Air leakage is a building level characteristic. The compliance software distributes the leakage over the envelope surfaces in accordance with the building configuration and constructs a pressure flow network to simulate the airflows between the conditioned zones, unconditioned zones, and outside.
The airflow through a blower door at 50 pascals (Pa) of pressure measured in cubic feet per minute is called CFM50. CFM50 multiplied by 60 minutes, divided by the volume of conditioned space, is the air changes per hour at 50 Pa, called ACH50. This method is used for multifamily buildings with three or fewer habitable stories.
Proposed Design
ACH50 defaults to 7 for multifamily building dwelling units and common areas.
Standard Design
The standard design for multifamily buildings shall have 7 ACH50.
Verification and Reporting
Due to the lack of an applicable measurement standard, the ACH50 for multifamily buildings is fixed at 7 and cannot be lowered. HERS verification is not required because the proposed design cannot be lowered beyond the standard design defined ACH50 of 7.
Multifamily buildings that have floors between dwelling units must define each floor as a separate zone, or each dwelling unit as a separate zone.