How to Insulate a Split-Level Basement

Split-level houses combine a two-level story with an attached single-story structure. One part of the house may have a full basement, and the other a crawlspace. Insulating basements requires attention to moisture, as it comes from inside and outside. Exterior walls should be dampproofed and insulated before the interior is insulated, so that moisture is allowed to escape to the interior as the concrete cures. Overall, when applied correctly, insulating the basement can improve moisture control and air quality for the house, reducing the potential for mold growth, and creating a more comfortable environment.

Extruded polystyrene board (foam board), 2-inch
Utility knife
2-by-4 lumber
Fiberglass batt insulation, minimum R-12
Polyethylene (poly) sheeting, minimum 4-mil
Polyethylene (poly) sheeting, minimum 6-mil
Staple gun and staples
Acoustic sealant
Tape for sealing poly, Tuck Tape or similar
Caulking
Caulking gun
Foam-in insulation (optional)
Gypsum board, or another fire-resistant material

Instructions

- 1
  Install extruded polystyrene board vertically on the exterior of the foundation wall, from the finish grade line to at least 2 feet depth below grade. Backfill to the top of the board.
- 2
  Lay the foam board against the interior basement foundation walls for the full height. Build a stud wall with the 2-by-4 and install against the wall. Fit fiberglass insulation snugly between the studs.
- 3
  Use the staple gun to seal the entire wall with 4-mil polyethylene material. Cover the penetrations of the staples, and any tears or cuts, with the tape or acoustic sealant. Finish the wall with gypsum board, or another fire-resistant material.
- 4
  Friction-fit foam board into the cavities of headers. Seal gaps with caulking or foam-in insulation.
- 5
  Insulate the heated crawlspace with the polystyrene board for the full height of the walls. Add foam board to the joist rim, and to the joists for the floor above.
- 6
  Close off vents after insulating heated crawlspace walls. Place 6-mil poly sheeting over the ground or concrete. Overlap seams, and use stones to weigh it down. Seal the edges.
- 7
  Increase the insulation for the floor above an unheated crawlspace to R-20. Install the 6-mil poly to the crawlspace floor.

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