You may be able to significantly reduce your heating and cooling expenses by investing in additional insulation.
Below is information about insulation from trusted sources.
American Council for an Energy-Efficient Economy, Consumer Guide to Home Energy Savings: Condensed On-line Version: Buttoning Up Your House
Department of Energy, Insulation Fact Sheet
Energy Efficiency and Renewable Energy Consumer's Guide, Insulation and Air Sealing
Energy Savers, Insulation and Sealing Air Leaks
Energy Star, Home Sealing
North American Insulation Manufacturers Association, Simply Insulate
The Rocky Mountain Institute, Home Energy Brief #1, Building Envelope (Registration is required to download this free publications.)
Washington State University Extension Energy Program, "Inspecting Attic Insulation."
Bonneville Power Administration, "Do-it-yourself Home Weatherization Guide," includes sections on how to insulate ceilings, floors, walls, pipes and ducts; weatherstripping and caulking; storm windows; vapor barriers and ventilation control.