Terminology & Calculations

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These terms, acronyms and abbreviations are used in energy advising and blower door testing:
Term Definition
ACH50 Air Changes per Hour @ 50 Pa
CFM Cubic feet per minute - air flow rate
CFM50 Standard airflow rating for blower door tests
CFMnatural Natural airflow in cubic feet per minute
50 Pa 50 Pascals - measure of air pressure
Pascal Metric measure of air pressure
CRF Cannot Reach Fifty (Pascals)
N-factor Estimate for naturally occurring leakage (usually around 20)
EqLA Equivalent Leakage Area
Defined by Canadian National Research Council as the area of a sharp edged orifice (a sharp, round hole cut in a thin plate) that would leak the same amount of air as the building does at a pressure of 10 Pascals.



To calculate Air Changes per Hour (ACH) =

Reading of CFM50 from manometer x 60 minutes / house volume

Example:

  • Reading of 3550 CFM50 on machine
  • 3550 x 60 minutes = 213,000
  • House is a ranch style 48' x 32' with 8' walls (no basement) = 12,288 cubic feet
  • 213,000 / 12,288 = 17 ACH


To calculate Equivalent Leakage Area ( EqLA in sq. inches) =

Reading of CFM50 from manometer / 10 (Pascals)

Example:

  • Reading of 3550 CFM50 on machine / 10 = EqLA of 355 sq. inches