Insulating Basement Concrete Walls with Foam Board and a 1×3 Grid

Tom Silva of This Old House plugs leaking steel form-tie holes in a basement wall with hydraulic cement before any insulation goes up. The rust marks he spotted around the ties suggested water might be leaking down, and the homeowner confirmed a little water in that corner of the basement.

Checking moisture first

The main concern in any basement job, Silva says, is moisture. A plastic panel had been installed a few weeks earlier to test for it. Condensation between the plastic and the foundation would mean the foundation is not sealed; condensation on the plastic surface would be another warning. Here the plastic was dry.

Plugging the form-tie holes

The steel ties hold the forms when a foundation is poured. When the forms come off, the ties are broken off and should be sealed from the outside. Several were letting water in. A center punch drove each tie in about 1/4 to 1/2 inch.

Silva mixed hydraulic cement to the consistency of peanut butter, mixing only as much as needed because it sets up fast. It expands inside the hole to fill the gap. He had the hole misted with water first so the concrete would not draw water from the cement, then forced the cement in and flattened it.

Foam board

The insulation is 2 ft wide by 8 ft high by 2 in. thick polystyrene with a tongue-and-groove joint. It is stuck to the foundation with foam board adhesive; an adhesive containing a solvent must be avoided because it would eat the foam. Around the carrying beam, Silva marked the beam's edges on a scrap piece, drew a level line across the bottom of the beam, extended reference lines up the sheet and cut the slot with a handsaw. The wall went up in about an hour.

A wood grid for the wallboard

The grid uses 1×3 spruce. Lines were set about 3 in. below the top of the foundation and about 3 in. above the floor, with the spaces divided in half to give five horizontal strips along chalk lines.

The strips are fastened with 4 in. spring spikes, which have a bend rather than a point and act like a spring to hold in the concrete. Each hole is drilled 5 in. deep with a hammer drill and a 3/16 masonry bit.

Room for electrical

Vertical 1x3s go over the horizontals with drywall screws, 16 in. on center, and the wallboard fastens to them. This builds the wall out and makes two chases, so wire can run down vertically or horizontally behind the uprights. A standard outlet box can be mounted by digging out a little insulation and setting it against the 1×3.