For a home improvement project that creates instant improvements to your energy efficiency, we recommend adding insulation in your attic. The energy savings you’ll experience depends on how much insulation is already in your attic and how old it is.
Insulation helps save energy because it acts as a barrier that keeps heat in during the winter and heat out during the summer. In order to maximize the efficiency of adding insulation, it’s best to air seal your attic to prevent air leaks first before adding the insulation. You can do this by caulking, sealing, and weather stripping all seams, cracks, and openings to the outside in your attic.
Referenced from BounceEnergy.com