Insulating finished exterior walls need not involve ripping out drywall. It can be done by injecting insulation into the stud cavities between exterior sheathing and interior drywall. The process is non-invasive, makes very little mess, and is easy to patch and refinish. It also costs much less.
Why Insulate Existing Walls?
Many homes constructed before 1970 were not insulated. Energy was cheap and plentiful. Building codes now require wall and attic insulation to meet certain standards. Insulated walls save on energy costs, help provide a more comfortable living environment, and reduce noise entering the home.
How To Insulate Existing Wall
Insulating finished exterior walls involves installing insulation between the sheathing and drywall. Holes are drilled into each stud cavity–either from outside or inside–and the insulation is blown into the cavity. The holes are patched and re-finished. Finished R-values vary from R-3.0 per inch to R-6.3 per inch.
The cost of insulating existing walls ranges from $1.00 per square foot to $4.00 per square foot of wall area. There are only a few types of material that accomplish the job properly.
Spray Foam Insulation
Spray foam insulation is the best choice for wall insulation. As it expands, it fills the cavity, seals cracks and gaps, and forces its way around protrusions like nails and screws, wires and pipes, and electrical boxes.
Spray foam is available as a closed cell or open cell product. DIY spray foam kits can be purchased online or from building supply outlets–closed cell foam only. Open cell foam is only installed by professional installers. It can expand up to 100 times its original size and push drywall off the studs. Closed cell foam is R-6.3 per inch and resists moisture and pests. Open cell foam is around R-3.8 per inch and will absorb moisture.
Cellulose and Fiberglass Loose-Fill Insulation
Both cellulose insulation and fiberglass insulation can be contractor-installed or a DIY project. At least two holes should be drilled in each stud cavity–one about 6” from the ceiling and one about 4’ from the floor–because these products are light and can get hung up on protrusions like nails, wires, and electrical boxes–leaving a void below that reduces R-values.
Cellulose and fiberglass absorb moisture. They are best used in dryer less humid locations or where the wall has an existing vapor barrier. (If there is a vapor barrier, drill the holes from the exterior.) Cellulose is R-3.4 per inch. Fiberglass is R-3.1 per inch. Both products may slump–leaving an uninsulated spot at the top of the cavities.
Retrofit Insulation Options
These options are more expensive than installing insulation into individual stud cavities. They should only be considered when planning a major home retrofit.
If planning an exterior renovation add a 2” continuous layer of extruded polystyrene insulation to the walls. Then install new siding or stucco. The rigid foam board insulation adds R-10 to the house and reduces thermal bridging. New siding and foam cost around $10.00 per square foot of wall area.
Removing the drywall to insulate walls provides full access to all of the stud cavities. Batt insulation and vapor barrier or spray foam can be properly installed around electrical wires and boxes, and plumbing pipes.
Once the insulation is done, new drywall is installed. Then taped, mudded, and painted. All the trim has to be replaced. It disrupts the home and is time-consuming and expensive. Removing and replacing drywall c/w finishing and painting adds between $1.50 and $3.00 per square foot of wall area to the cost of insulating.