After defining the building envelope, select a suitably sized mechanical system. If you do not need to pay to install, operate, and maintain a larger system then why bother.

Building science is greatly focused on the movement of heat, air, and moisture thru the building envelope. Heat always moves from hot to cold. Air & water (always a pair) move from high pressure to low pressure. Moisture migrates from wet to dry.

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.