Adding Insulation to Basement Walls
If you live in South Carolina, Alabama, Oklahoma, Southern California, or anywhere colder, your basement walls should be insulated. In climate zones 3 and higher, basement insulation is required by the 2012 International Residential Code as follows: R-5 in climate zone 3, R-10 in climate zone 4 (except marine zone 4), and R-15 in marine zone 4 and climate zones 5, 6, 7, and 8.
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.
Make sure your basement is dry
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.
Use foam insulation
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.
Building codes require most types of foam insulation to be protected by a layer of gypsum drywall. Many builders put up a 2×4 wall on the interior side of the foam insulation; the studs provide a convenient wiring chase and make drywall installation simple. (If you frame a 2×4 wall, don’t forget to install fire blocking at the top of the wall.)
If your basement has stone-and-mortar walls, you can’t insulate them with rigid foam. The only type of insulation that makes sense for stone-and-mortar walls is closed-cell spray polyurethane foam.
If you plan to insulate your basement walls with spray foam, the best approach is to frame your 2×4 walls before the foam is sprayed, leaving a gap of 1-1/2 in. to 2 in. between the back of the studs and the concrete wall. The gap will be filled later with spray foam. If you live in an area where termites are a problem, your local building code may require that you leave a 3-in.-high termite-inspection strip of bare concrete near the top of your basement wall.
While reduced costs might tempt you to use fibrous insulation such as fiberglass batts, mineral-wool batts, or cellulose, these materials are air permeable and should never be installed against a below-grade concrete wall. When this type of insulation is installed in contact with concrete, moisture in the interior air can condense against the cold concrete surface, potentially leading to mold and rot.
Don’t worry about inward drying
Some people mistakenly believe that a damp concrete wall should be able to dry toward the interior—in other words, that any insulation on the interior of a basement wall should be vapor permeable. In fact, you don’t want to encourage any moisture to enter your home by this route. Don’t worry about your concrete wall; it can stay damp for a century without suffering any problems or deterioration.
Avoid polyethylene vapor barriers
Basement wall systems should never include polyethylene. You don’t need any poly between the concrete and the foam insulation, nor do you want poly between gypsum drywall and the insulation. If your wall assembly includes studs or furring strips, polyethylene can trap moisture, leading to mold or rot.
Basement insulation is cost-effective
If you live in climate zone 3 or anywhere colder, installing basement-wall insulation will almost always save you money through lower energy bills. It will also provide an important side benefit: Insulated walls are less susceptible to condensation and mold. This means that insulated basements stay drier and smell better than uninsulated basements.
Drawings: Steve Baczek, Architect