The purpose of exterior wall insulation is to separate the warm interior of a house from the cold exterior, or, in hot climates, the cool interior from the hot exterior. This is done through the addition of R-value (a measure of thermal resistance) in the form of insulation and the reduction of drafts by minimising air flow between interior and exterior. By separating interior from exterior, insulation reduces the flow of heat to the outside, thus saving both energy and money.
Most houses are insulated with insulation batts that fit between the studs of the wall. These batts can be made of fibreglass, or Roxul, a product manufactured from stone wool. To fit different walls, batts are made in 4 and 6 inch thicknesses, 16 and 24 inch widths and 4 or 8 foot lengths. Batts are easily installed in new construction, but can only be added to existing houses by removing either the interior or exterior walls.
Blown In Cellulose
Blown in cellulose is an insulating product made of recycled newspaper that has had boric acid added to it to deter insects. Blown in cellulose is used primarily in attics, where it sits loose on top of the ceiling joists, and as a retrofit insulation in existing walls. Its advantage over batts is that it can be blown into the walls by drilling a series of holes in the wall, rather than removing the entire wall sheathing. The disadvantage of blown in cellulose in walls is that it can get hung up on wires and other obstructions, leaving unseen gaps in the wall's insulation and it will settle over time, leaving openings for cold at the top of the wall.
Rigid Foam Panels
Rigid foam panels provide high quality insulation, and reduce drafts by extending over the joints between batts and studs that inside the wall. The retrofitting of rigid foam panels to the exterior walls of an existing house is a fairly labour intensive process, as it requires new siding on the outside of the panels, as well as extension of door and window jambs. On new houses, rigid foam can be included in the building process with little extra work. The advantages of this type of insulation are that it will not settle and it is impervious to dampness, water and insects.
There are various green methods of building, including straw bale and cob, that provide integral insulative value in the building material itself. The thickness and porosity of straw bale walls provide heat retention that is equal to more conventional means, using materials that do far less damage to the environment. Cob, which is a mixture of straw, soil, sand and water, provides walls with enormous thermal mass, so they absorb heat throughout the day and release it into the house during the cold night. In hot climates, cob does the opposite, storing the cool during the night and releasing it during the hot day, thus cooling the house without air conditioning.