Insulating Basement Walls

Basement walls are made of solid masonry, which readily transfers heat and cold from the outside. This makes adding insulation a standard part of a typical basement renovation. If your winters are mild, you can get by with an R-value of 7; in colder areas, aim for at least R-11. To achieve these R-values with batts or blankets, frame out 2x3 or 2x4 stud walls over the masonry walls. Then staple the insulation between the studs of your new built-out walls. Cover the wall surface with drywall or paneling.

Rigid foam insulation planks allow you keep a new wall’s thickness to a minimum by furring out the walls with 1x lumber. Solve moisture problems before you begin. Seepage or leaks render insulating materials useless—even if your basement leaks or floods only once every 10 years. Don’t worry about mild condensation; insulation and a vapor barrier will eliminate this. Wall spaces above ground level are more in need of insulation, so pay particular attention there. You may choose to replace a window with glass block. As with crawlspace walls, these insulating techniques could cause frost-heave problems in extremely cold regions. Check your local building codes for approved insulation procedures.

Tools: Hammer, saw, utility knife, caulking gun, stapler.

1. Staple insulation to studs. If you choose fiberglass batt or blanket insulation, frame up new stud walls. Staple insulation between the studs. Don’t skimp on the staples: Drive one every 10 or 12 inches.

2. Insulate the sill. Above the sill, fit in small pieces of insulation. Allow the pieces to drape over the top of the stud wall for complete coverage. Secure the insulation with tape, staples, or wood strips.

1. To install rigid foam, make a grid and glue pieces to the wall. For rigid foam insulation, attach a grid of 1x2s or 1x3s to the basement wall. Use construction adhesive to cement the planks to the walls between the furring strips. Measure as you go to ensure tight fits. Cover the framing and insulation with drywall.

2. Insulate the sill. Cut pieces of batts or blankets to fit snugly in the spaces over the top of the sill. Have the vapor barrier face inside.

Insulating Existing Exterior Walls

An older wood-frame home may lack insulation in some or all of its exterior walls. The solution is to have a pro blow in insulation. This is a tedious and expensive process, but usually pays off in the long run. (However, adding insulation to a home with double-brick construction is seldom cost-effective.) Unless you choose liquid foam, you’ll need a vapor barrier to protect the new insulation from condensation.

Tools: Drill, tape measure, wall-refinishing tools, and specialized tools for blowing insulation into wall cavities.

1. Check the wall cavity. Determine whether the wall cavities run unimpeded from the attic to the basement by dropping down a weighted string. If they do, you can pour in loose-fill insulation from the attic.

2. Identify the framing members. Most wall cavities have headers, sills, and blocking, so you can remove exterior siding and bore a series of holes to gain access. To insulate an entire wall, all the framing members must be identified, and a hole must be drilled into every cavity.

Improving Existing Insulation - Existing insulation in an older home may have compressed to the point where it does little good. If the walls still feel cold after weather-stripping and sealing your home, it may make economic sense to upgrade their R-value. Get several written bids that specify the R-values as well as the amounts of material needed. A properly insulated 2x4 wall should have a value of at least R-10.

Provide a vapor barrier. Without a vapor barrier, all insulation, except foam, will quickly turn into a soggy mess. Unfortunately, it isn’t feasible to get a continuous waterproof membrane into a finished wall. However, you can seal the wall’s interior surface for the same effect. Caulk any cracks you find at the floor and ceiling and around doors or windows. Then apply at least two coats of waterproof oil-or alkyd-base paint, or use a paint especially formulated for this purpose.

3. Remove siding and drill holes. If the siding is horizontal, remove strips at the top, under windows, and possibly at the 4-foot level, just under fire blocking. After peeling back the building paper, use a hole saw to bore holes into each cavity. If the house has shingle siding, individual shingles can be easily removed and replaced. With other types of siding, it is common to drill holes with a hole saw and fill them later with the circular plug that was cut out by the hole saw.

4. Probe the wall. Probe with a steel tape or a plumb bob to see if there are obstructions that might create uninsulated pockets. For a complete job, this probing should be done in every wall cavity.

5. Blow in insulation. First blow material into the cavity to the level of the inlet hole, then flip the nozzle up to fill the space above.

6. Plug the holes. After filling each cavity, insert a snap-in plug to seal the hole. Then apply building wrap or paper over the sheathing.

7. Replace the siding. Finally replace siding materials. Use the same installation techniques described for siding repairs.

Insulating Outside Foundation Walls

Surprisingly the average basement accounts for 20 percent of a home’s heat loss. The best location for foundation insulation is along the outside of a masonry wall, where it can protect the masonry from damaging frost. The mass of the masonry acts to moderate the temperature swings of the outdoor air. The insulation keeps the wall warmer in summer, reducing condensation and basement humidity.

Tools: Shovel, wire brush, hammer, pry bar, utility knife, square, wide putty knife, paintbrush, roller.

1. Remove siding and excavate. Remove several bottom courses of siding. Save the siding for reinstallation. Excavate a trench around the foundation 2 feet below the sill and 2 feet wide. Place the excavated soil onto sheets of polyethylene.

2. Attach polystyrene insulation. Fasten 1-inch-thick sheets of foam insulation to the foundation, using fender washers and masonry nails. Install L-shaped aluminum window flashing against the building sheathing and over the top edge of the foam.

3. Tape the joints. Scratch the surface of the foam with a wire brush to give the protective coating a “tooth” to adhere to. Apply self-adhesive fiberglass drywall tape to all the joints and corners.

4. Stucco the surface. Brush on fiberglass-reinforced latex coating or surface bonding cement. Cover an area from the flashing to several inches below the final ground level.

5. Lay sloping insulation. Lay horizontal 2x 8-foot sheets of foam so they slope away from the foundation at the rate of 1 inch per foot minimum. Replace the soil and grade the area so it slopes away from the building. Replace the bottom courses of siding. Plant grass seed or lay sod.

Log in to comment