To balance budget with benefits, we recommend insulating basement walls to 2 feet below grade, which is the coldest part of the wall. Going deeper than that will help, but you’ll get diminishing returns because the wall gets warmer further down. If you live in a very cold area, insulating more can help keep your home comfortable.

Below-grade walls require continuous insulation (CI) with the correct R-value for basement walls. Typically, builders install insulation (sprayed foam, foam board, and/or fiberglass) on the exterior of the wall, which helps insulate the whole house.

The best insulations to use are foam based and should allow the foundation wall assembly to dry inwards. The foam insulation layer should generally be vapor semi impermeable (greater than 0.1 perm), vapor semi permeable (greater than 1.0 perm) or vapor permeable (greater than 10 perm) (Lstiburek, 2004).

Similar to monolithic concrete, a combination of foam board and spray foam is recommended, or taped foam board to air seal and insulate the basement walls. Double brick wall with rubble fill or concrete block. Closed-cell SPF is the easiest way to provide interior insulation and air sealing.