Insulating Foundation Walls

By: Scott Gibson , Contributing Writer
In: Old House Construction, Home Improvement Tips

A reader writes: We have a 150-year-old brick house with a stone foundation. There also is an addition with a concrete block foundation on the back. Both of the basements are damp and musty. We want to tighten up the house and insulate the basements, but we get conflicting advice. Is this a good idea? The sill area would be particularly hard to insulate.

Old houses often have lots of air leaks and rarely enough insulation, a combination that makes heating and cooling expensive. Sealing air leaks and adding insulation is always money well spent. This includes foundation walls. But what you want to avoid in the process is making the problem worse by trapping moisture and giving mold a chance to grow.

Foundation walls can be insulated effectively from either the inside or the outside. Several inches of extruded polystyrene foam on the outside keeps the foundation warm and helps prevent condensation inside.

Foam board is easy to add when the house is being built and the foundation walls are exposed. It’s obviously much more difficult after the fact, and the foam can give termites and carpenter ants a convenient way into the house. For these reasons, it’s probably easier to add insulation to the inside of the foundation walls.

One approach comes from the Building Science Corporation, a consulting and design firm that specializes in energy efficiency and building performance. Its solution is to add rigid foam insulation to the inside of the foundation and seal the seams with housewrap tape. This keeps most of the warm, moist air in the basement away from cooler foundation walls.
Add strapping over the top of the insulation and then gypsum drywall. This wall allows any moisture that does accumulate against the foundation wall to dry to the inside.

A suggestion you might hear is to build a wood frame wall on the inside of the foundation, insulate it with fiberglass batts and add a polyethylene vapor barrier to keep moisture away from the foundation. Don’t do it. Although the plastic keeps some moisture out of the wall, some leaks are inevitable, and the plastic traps moisture inside. This leads to rot and mold. Building Science maintains an excellent website where you can read more on this and other energy subjects (www.buildingscience.com).

Insulating the sill (also called the rim joist or band joist) is difficult. You can stuff fiberglass insulation into the framing and cover it with a plastic vapor barrier. But this is not going to do a great job. Spray-in polyurethane insulation would be much more effective. Closed-cell foam not only has very high R-values, but it also makes a good air and vapor barrier at the same time. The two-part foam fills nooks and crannies and seals off leaks better than anything else.

Polyurethane foam is usually applied by trained insulation contractors, and it’s fairly expensive. For smaller jobs, you can buy a kit that allows you to do the work yourself (one such company is at www.fomofoam.com).