What is ACH50 and why is it important in insulation practice?

• A. ACH50 is the amount of moisture in the air.
• B. ACH50 is air changes per hour at 50 Pa; measures building airtightness; lower ACH50 indicates tighter envelopes with better insulation performance.
• C. ACH50 measures the depth of insulation.
• D. ACH50 is the speed at which air leaks occur.

Answer: (B)

ACH50 uses the blower door test to express how much air leaks from a building when there is a standardized pressure difference of 50 pascals. It tells you the rate of air leakage as a change per hour at that fixed pressure. This provides a consistent way to compare airtightness between buildings and track improvements. In insulation practice, a lower ACH50 means the envelope is tighter, so there’s less unwanted air flow that can carry heat in or out. That makes the insulation work more effectively, reduces energy use, and helps with moisture and comfort control. It isn’t a moisture measurement, it doesn’t indicate how thick the insulation is, and it doesn’t describe how fast leaks occur—those are separate concepts.