Once the necessary Water Control, Air Control, and Vapor Control are achieved for the building enclosure, then it is appropriate to focus on reducing conduction heat flow through the building enclosure. Conduction heat flow is reduced through the use of insulation. More insulation results in less heat conduction. Less heat conduction means the home is better at retaining heat in winter and keeping excess heat out in summer.

While thermal control is subordinate in importance to vapor control, insulation sometimes provides the necessary vapor control strategies by controlling the temperature of moisture-sensitive materials. Sometimes vapor control strategies dictate higher levels of insulation than would be needed to meet thermal control or energy performance goals.