How to Insulate Below Grade Basement Walls


Below grade basement walls can remain much colder than walls that are located above grade because of the dampness in the surrounding soil. It is important that you use several methods of insulation to properly seal and keep the moisture out of your basement. Properly insulating your below grade basement walls can help reduce your heating and cooling bills, as well as make your basement more comfortable. This method should be used to insulate an existing basement.

Things You'll Need

  • Shovel
  • Measuring tape
  • Bituminous coating
  • Paintbrush
  • Waterproof sealant
  • Flashing
  • Expanded polystyrene insulation board
  • Construction adhesive
  • 2x4 lumber
  • Exterior grade plywood
  • Galvanized nails
  • Hammer
  • Dig a trench about 18 inches deep and two feet wide around the foundation of your below grade basement. Clean the exposed wall with a stiff brush to remove dirt and debris.

  • Ensure that the previously installed damp proofing is in good shape. Apply a layer of bituminous coating, available from a building supply store, to the damp proofing material with a large paintbrush to repair any damaged or cracked areas. Allow the bituminous coating to dry for approximately four to six hours before continuing.

  • Seal around all exterior water taps, electrical outlets, exhaust vents and gas lines using a waterproof sealant appropriate for your basement walls. The sealant should be applied all the way around the fixture. Allow the sealant to dry for two to four hours.

  • Loosen the bottom edge of your house siding so a piece of flashing can be installed. Slide the flashing under the edge of the vinyl, so it will form an additional barrier against moisture.

  • Install a layer of wall insulation around the basement foundation. Select a high density expanded polystyrene that is rated at a level R-5. Secure the expanded polystyrene to the concrete foundation with a heavy-duty construction adhesive or small galvanized nails.

  • Lay a layer of wall insulation flat against the bottom of your trench around the basement foundation and press it against the soil. This horizontal layer of insulation is called the apron method, the most cost-efficient method of insulating a pre-built below grade basement.

  • Attach pressure treated 2x4s to the outside bottom of the extruded polystyrene to act as a ledge, helping keep the insulation board upright.

  • Apply exterior grade plywood over the extruded polystyrene to protect the insulation board from moisture using galvanized nails. The plywood should reach at least six inches below ground level.

  • Replace the dirt around the foundation and tamp it down firmly. The soil should slope down one inch for every 18 inches that it moves away from the basement. The sloped soil is important to direct water away from the house and basement.

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