Airtightness is measured by ACH @ 50 Pa, ELA (Equivalent Leakage Area), & NLA (Normalized Leakage Area). The ACH is affected by the SV ratio (surface area to volume), air barrier system and location, building envelope, and any penetrations thru the building envelope.

The building envelope is a primary component of any building. It is the interface between interior and exterior environments. The building envelope contributes to energy efficiency, air quality, moisture control, fire resistance, durability, cost to operate, and aesthetics.