Air sealing an existing house involves identifying and sealing air leaks throughout the home. The process typically involves sealing gaps and cracks in walls, floors, and ceilings with caulking or weatherstripping, and insulating around doors, windows, and other penetrations. Common areas where air leaks occur include around electrical outlets, plumbing fixtures, and ventilation fans. It's important to use high-quality materials and work with a professional to ensure that the air sealing is done correctly and safely. This can help improve indoor air quality, reduce energy bills, and increase overall comfort in the home.