You wouldn't think of leaving your doors or windows open during the winter when you're trying to heat your home, right? Your wouldn't leave them open in the summer, either, when you are running the air conditioner. But, having a home without insulation is doing just that - leaving the house open year round.
Materials that insulate well do so because they are poor conductors of heat. Instead of passing heat, they form barriers between interior and exterior spaces - between warmed interiors and cool weather, or cool interiors and hot weather. Since insulation is as effective in hot conditions as in cold conditions, less energy is required to cool homes in hot weather or heat homes in cool weather. Well-insulated, energy-efficient houses maintain an even temperature year-round.
Ideally, you should insulate every surface between your house and the outside world - any surface that meets an unheated or uncooled portion of the indoors on one side and your living area on the other. This means every exterior wall and every wall, floor and ceiling between your living area an unconditioned portion of the indoors, such as a porch, garage, attic, basement, utility room, crawlspace and cellar.
There are lots of choices for insulation - from loose fill, batts or rolls of the "pink stuff," to rigid boards and foam-in-place products. Some materials insulate better than others, but all insulation saves energy and money. Check out Home Construction for more information of types of insulation.