How to Insulate Exterior Walls


Insulating exterior walls is an easy do-it-yourself project if you're working with a new construction, a room addition or if you're installing siding. Insulating exterior walls will save you money by reducing heating and cooling costs, and they can also mute unwanted outside noise.

New Siding Application

  • Determine how many sheets of foam insulation are needed to cover the exterior of your home; measure the width and height of your home first. This is not done to determine square feet needed, but to determine how many sheets are needed to fill the space between furring strips installed for the new siding. For example, if furring strips are placed every 2 feet and the foam sheet is 4 foot wide, then a 4 foot wide sheet will cover two sections.

  • Research the different types of foam insulation available; find out the thickness of the sheets and the R-factor, find out if the foam sheets can be secured with adhesives, check mold and insect resistance.

  • Install solid sheets of foam insulation between the furring strips you've installed for the new siding. Fill in all voids with smaller pieces of foam and use caulking to fill in gaps around windows and doors.

New Construction

  • Use batts of fiberglass insulation for new construction or a room addition. A batt is similar to a blanket, but a batt is pre-cut in specific lengths. A blanket comes in certain widths but the length must be cut and is best for a wall without studs, like a basement wall.

  • Determine the correct R-value you need for exterior walls in your location; check the Department of Energy and your own state guidelines as well as the manufacturer recommendations.

  • Install the batt between the wall studs and secure the edges with a stapler or nails; make sure the space is totally filled with insulation.

Adding Insulation to Existing Exterior Walls

  • Use a stud finder to locate and mark the studs on the exterior walls all around the house.

  • Drill circular holes between the studs using a special paddle type bit or a hole saw; vary the height so the holes will be less noticeable.

  • Make at least two holes in every section between studs, one close to the top and one close to the bottom for 8-foot high walls. If the wall is higher, add another hole. Make sure you save the wood that comes out of each hole.

  • Insert the nozzle of expanding foam into the lowest hole and pull the trigger easy; slowly move the nozzle back and forth while pulling it upwards. Continue to the next hole above and repeat the procedure until the section is filled with expanding foam insulation.

  • Cut off any expanding foam that has protruded outward with a serrated knife after the foam has dried.

  • Replace the cutouts from making holes; use caulking or adhesives to adhere the cutout to the hole.

Tips & Warnings

  • If you've used the hole cutout method, sand the hole area and repaint the house for a finished look.
  • Rent a machine at the local rental center to spray in loose cellulose insulation instead of expanding foam.
  • Hire a reputable contractor to insulate existing exterior walls if the job is too big for a do-it-yourself project.
  • Never crush the insulation or pat it in, this reduces the R-value significantly.
  • Make sure the facing on the insulation is installed correctly as it differs in hot or cold climates; check with the manufacturer.
  • Always cover or finish the insulated walls with drywall or similar wallboard; foil or paper facing is a fire hazard and can catch on fire.

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