To begin with, what is a "building envelope?" Is it the same as a "building enclosure?" And what about the building's "shell?" Is it the same as a "thermal boundary?" OK, hang with me here. These terms are often used to refer to the same thing, but let's parse a little.
Every home has a boundary between indoors (conditioned space), and outdoors (unconditioned space). Some homes' boundaries are intact, many are not; sometimes the boundary between indoors and outdoors is so leaky, the conditioned space is referred to as "well-connected" to unconditioned space. Attics and crawlspaces that are ventilated with simple vents are considered unconditioned space; if it is encapsulated and heated/cooled, it is considered conditioned.
Typically, insulation is installed along the boundary between inside and out. When it is installed properly, it is in continuous contact with an air barrier. An air barrier is a durable material that resists the passage of air; examples include sheetrock, rigid foam insulation, metal flashing, plywood, OSB, and wood. (Fiberglass insulation is an air filter, but not an air barrier.) Such a barrier prevents airflow from circumventing insulation. Where insulation and an air barrier are installed together, they are called a "thermal boundary."
In a simple house, the building envelope or building enclosure is formed by the floor, walls, and ceiling assemblies, the boundary between indoors and outdoors. (Ideally, this boundary is also a thermal boundary.) The older term for these components is building envelope; more and more building scientists and practitioners are using the term enclosure.
Most people would consider the roof a part of the protective "shell" of the house. In the case of a sealed attic, where the underside of the roof deck is insulated instead of the space above the top floor's ceiling, the term shell would work as an equivalent for enclosure. However, this is usually not the case and can lead to confusion.
In my reports, I will for now continue to use the terms envelope, enclosure, and envelope/enclosure. Know that they are the same thing. Over time, I will move toward the term enclosure. Now, let's get on with the primer.
Envelope/enclosure leakage is a very important and multifaceted issue. Since you pay to heat the air in your house in winter and to condition (cool + dehumidify) it in the summer, loss of this indoor air (exfiltration) is a major waste of energy and money. Homes that have not been air sealed can waste up to 30% of their energy use in this way.
Also, with exfiltration comes infiltration. Since nature abhors a vacuum, air that leaks out is replaced by an equivalent amount of air that leaks into the home. This outdoor air is typically hotter or cooler than indoor air, depending on the season, and this causes drafts, hot/cool spots, and other comfort issues. In addition, most of this infiltration tends to come from the lower segments of the home, often a crawlspace, basement and/or garage. When this occurs, odors, fumes, radon, mold spores, and other pollutants come along for the ride, causing indoor air quality issues. The air drawn in from higher levels often comes via the dusty and hot attic, with all that entails.
Further, air contains moisture. Indoor air gains humidity from cooking, bathing, and even occupants' breathing. When this humid air leaks into a building cavity such as a wall or ceiling, the moisture will condense on the cooler building materials it contacts, such as wood and sheetrock. This sets the stage for potentially serious levels of mildew, mold and even rot. This is not good!
Where do air leaks tend to occur? While there are lots of possible locations, some are more common. These include leaks in duct work, open fireplace flues, ceiling light fixtures, loosely fitting windows, doors, and attic access panels, and penetrations where electrical wires and plumbing pipes go through floors, walls, and ceilings. Other locations are less obvious, happening behind the sheetrock where unseen gaps and holes exist. This air can show up in seemingly odd places, like around outlets and switch plates. In this instance, the air is finding its way into the walls, moving between studs through holes made for electrical wires, plumbing pipes, etc., and escaping around the gap between the switch box and sheetrock.
Experienced do-it-yourselfers can find and seal some of these leakage sites. A safer approach is getting an energy audit to help find them and hire an experienced crew to seal them.