One way to insulate the space between existing walls is to blow in cellulose fiber loose-fill insulation by way of pneumatic equipment. This method is a fast, effective way to fill the stud cavities of houses built with regularly spaced studs. It is less effective for post and beam houses, which have irregular framing that results in odd-shaped spaces (such as triangular spaces between the braces) that are hard to fill.
Blown-in insulation works best where the wall cavities are empty. You won't gain much insulating value if existing insulation already fills most of the cavity. But even when there is no insulation in the wall, some cavities will always be partly blocked by pipes, wires, and built-in obstructions such as horizontal fire-stops. Sometimes, the blown-in insulation will fill around the blockage. Usually, however, a new hole will have to be drilled higher on the wall to feed insulation into the blocked section.
The installation of blown insulation requires special expertise and pressurized equipment, so hiring an insulating contractor may be the best bet. A contractor can fill the empty spaces by cutting small holes through the drywall, inserting a hose, and pumping insulation into the bay between each pair of framing members. Many times, however, it's easier for the contractor to gain access from the outside of the house, by removing courses of clapboards and cutting holes in the sheathing (see below). It depends on which way into the wall cavity causes the least damage while providing the best access. In most cases, it's simpler to remove and replace exterior siding than it is to patch and repaint dozens of small holes in an interior wall. But even working blind with a hose through a hole, experienced contractors should be able to gauge how much insulation the cavity should take and know when the flow of loose fill has been blocked by something like a construction brace or plumbing. In those cases, they may have to make a second hole to be sure that the bay is completely filled.
Although this job is best done by a contractor, those who have experience working with this material may still opt to do it themselves. Some building supply centers rent out the necessary equipment.