How to Blow Insulation Into Old Plaster Walls

Chris Deziel

If your house is old enough to have plaster walls, chances are that it is also too old to have sufficient insulation. The only way to insulate these walls without removing the plaster is to blow insulation into them. Fortunately, this is easy to do, but you should be aware that plaster walls usually do not have a moisture barrier. Without one, the insulation may get wet and clump at the bottom of the wall. Therefore, you should also paint the walls with a moisture-proofing paint and caulk all crevices in doors, windows and exterior siding.

Plaster walls usually lack a moisture barrier.
  1. Find the studs in the wall and make holes at the top of the wall in the bay between each pair of studs. Since old plaster walls often have cross-bracing, you'll also have to make holes at the bottom of each bay. Cut these holes with a hole saw attached to a drill and make them just large enough for the nozzle of the sprayer.

  2. Make sure the floor and all furniture are covered with plastic sheeting, then spray insulation into each hole until the bay is full. You'll be able to gauge this by the pressure of the sprayer. Tap the walls as you spray to dislodge insulation from pipes and wires in the wall.

  3. Spray foam insulation into each hole when you are finished blowing. Spray the hole about half-full and allow the foam to expand and bulge out of the hole. When it has set, cut it flush to the wall with a drywall saw. The dried foam will give you a firm surface on which to spread plaster.

  4. Use a drywall blade to pat the holes with plaster or drywall joint compound. Apply two or three coats, sanding between each application, until the holes are blended with the wall.

  5. Apply a moisture-guard paint to the interior and exterior of the wall after first caulking all gaps, cracks and holes with a moisture-resistant caulk.