Cold air seeps into your house through small holes and cracks, while heated air leaks out. Both can cost you plenty, since you pay to heat the cold air coming in and you have already paid to heat the warm air leaking out.
Install Rope-Caulk Weather-Stripping
It's a cheap, flexible, clay-like material that is easy to install. Press it in the cracks between the window sash and frame. It dries over time, so check it once or twice during the heating season to be sure it's still tight. It should be removed in the spring and replaced in the fall. If you keep your windows closed during the summer, replace the old caulk in the spring.
Fill in cracks with caulk where:
Any two different outside materials meet
Dryer vents and fan covers pass through the wall
The fireplace chimney meets the siding
Plumbing pipes and telephone wires enter the house
The wooden sill of the house meets the foundation
Use compounds that are flexible over large temperature changes and will last for many years. They may cost a little more, but they're worth it.
Compounds can be used to:
Install switch and outlet gaskets
Seal air leaks into the attic
Stop drafts around electric light switches and wall outlets with inexpensive styrofoam or foam rubber gaskets which fit behind the cover plates
Stuff gaps around chimneys with unfaced fiberglass batt. Seal any connections between the heated space and the attic, such as plumbing, vent stack openings and the tops of interior and exterior walls or stairway framing.
Weather-strip around the edges of the attic hatch or door to reduce the amount of warm air leaking into the attic from your living space. Also insulate the back of the hatch or door with a piece of fiberglass or rigid board insulation.