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air leakage calculator

What is SLA?

SLA stands for specific leakage area and is a method of describing how much air enters or escapes from a building (building infiltration). The lower the number the tighter (less leaky) a building is. Air Leakage is important as the air escaping from the building represents energy loss. Specific leakage area is calculated using inputs from a blower door test. Typically blower door results are expressed in one of these forms.

  • CFM50 = cubic feet per minute at 50 pascals – this is direct measurement from the blower door.
  • ACH = Air changes per hour at 50 pascals – how long it takes to replace all the air in a home.
  • ELA = Effective leakage area – the size of a hole that would leak at the same rate as the building at 4 pascals.
  • SLA = Specific leakage area. Calculated by dividing the ELA by the square footage of the conditioned space. In essence, SLA represents the ratio of how big a hole the infiltration represents.

In California, the average residential building leakage is estimated to be SLA 4.9. Building with high SLAs are drafty, uncomfortable, and hard to heat and cool. Conversely, a building with too low of an SLA can experience problems with moisture, mold and have overall poor indoor air quality.