Although the emphasis on saving home heating and cooling energy has been on attic insulation, exterior walls have a similar surface area that transmits energy.
Homes built prior to 1960 had no insulation or at best R-5. Between 1960 and 1980’s average R values increased to about R-11.
Insulating wall cavities that were empty or under-insulated will save energy. A 2″ x 4″ stud framed wall filled with expanding foam insulation will provide an insulating value of R-22.
Before insulating side walls you need to find out what is in there. Visual inspection through openings may reveal what is already in place, if anything, or an infrared thermal imaging will reveal heat loss in areas that are hidden or have varying insulating value.
Sometimes cellulose, fiberglass, or rock wool may have been blown in, and either not installed at the proper density or settled and lost effectiveness.
Adding insulation to partially filled wall cavities is difficult and a job for a well-qualified installer. Expanding foam insulation may be the best solution when done correctly. Beware of old knob-and-tube wiring because of the possibility of creating a fire hazard. Be sure to check with a licensed electrician.
If you are replacing exterior siding, it may be a good time to add insulation. Installing rigid board under new siding may not be the most cost effective method and complicates window and door opening depths and sills. Injecting expanding foam wall insulation could be your best choice and not require modifying window sills and trim.
If exterior walls are opened for remodeling consider in place expanding spray foam insulation for best R-value and reduced air infiltration.
Technicians with special equipment can test your house for air leakage. This test measures the air leakage of your structure at all localities and can reveal where hidden air flow occurs around framing, wall penetrations for plumbing and wiring, and chimneys and attic access.
Insulating wall cavities not only saves energy, but also improves comfort in winter and summer, drafts are reduced, and your home will be quieter with expanding foam wall insulation.