Heating and cooling account for 50 to 70% of the energy used in the average American home.4 Heat loss through the basement accounts for 15% – 30% of a two-story home’s total heat loss3, and the attic 25%. R-10 perimeter insulation from siding to footing (with one foot of exposed foundation) will cut basement heat loss by 70%.3 In mild winter climates, this saves $50 – $60/month for an 1800 sqft house with R-10 insulation. The cost is $300 – $600, with a payback of 5 – 10 years. Underground walls are assumed to have no infiltration. For aboveground walls, in winter, sensible heat loss due to infiltration is two to three times the heat loss due to conduction, so air sealing is important. The infiltration losses are halved in summer cooling.


The two main types of insulation are continuous and cavity. Continuous insulation lies continuously alongside structural members, so it is not unaffected by thermal bridging. Continuous insulation is usually foam board (i.e. rigid foam or insulation boards) made of either polystyrene (extruded or expanded), polyisocyanurate, or polyurethane material. Cavity insulation is fitted between wood or metal structural members, such as studs, joists (including rim joists), and beams. It is a blanket type that comes in either batts or rolls that is attached inside the walls. Rigid fiberboard (i.e. rigid fiber or fibrous board) consists of flexible fibers, most commonly fiberglass. Mineral wool (i.e. ROCKWOOL) has a similar R-value but is a better fire retarder, moisture-resistant, and environmentally friendly. There are three sections of the basement walls that need to be taken into account: the aboveground section; the belowground and above the frost line; and the belowground and below the frost line. Whereas an outside wall is exposed to uniform outside air temperature, a belowground slab is exposed to the soil, which varies in temperature depending on the depth below grade. Calculating heat loss is more complicated, as the soil temperature varies depending on the season, climate, soil condition, moisture content in the soil, depth below grade, and whether there is snow on the ground.

Construction Techniques

Cores in concrete blocks promote vertical convection, which is advantageous. However, an 8-inch concrete wall without insulation still has low thermal resistance (R-1.11 & R-1.49 with air films) and warrants adding insulation. Heat loss is reduced by 93% for a 20″ x 30″ basement covered in a 2-inch beadboard (R-8). Using 3.5-inch fiberglass batting (R-11) reduces heat loss to 95%. In colder climates, 4″ – 6″ (R-30) insulation is used. Adding a reflective barrier to the wall adds 16% to the R-value and 50% if the air space is reflective. For slab insulation below a 2-foot depth, more than R-20 is not justified, because the heat would travel around the insulation. Due to earth coupling, insulating an 8-foot wall halfway down with R-10 is practically the same as insulating the 8-foot wall with R-5, with less digging and insulation required.

Where It Goes

Attaching insulation to the exterior belowground has the following advantages:
  • It is less expensive, as it can be exposed and no fire retarder (i.e. 5/8-inch drywall) is required.
  • A proper installation prevents the major air leak at the sill wall.
  • The thermal mass remains available to heat or cool the house.
  • The foundation is protected from thermal stress.
Interior insulation prevents frost heaves from developing because the heat of the house warms up the soil. Frost heaves are problematic for block and stone foundations that cannot resist lateral forces, and spalling can occur. However, foam insulation and backfilling with clean granular fill can prevent cracks caused by freezing and thawing. Avoiding the possibility that an unheated basement falls below the freezing point can be important to prevent freezing pipes. Heat is added by underground furnaces and boilers. Water heaters and laundry equipment, a source of heat, can also be found in the basement. The basement temperature should be halfway between the outside and inside temperatures. Moisture control is important, so mold or bacteria do not grow. In all but arid climates, a vapor barrier is installed on the warm side of the wall. The barrier is usually a thin sheet of PET plastic. It prevents moisture from entering the wall, where it condenses and reduces the R-values of insulation and reflective barriers. The insulating material should not degrade after long-term exposure to the elements.

Calculating Heat Loss

If you use a program (i.e. REScheck) to calculate the loss through a basement wall, at least 50% of the wall must be belowground. Basement walls that enclose heated rooms are part of the building envelope. The wall area should exclude windows and doors. To achieve greater accuracy, the thickness of the exterior wall should be subtracted from the basement floor area. An approximate method for calculating heat loss through a basement is given by the equation: qem = EF x U x A, where EF is a correction factor for a belowground wall and A is the area of the basement walls. Heat loss coefficients for aboveground basement walls are given as U-factors and belowground as F-factors [Btu / (hr-ft-F)].

For the foundation, the heat loss is conducted away from the center out to the perimeter of the slab according to:

Qsc = F x P

  • Qsc = slab edge transmission heat loss
  • P = the perimeter of the slab edge in linear feet
  • F = transmission heat loss per linear foot of slab edge