One common way to install foam insulation in an existing wall without tearing it down is to inject spray foam into the wall from the outside. Spray foam expands upon contact with air, causing it to fill spaces on its own and thus keeping interior walls intact.
Many older homes contain batt insulation, which is made of materials such as fiberglass or cotton and requires exact measurements and cutting to create a piece that fits in an exact area. When installing this into a wall, the interior of the wall must be open so that the insulation can fit inside, which often leads to excessive construction on an existing home. To avoid this, many home owners use expanding spray foam to fill wall space, as it only requires a small access point into the wall cavity.
Spray foam is a poly material, which means it consists of many different elements that react with its surroundings to form the final product. Contractors commonly install this type of foam by cutting a hole near the base of an exterior wall, in between the studs, and spraying the foam into the space. As it mixes with the air, it grows to fill the space. The contract is then able to fill the hole on the outside and preserve the appearance of the internal wall.