Insulating an older home presents several problems
A: The tar paper on older homes is an excellent air and moisture barrier, much like the house wraps used today. Insulating older homes presents several problems for the average homeowner. For instance, using a rigid foam insulation provides resistance to thermal transfer, but the foam board cannot be left uncovered.
An ignition barrier needs to be installed over exposed foam boards to protect the foam from igniting, which could give off poisonous fumes. Spray foam also needs to be covered when exposed to areas of the home where there are lights, electrical connections, heating systems, water heaters, dryers, etc. — anything that can generate a flame or a spark. Kraft faced fiberglass insulation requires an ignition barrier because the infused Kraft paper is flammable.
The stud cavity in older homes can be filled with loose-fill cellulose insulation or low-expanding spray foam. Blown-in cellulose insulation has been extensively used, but the insulation may settle over time, leaving underinsulated cavities inside the walls. Spray foam will provide a better thermal barrier, with fewer, if any, voids inside the walls. The contractor will drill holes on either the inside of the home or the exterior, depending on the damage that may be done to the exterior wall covering. Foam insulation is injected into each wall cavity, and then the holes are filled.
On the interior, the holes can be sanded and painted to blend in with the decor. On the exterior, some holes are filled with plastic or wood fillers, and some are capped with air vents to allow the wall cavity to vent moisture. With cellulose or expanding foam there will not be a vapor barrier. Ask your paint dealer about certain paints that can provide a vapor barrier to prevent moisture migration from the warm side of the wall to the wall cavity.
Another project to help stop airflow and loss of heat would be to insulate the gaps and voids around windows and doors. Depending on the style of windows, the trim around the window is removed, the gaps are filled with low-expanding foam, and the trim is reinstalled. Older windows that use sash cords with counterweights cannot be insulated in this way.
This is a brief description of methods of insulation for an older home, but your insulation contractor should be able to advise you on the need for a vapor barrier, depending on your climate zone.