It's not unusual for old single-family wood-frame homes to lose as much heat through air leakage as they do because of lack of insulation.
Airtightness and high levels of insulation are both critical for efficiency and energy improvement.
Insulation and Sealing complete each Other
Most insulation materials do not prevent air from entering or leaving the house through cracks and holes
On the other hand, air sealing doesn't prevent heat from flowing through the wood, concrete, steel or glass and other materials on the home's shell.
Insulation is also required.
Sealing and insulation are necessary to each other, in order to perform their common goal: to keep the home comfortable and to reduce the energy requirements.
Sealing and insulation use different materials
Air sealing materials are not just the caulks and the foams that we associate to small and medium gaps around windows, doors, light fixtures, plumbing and wiring, or chimneys and fireplaces.
Air sealing materials comprise also air barriers: sheets of materials like OSB, plywood, drywood, housewraps, polyethylene, or concrete and some types of bricks in masonry houses.
Insulation vs. air sealing materials
Many insulation products are poor sealing materials, especially fiberglass insulation.
When people see dirty fiberglass insulation in the stud cavities of their attics, that's a proof that the insulation material is not preventing heat from escaping or entering their houses.
People often think that if they install a lot of insulation in the attic or other parts of the house it will make the house airtight; but that’s wrong, though there are some insulation products, with good sealing properties: spray foams but also dense rigid foams and cellulose.
Basics on air sealing and insulation
Air sealing materials are intended to prevent air from flowing through holes, gaps and cavities in the attic, walls and floors.
Insulation materials are intended to prevent heat from moving through the wall, ceiling and floor materials, or from radiating. Heat can move through walls, ceilings and floors event when they haven't any gaps or holes; to stop it, we use insulation materials.
In cold weather, the goal of insulation materials is too keep warm air inside the house; in hot weather the goal of insulation materials is to keep the outside warm air out.
As to air sealing materials, their goal in cold weather is to to prevent inside hot air from escaping through holes and gaps in the attic and upper parts of the house (ant to prevent outside colder air from entering through gaps and holes in the basement and other lower parts of the house); in the summer, that's the reverse.