How to install cavity wall insulation

Updated February 21, 2017

An easy way for a do it yourself homeowner to insulate an existing home is to use blown-in cavity wall insulation. No demolition is needed. Holes are drilled into the interior walls that can easily be patched later. You and a helper can insulate a small to average size single family home in a weekend, and for a fraction of what you would pay a contractor.

Locate and mark the wall studs along all of your exterior walls. Use an electronic stud finder to locate them. Mark each edge of the studs with a pencil about 6 inches from the ceiling. The areas between the studs, called the stud bays, are where the insulation will be blown in. Do not install blown-in cavity wall insulation in stud bays containing light fixtures or chimneys. The heat from these items causes a fire hazard when combined with insulation.

Drill a hole into the centre of every stud bay--half way between every marked stud--about 6 inches from the ceiling. Make the holes about 1/8 inch larger than the nozzle on the insulation blower with a hole saw.

Put the nozzle of the blower into one of the holes in the stud bays. Wrap a rag around the nozzle right next to the wall, sealing it to the wall. This prevents the insulation from blowing back out of the hole. Holding the rag and nozzle securely in place, have your helper start the machine. When the machine sounds like it is struggling and the machine stops blowing insulation, the cavity is full. Tell your helper to turn off the machine.

Repeat Step 3 for every stud bay along your walls.

Patch the holes. First use your finger to push the insulation back into the cavity away from the hole, making a slight indentation. Insert the nozzle of the can of expanding foam sealer into the hole and fill it. Keep filling until the foam mushrooms up outside of the hole.

After the foam is completely dry, use a serrated bread knife to saw off the mushroom part of the foam flush with the surface of the wall.

Cover the foam with several thin coats of drywall mud. Use a drywall putty knife to apply it to the drywall. Fill in any indentations or irregularities in the foam until you have a flat surface that is level with the wall.

After the drywall mud is completely dry, sand lightly with medium-grade sandpaper.

Prime and paint over the patched holes.


Blown-in cavity wall insulation is installed through holes near the ceiling on every floor of your home. Place the blower outside the house because it generates a lot of dust when pumping the insulation through the installation hose. You can rent a mechanical insulation blower from retailers that sell blown-in cavity wall insulation.

Things You'll Need

  • Protective clothing
  • Mask
  • Safety glasses
  • Electronic stud finder
  • Hole saw
  • Mechanical insulation blower
  • Assistant
  • Aerosol expanding foam sealant
  • Drywall mud
  • Sandpaper
  • Wall primer and paint
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About the Author

Sharon Sweeny has a college degree in general studies and worked as an administrative and legal assistant for 20 years before becoming a professional writer in 2008. She specializes in writing about home improvement, self-sufficient lifestyles and gardening.