Foundation Insulation: Worth the Efficiency Boost?

Foundation insulation is installed on the exterior or interior of concrete or concrete block walls. Typically contractors and DIYers use rigid foam boards or rigid mineral wool. Insulated foundation covering panels are used between the grade and siding or stucco.

Foundation Insulation

Exterior Foundation Insulation

Any insulation product used on the exterior of foundation walls must be rigid for proper installation. It has to be able to withstand compression from the lateral pressure of the earth pushing against it. The product also has to be moisture and pest resistant.

Exterior insulation involves

  • excavation
  • waterproofing
  • insulation
  • drainage system and backfill
  • Protective coating and flashing

Exterior Foundation Insulation Products

The following products are the most popular and efficient exterior foundation products. All rigid foam insulation products are priced and sold by the board foot. A board foot is one square foot of material one inch thick.

  • Expanded Polystyrene (EPS):Approximately R-3.6 per inch. Closed cell foam. Approximately $0.25 – $0.40 per board foot. Used to construct Structural Insulated Panels and Insulated Concrete Forms.
  • Extruded Polystyrene (XPS): R-5.0 per inch. Commonly called Styrofoam. Most widely used type of foundation insulation. Costs approximately $0.40 – $0.50 per board foot.
  • Polyisocyanurate (ISO): R-6.5 per inch. Foil or kraft paper faced on both sides. Costs approximately $0.40 – $0.60 per board foot.
  • Rigid Mineral Wool: R-4.0 – R-4.3 per inch. Approximately half the price of XPS. Not widely distributed because it is a relatively new product.

New Construction

The most cost-efficient time to insulate exterior foundation walls is during the construction of a new building. The excavation is done and the walls are new, clean, and easy to work on. Most builders are willing and able to insulate foundation walls. Make sure that foundation insulation costs are included in the estimate.

Building with insulated concrete forms (ICF) provides two inches of insulation on each side of the concrete. ICF construction is becoming more popular as heating costs rise.

Adding Insulation To an Existing Basement

People add insulation to existing foundations when fixing other problems like drainage, installing weeping tile, sealing leaks, or repairing cracked concrete. The excavating is already done. Why not take the opportunity to make the house warmer by adding insulation? The cost to only add insulation can be prohibitive because of the excavating, backfilling, and landscaping. The return on investment is relatively low.

Once the excavating is done and the original problems are solved, add the insulation of your choice. Make sure that the concrete is clean. Typically a pressure washer will remove all of the dirt. Hammer off any concrete lumps–usually left by the form ties.

The insulation is attached with a compatible adhesive, Tapcon screws, or self-impaling clips. Seal any gaps and joints with spray foam or caulking. (Do not use acoustic caulking. It never dries and dirt will penetrate the seal.) Ensure that the top of the foam is waterproof and pest-proof by caulking to the concrete and flashing it.

Any foam protruding above grade must be protected from sunlight to prevent deterioration. Backfill the trench carefully to avoid damaging or tearing off the insulation.

Self-impaling clips are 2” x 2” square metal plates glued onto the concrete. They have a roofing nail protruding from the center. Insulation is pushed onto the nails tight to the foundation wall and a square retainer is pushed onto the nail.

Foundation Covering Panels

The above-grade exposed portion of most basements is often finished with parging–a concrete mixture applied directly to the concrete. Or left unfinished. This area–which can be two feet high or more–accounts for up to 22% of house heat loss or heat gain. Especially if the interior of the wall is uninsulated. The R-value of an 8” concrete wall is R-1.35 which provides almost zero thermal resistance.

Foundation covering panels not only add insulation; they also provide a finished look. Most panels are manufactured of extruded polystyrene with a stucco-like finish available in multiple colors and finishes. They are even made to look like stone. Many of them are hard enough to resist weed whacker string and lawnmower scrapes.

Available with R-values up to R-10, they are a simple DIY project that adds a finished look and keeps the house more comfortable while saving heating and cooling costs.

Panels are glued to smooth surfaces with rigid foam adhesive. Use Tapcon screws and/or J-channel to install on rougher walls like parging. Follow the manufacturer’s directions, but make sure the panels extend up to a foot below grade.

Interior Foundation Insulation

Insulating the exterior of an existing 8’ foundation wall is expensive. All of the soil must be removed down to the footings and at least 3’ away from the foundation. Then install the insulation and replace the dirt.

Insulating the interior of the walls with rigid foam insulation–applied directly to the wall, between the studs, or over the studs–is a better and cheaper option. Rigid foam is glued to the walls. The gaps and seams are filled with spray foam or acoustic caulking. Two-inch thick extruded polystyrene or polyisocyanurate also provides a vapor barrier.

Other interior insulation options include professionally installed closed-cell spray foam or you can buy DIY spray foam kits. Or build a stud-framed frost wall insulated with fiberglass batts or mineral wool batts.

Any interior foundation insulation project is less expensive, quicker, and more DIY-friendly than working on the exterior of an existing basement.