Insulating Foundation Walls

Foundation walls give up a significant amount of heat. Insulate poured-concrete and block foundation walls with a minimum of 1 in. of extruded polystyrene insulation. Expanded polystyrene (EPS), polyisocyanurate, and polyurethane rigid insulations are not recommended for below-grade applications.

Cold-Climate Foundation Insulation

Install insulation running the full height of the foundation wall. In cold-climates, “taper” exterior foundation insulation as follows:

  • Install 1-in.-thick 2x8-ft. tongue-and-groove sheets vertically from footing to sill plate.
  • Next, place a second layer of half-sheets (2x4-ft.) extending down 4 ft. with joints offset.
  • Follow with a third layer of 2x8 sheets running horizontally across the top of the foundation.

Insulation Protection

Protect vertical insulation on the exterior of foundation walls with 3/8- or 1/2-in. pressure-treated plywood or a parging coat of Type M or S mortar applied over expanded-wire metal lath (see Stucco).

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