How to Insulate Exterior Basement Walls


The advantages of properly insulating the exterior walls of a basement can outweigh the cost. Energy costs and moisture reduction are two of those advantages. The typical basement accounts for an average of 20 percent of the heat loss in a home. Insulation on exterior masonry walls helps regulate any temperature changes from outside air. The insulation keeps the walls warm in summer and reduces condensation build-up and humidity in the basement. Most do-it-yourselfers can insulate exterior basement walls with the proper insulation and equipment.

Things You'll Need

  • Shovel
  • Clear plastic
  • Waterproofing material
  • Paint sprayer
  • Tape measure
  • Gloves
  • Utility knife
  • Adhesive
  • Nails with plastic stop washers
  • Tape, foil-faced
  • Caulk
  • Wire brush
  • Aluminum flashing
  • Protective coating
  • Paint brush
  • Remove the bottom four to five rows of siding. Save them to reinstall when the project is finished.

  • Dig a two-foot wide trench two feet with a shovel below the sill around the foundation. Place soil on clear plastic for ease in refilling the trench.

  • Inspect the basement exterior walls for cracks and dampness. Apply two coats of waterproofing with a paint sprayer if continued dampness on basement walls is found. Use a urethane, clay-based or rubber-based waterproofing that holds its elasticity and is self-sealing.

  • Measure the square footage of the exterior basement walls by measuring the width and the height of the walls with a measuring tape. Multiply those measurements to determine the square footage.

  • Choose an insulation rated for outside walls. Solid foam sheet insulation is an excellent choice for basement exterior walls since foam will not absorb dampness. Insulation blankets are also available. Most insulation blankets have facings that work like a vapor barrier.

  • Wear gloves. Cut foam sheets to the height of the wall from the top of the footings to the flashing using a utility knife. Starting at one corner, attach the sheet with adhesive made for foam. Tap nails with plastic stop washer into the studs or masonry walls with a hammer until the washer is snug against the foam insulation. Continue around entire basement.

  • Tape the seams with a foil-faced tape where foam sheets join each other. The foil-facing acts as both a vapor barrier and a air barrier. Alternatively, a rubberized caulk can be used to seal the seams.

  • Scratch the foam surface with a wire brush. This gives the foam teeth for the protective coating stick to. The protective coating protects the exposed insulation from the environment.

  • Attach aluminum window flashing against the building over the foam's top edge.

  • Brush on a coat of fiberglass reinforced coating with a paintbrush from the flashing to six inches below ground level.

  • Back-fill the trench with a shovel. Slope the soil away from the foundation walls. Replace bottom rows of siding.

Tips & Warnings

  • Furring strips can be attached beforehand to the basement wall and the foam sheets can be attached to the furring strips. A paint sprayer can be rented at most home improvement stores. Rigid foam insulation boards should be butted tight against each other.

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