The goal of a room air distribution system is to provide thermal comfort and a healthy living environment for occupants in the space. This article looks at designing such assemblies for comfort and indoor air quality (IAQ) per ASHRAE 55 and 62.1. It examines the common method of overhead distribution, but focuses on partially mixed (i.e. most under floor air distribution [UFAD]) and fully stratified (i.e. displacement ventilation) systems.