Insulating perimeter basement walls can domesticate the space inexpensively by separating the perpetually cool outside walls from the mild interior and creating a much more comfortable living area. Evenly heating and cooling the basement then becomes easier — and cheaper. According to the U.S. Department of Energy, typical annual energy savings achieved by insulating a 1,500-square-foot basement to R-10 range from $250 in St. Louis and Washington to $400 in Minneapolis.
But contractors often install basement insulation the same way as above-grade room insulation — interior stud wall framing with fiberglass batts — so many insulated basements develop varying degrees of moisture, decay and odor problems. Wet insulation dries more slowly than an uninsulated surface, which can increase the likelihood of mold and damage adjacent materials. Dampness also compromises the insulation's thermal barrier.
Controlling basement moisture
"The first step in controlling basement moisture is to make sure the yard is graded and that gutters and downspouts work properly to divert rainwater away from the foundation," says Nathan Yost of Building Science Corp., who has studied basement insulation in new and remodeled homes for the DOE's Building America program.
A capillary break, such as waterproofing or rigid insulation, that separates the foundation from the soil minimizes wetting of the foundation wall, but in a remodeling job, you're often stuck with what you have.
Basement walls probably will get wet sooner or later, whether from internal moisture caused by vapor condensation or spills, leaks or pipe bursts, or from external moisture caused by leaking or damp foundation walls. Anything that covers the walls must allow the walls to dry. A basement insulation system must have the following characteristics:
Yost points out that vapor barriers actually can cause mold growth and even building failures. A misused vapor barrier can trap condensed vapor within a wall assembly and prevent that wall from drying. PATH recommends that vapor barriers be used only on the 100% above-ground wall in a walk-out basement, but not in humid Southern climates.
Insulation systems that work
A Finishing System
Hold the drywall. There's more than one way to finish a basement.
Owens Corning's Basement Finishing System, an integrated wall and insulation system, goes up without drywall dust, tape or paint, and provides R-11 thermal performance with a 0.95 noise reduction coefficient. The panel system and PVC framing accommodate traditional wiring. Panels remove easily for access to foundation walls, pipes and wiring, making the system ideal for homeowners concerned about basement flooding. Unlike drywall, the fiberglass system can be removed, set out to dry and easily put back into place.
But the system remains relatively new and untried. And the panels let air pass through to the basement wall, so moisture problems could develop from vapor condensation at the walls.
Still, early results look promising. Christie Schmidt, who had the system installed in the fall of 2002 in her family's Mason, Ohio, home, says it provides peace of mind. "It's nice to be able to pull the panels off and check for moisture after a big rainstorm," she says. Schmidt adds that despite more than a year of abuse by three children, the walls still look new, and the soundproofing works well.
Here's what a successful basement insulation system looks like:
For dry walls: Insulating systems for basement walls that remain dry even when the ground is soaked are obviously the most practical and cost-effective (see Fig. 1). Install extruded or expanded polystyrene insulation directly against the foundation wall. Seal the insulation joints with mastic and mesh tape so the warm, moist basement air will not contact and condense against the cool basement wall. Cover the polystyrene with 1/2-inch gypsum board to achieve the required fire protection. Attach the gypsum board directly to the polystyrene or to a 2x3-foot frame wall, 24 inches on center, leaving a cavity for additional fibrous insulation and electrical service. Paint the gypsum board with semipermeable latex paint, which will allow any dampness to dry to the interior.
An optional insulated concrete slab would improve comfort and add insurance against moisture-related problems, but at significant cost — probably more than $5 per square foot of floor space.
For wet or damp walls: Insulating basement walls that are damp or become wet under extreme weather conditions is more complicated and can be twice as expensive. These walls (see Fig. 2) would have an insulation value limited to R-8.
Damp walls should be treated the same as wet walls to ensure that they will not be compromised by excessive moisture. These installations require cutting and removing the perimeter of the concrete slab and installing perforated drain tile. Install grooved extruded polystyrene against the basement wall and then install a drainage mat to direct water from the wall to the perimeter drainage system. Pour concrete to replace the perimeter concrete that was removed. Finish the basement wall with a 2x3-foot frame wall, 24 inches on center, attached to the floor and the underside of the floor joists. Use the cavity of this frame wall for electrical service but not for additional insulation because the cavity insulation would increase the possibility of moisture accumulation and mold growth. Finish with gypsum board at least 1/2 inch above the floor to ensure that it stays dry if the floor gets wet.
Alternatively, insulate with material such as spray foam insulation that retards but does not block the inward movement of water vapor. The spray foam insulation would extend over the top of the wall into the rim joist area.
Each house is unique
Those are broad guidelines. Every house is different. When insulating a daylight or walk-out basement, consider its orientation to the sun. In a daylight basement with dry walls, baseboard heat along the perimeter of the three walls that are not completely below grade might be enough to keep those walls dry enough for conventional stud framing or furring strips. Then fill the void with batt insulation and cover with gypsum board or paneling. The area that includes the northern wall could remain unfinished and would be a good place for a laundry room. Waste heat from the mechanical equipment often keeps the area as warm as needed.
Glen Salas is a senior engineer at D&R International, an environmental consulting firm in Silver Spring, Md. PATH is a public-private partnership dedicated to the development and adoption of advanced building technologies.