I do a lot of energy audits for Harrisonburg Electric Commission, they offer them free to their customers. The problems I find in most houses are very similar and are caused by lack of building science understanding. Air leaks in your thermal envelope are the biggest source of comfort issues and wasted energy for heating and cooling. You can find air leaks in your thermal envelope by looking at fiberglass insulation.
Find places where your pink fiberglass is black and this is an air leak.
This yellow insulation shows air moving through the wall from an unconditioned attic into a conditioned space. Stop the leaks and you will improve comfort and reduce wasted energy. It is easy to fix this kind of leak. You can remove the insulation and spray foam the cavity or you can cover the leak with a solid surface material and caulk the joints. These leaks add up quick and cost you a ton of comfort and money. You may hear from framers that you don’t want a house that is too tight. That is true, but you don’t want random air leaks around your house that add up to the size of a window. Control your air intake through your HVAC system and plug the air leaks in your home.