performance and energy efficiency. Air leakage allows conditioned air to escape and unconditioned outside air to enter, leading to energy loss, moisture problems, and reduced occupant comfort.
What is CFM50?
CFM50 stands for Cubic Feet per Minute at 50 Pascals. It is the standard measurement obtained from a Blower Door Test. During this test, a fan depressurizes the building to a pressure difference of 50 Pascals with respect to the outside. This simulates a roughly 20 mph wind blowing against all sides of the building simultaneously.
The ACH formula
To understand how "leaky" a home is relative to its size, we calculate Air Changes per Hour (ACH). The standard metric derived from the blower door test is ACH50.
Formula: ACH50 = (CFM50 × 60) / Building Volume
- CFM50: The airflow reading from the blower door test.
- 60: Converts minutes to hours.
- Volume: The cubic footage of the conditioned space (Floor Area × Ceiling Height).
ACHnat: Natural Air Infiltration
While ACH50 represents a high-stress scenario (simulated storm), ACHnat estimates the air leakage under normal, natural conditions. This is calculated using the LBL (Lawrence Berkeley Laboratory) Factor, often referred to as the N-Factor.
Formula: ACHnat = ACH50 / N-Factor
The N-Factor depends on your geographic location (climate zone), the height of the building, and the shielding provided by surrounding trees or structures. It typically ranges from 10 (very leaky/exposed) to 30 (very sheltered), with 20 being a common standard.
Interpreting the Results
Different building codes and standards define what constitutes a "tight" or "leaky" house:
- Passive House Standard: Less than 0.6 ACH50. Extremely airtight. requires mechanical ventilation.
- Modern Energy Code (2015+ IECC): Typically requires 3.0 to 5.0 ACH50 or less, depending on the climate zone.
- Older Homes: Often range from 10 to 20 ACH50, indicating significant energy waste.
If your calculator result shows a low ACHnat (under 0.35), verify that the building has adequate mechanical ventilation (HRV or ERV) to ensure healthy indoor air quality.