This refers to the building as a singular unit or system. Closing the “envelope” of a home means that we are eliminating any easy access points for the hot, humid Florida air. By doing so, your house systems (wiring, lighting, HVAC etc) should have an extended life. It also improves air quality by greatly limiting the possibility of mold or mildew growing in your attic space. A closed envelope is thought to be more resistant to the extreme pressure changes that occur during a hurricane as well.