A good starting point is to always require and verify that a building enclosure meets the code and ASHRAE standards. Setting more aggressive airtightness standards to reduce energy use requires experience, but can easily achieve dramatic improvements in overall building performance.
An air barrier commissioning agent selects building materials that are airtight, then details the connections between these materials until the whole-building air leakage rate is at or below the standard that is going to be used in the mechanical system design. In addition to a good air barrier design, establishing and implementing quality assurance protocols and inspections during the construction process allows air barrier values well below the code and ASHRAE maximum leakage values. Whole-building blower door compliance testing is used to verify air barrier performance.
See Setting Air Leakage Standards for Buildings (ASHRAE Journal, September 2005) – Authors: H C Fennell & Jon Haehnel