How to Insulate my Basement Walls?

If you want to insulate the interior of your basement wall with Spray Foam, specify closed-cell Spray Foam, not open-cell foam. Closed-cell foam does a better job of stopping the diffusion of moisture from the damp concrete to the interior. Frame the 2×4 wall before the Spray Foam is installed, with a gap of about 2 in. between the 2x4s and the concrete.

If your home lacks basement-wall insulation, it’s much easier to install interior insulation than exterior insulation. Here’s how to do it correctly.

Before installing any interior-wall insulation, verify that your basement doesn’t have a water-entry problem. Diagnosing and fixing water-entry problems in existing basements is too big a topic to be discussed here (but see “Build a Risk-Free Finished Basement,” FHB #248). Suffice it to say that if your basement walls get wet every spring or every time you get a heavy rain, the walls should not be insulated until the water-entry problem is solved.

The best way to insulate the interior side of a basement wall is with foam insulation that is adhered to or sprayed directly on the concrete. Any of the following insulation materials are acceptable for this purpose: closed-cell spray polyurethane foam or either XPS, EPS, or polyisocyanurate rigid foam. Rigid foam can be adhered to a poured-concrete or concrete-block wall with foam-compatible adhesive or with special plastic fasteners such as Hilti IDPs or Rodenhouse Plasti-Grip PMFs. To prevent interior air from reaching the cold concrete, seal the perimeter of each piece of rigid foam with adhesive, caulk, high quality flashing tape, or canned Spray Foam.