Adding insulation to a wall is a bit more difficult than adding it to an attic because, in a wall, the insulation is hidden between the interior and exterior wallcoverings. Removing the wallcovering to insulate the wall cavity just isn’t cost effective. It’s cheaper and easier to create small penetrations in the wall so that the insulation can be blown in. For that reason, blown insulation is usually the preferred choice when you have to re-insulate a completed wall.
Your best bet is insulating from the outside. Attacking the project from the interior is a bit messier but also less expensive. But, a perfect repair normally can’t be made to the exterior wallcovering — regardless of type
Insulating a completed wall with an insulation blower is easy:
Use a stud finder to locate each stud and any horizontal blocks that exist at or near the center of the wall’s height; then mark each stud center and horizontal blocks with blue masking tape.
Some homes have a horizontal block midway between the top and bottom of a wall cavity. Some have a diagonal row of blocks. Others have none.
Use a drill and hole saw to make a penetration (on center between each pair of studs) into the top of the cavity, approximately 6 inches below the ceiling.
Save the discs of wall-covering material removed by the hole saw. When the project is complete, you (or a wallboard contractor) can use them to patch the holes.
If the wall has horizontal or diagonal blocking, make an additional (second) penetration (on center between each pair of studs) an inch or so beneath the block.
Because studs are spaced every 16 inches or so, penetrations to add insulation must be made at the same intervals, filling one cavity at a time.
Remove old compacted insulation with a vacuum.
Blow in the new insulation.
You can rent insulation blowers, but some stores provide the machine at no charge when you purchase your insulation from them.
Repair the holes and clean up the mess.
Consider hiring a wallboard contractor to repair the holes. In an average home, they can be repaired for a few hundred dollars, and you’ll never be able to tell that the holes were there.