Siding material is not considered insulation. Typically, siding is not a good insulator. Siding sold with thin, insulated panels can provide a little insulation value, but not enough to be calculated as a significant energy-saving benefit. True insulation building materials include fiberglass, cellulose, foam, and ridge sheet siding.
For houses that were constructed in the 1950s, insulated siding was considered a big benefit because the walls of these older homes were barely insulated. Nowadays, the insulation value of siding materials is minor compared to a modern home with properly, well-insulated walls.
Some construction practices include installing large, rigid sheets of insulation behind the siding that is being installed. The insulation may be made of polystyrene, polyurethane or polyisocyanurate. The insulation sheets will noticeably increase the insulation value of the wall.
Careful consideration must be made when deciding to install large, rigid sheets of insulation on older houses that do not have a continuous interior vapor-retarder. The insulation may trap moisture in the wall behind the insulation, eventually causing water damage. The moisture problem is reduced if very thick, rigid sheets of insulation are installed instead of thin ones. The thicker insulation will keep the wall warm and prevent condensation formation inside the wall. A Canadian practice is to add this type of insulation with an R-value that is two times that of the value of the wall.
There have been problems with installing some siding materials over large, rigid sheets of insulation. Wood siding can crack if not installed properly over this type of insulation. The manufacturer’s recommendation must be considered when the siding is installed.