Examples of where to insulate. 1. In unfinished attic spaces, insulate between and over the floor joists to seal off living spaces below. (1A) attic access door 2. In finished attic rooms with or without dormer, insulate (2A) between the studs of “knee” walls, (2B) between the studs and rafters of exterior walls and roof, (2C) and ceilings with cold spaces above. (2D) Extend insulation into joist space to reduce air flows. 3. All exterior walls, including (3A) walls between living spaces and unheated garages, shed roofs, or storage areas; (3B) foundation walls above ground level; (3C) foundation walls in heated basements, full wall either interior or exterior. 4. Floors above cold spaces, such as vented crawl spaces and unheated garages. Also insulate (4A) any portion of the floor in a room that is cantilevered beyond the exterior wall below; (4B) slab floors built directly on the ground; (4C) as an alternative to floor insulation, foundation walls of unvented crawl spaces. (4D) Extend insulation into joist space to reduce air flows. 5. Band joists. 6. Replacement or storm windows and caulk and seal around all windows and doors. Source: Oak Ridge National Laboratory.
Here is a link to a longer article on the benefits of insulation & Where to Insulate Your Home by the Department of Energy.