There are many types of insulation and materials; their efficiency is measured precisely by the R-value (the insulation’s resistance to heath transfer). The US Department of Energy provides the recommended R-value for each area of the country; obviously, if the climate stays warm year round, you do not spend so much on energy, so the insulation requirements will be different from those in colder areas.
But wherever you are, you should be able to add adequate new insulation or make effective upgrades to the existing level of insulation.
Insulation can be interior or exterior and may be added to the attic or to the walls.
Generally, the attic insulation is 12-15″ thick and must hide all the ceiling joints, in order to be efficient. As about the walls in your house, they need as much insulation as possible, from the floor to the ceiling, wrapped around plumbing, ductwork and electrical boxes. In order to increase the R-value in exterior walls, specialists recommend foam insulation, underneath the siding.