Once the mechanical rough-ins are complete, insulation will be installed in the wall cavities. Insulation is relatively new—few houses built before 1940 had what we today would recognize as insulation. But in our energy-conscious age, insulation is a given.
Thermal building insulation resists the flow of heat: During the heating season, less warm air escapes and, during hot weather, less overheated air enters through properly insulated ceilings, floors, and extertor walls. Most insulation is made of airy, diffuse materials full of dead air spaces because air is a poor conductor of heat.
There are three basic categories of insulation that may be used in your home.
Batts and blankets. Thick blankets or batts of fiberglass, “glass wool,” or cotton insulation are common in new construction and in retrofitting old where the wall cavities are open and accessible. The insulation, which comes in thicknesses of 1 to 13 inches, is stapled between the studs in the walls or the joists or rafters in the ceiling or floor.
Loose fill. Cellulose (typically made of recycled paper products), fiberglass, cotton, or even perlite or vermiculite are used as loose fill insulation. That means they can be poured by hand or machine blown into ceiling cavities or, in instances where the interior wall surfaces have not been removed, between wall studs. The loose fills tend to flow readily into nooks and crannies. A variation of this is a mix of loose fill with a binder or glue that enables the material to be pumped in and, when the glue hardens, to hold its shape. Foams like polyicynene are installed in the same way
Rigid plastic boards. Typically of extruded or expanded polystyrene, polyisocyanu- rate, or fiberglass, rigid plastic boards are often used in new construction as exterior sheathing for walls or roof decks or interior masonry walls. Rigid insulation is available in a variety of thicknesses, from Va inch up to 10 inches.
Fiberglass batts or blankets must be firmly attached to studs or joists to hold them in place. The installer needs to work the insulation around electric wires in the walls by separating some of the insulation along its length and sliding it behind the wire. In the same way, the insulation must be carefully worked around wall receptacles. If the insulation is compressed, its insulation efficiency is greatly reduced.
When rigid plastic boards are used on interior surfaces, most building codes require that they be covered with a fire barrier, usually a ‘/2-inch layer of drywall. This is necessary because, in the event of fire, the plastic boards give off toxic gases when ignited.
The Vapor Barrier
Once the insulation is in place on the walls and ceilings, the vapor barrier comes next. Typically the barrier is a sheet of polyethylene film, asphalt-coated kraft paper, or aluminum foil. When a vapor barrier is installed in renovated portions of the house and there is no barrier in unrenovated areas, you may want to discuss with your contractor painting at least the ceilings in those areas with a specially formulated paint to form a vapor barrier.
The vapor barrier serves two purposes that are especially important in colder climates. The vapor barrier, as its name suggests, prevents moisture in the air of the home from traveling outward through the walls and condensing when it meets colder air. Moisture in the walls could reduce the effectiveness of the insulation, but more important, it could also lead to decay of the wood structure. The other value of the vapor barrier is to prevent the infiltration of cold air from the outside through any chinks in the walls.