The most common air leaks or drafts into a house are through what seem to be small openings around doors and window frames and through fireplaces and chimneys. Air also enters the living space from unheated parts of the house, such as attics, basements, or crawl spaces. The air travels through any openings in your walls, floors, or ceilings, such as;

• Cracks where two walls meet
• Where the wall meets the ceiling
• Near interior doorframes. 
• Gaps around exterior doors and windows
• Gaps around electrical outlets and switch boxes
• Recessed fixtures and cabinets
• Pull-down stairs
• False ceilings such as kitchen or bathroom soffits 
• Floor cavities of finished attics adjacent to unconditioned attic spaces
• Plumbing connections.

These leaks between the living space and other parts of the house are often much greater than the obvious leaks around windows and doors. Since many of these leakage paths are driven by the tendency for warm air to rise and cool air to fall, the attic is often the best place to stop them.

It's important to stop these air leaks before adding attic insulation because the insulation may hide them and make them less accessible. Usually, the attic insulation itself will not stop these leaks as air continues flowing through the insulation. Sometimes these leak locations are visible because the existing insulation has been stained by dust carried by the airflow.