How to Insulate Outside Foundation Walls


Insulating outside foundation walls can trim energy costs and reduce the moisture in your basement or crawlspace. Installing insulation on outside (exterior) foundation walls is a do-it- yourself project for most. A few useful tips will help the job go much faster.

Pick the Type of Insulation

  • Look closely at the outside foundation walls; determine if dampness is a continuing problem on the foundation walls.

  • Measure the square footage of your walls to determine how much insulation you will need. Measure the width and the height of the walls in total and multiply them. For example if the wall is 6 foot high and 20 feet long, multiply 6 x 20 to get 120 square feet.

  • Price different types of insulation suggested for outside foundation walls. Insulation blankets are large blankets with a facing on them; some facings can be used as a vapor barrier, while others may need a separate vapor barrier. Solid foam sheet insulation may be the best insulation for outside foundation walls. If the walls are prone to dampness, the foam sheet will not wick the dampness and will maintain it's insulation value regardless of dampness or chemicals.

  • After buying the insulation, remember to get tape, gloves and a utility knife for cutting. Decide on either adhesive glue or nails that can pierce the thickness of the insulation into the furring strips. If you use adhesive for foam sheets, make sure it's the type recommended by the insulation manufacturer.

Installing Foam Sheet Insulation

  • Cut the foam sheet the full height of the wall, from the top plate to the floor. If there are wall studs installed, cut the foam sheet the same width as the space between the studs.

  • Install the foam sheets butt against each other if there are no studs on the foundation walls.

  • Apply adhesive to the foam sheets, according to the manufacturer directions; make sure the sheet has been cut to fit first.

  • Use nails that will pierce the wood furring strips or studs by at least an inch; there are special nails with a plastic stop washer that protect the foam sheeting from large holes.

  • Tape the seams where the foam butts against each other after securing it to the foundation wall.

Installing Blanket Insulation

  • Buy enough furring strips to string across the top and bottom of the foundation wall; install furring strips across the center of the wall also, where the blanket seams will meet. For example, if the blanket is 3 foot wide, measure down 3 feet from the top and install the center wall furring strip.

  • Install the blanket insulation horizontally, securing it first at the top plate across the length of the wall. Roll out and install the second horizontal row, securing it to the center furring strip.

  • Attach the bottom of the first row to the same furring strip so they both meet at the furring strip.

  • Tape the seams where the blanket insulation meets.

Tips & Warnings

  • Use a vapor barrier on an earthen floor if you're installing blanket insulation; cut the blanket insulation about 2 inches above the floor level.
  • Fill openings around pipes and windows with caulking before insulating.
  • Make sure the facing on the insulation is installed correctly for your climate. Consult the manufacturer for the appropriate insulation for your area.
  • Never install insulation too close to a light fixture or heat source unless it is specially rated.
  • Always cover any insulation with a paper or foil facing with drywall or finished wallboard. Paper and foil facings are fire hazards.

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