Know what you're working with, before buying materials or hiring a contractor.
Modern Building Science --- More Than R-Factors
You cannot know how best to tighten the "building envelope" (foundation, walls and roof) of a conditioned structure without understanding two project aspects: modern building science and how your building was constructed.
How Your Building Is Constructed
Regardless of our attention to detail, we can't see inside closed cavities, such as walls, finished attics, vaulted ceilings, interior soffits, etc. The only true way to determine how leaky your structure's envelope is and how best to tighten it, is to hire a building performance auditor to use an infiltrometer (blower-door) and infrared (thermographic) camera to identify hidden gaps, holes and voids. Ask for the photos so you can methodically address them.
Modern Building Science
For decades, the focus was on installing as much insulation --- measured by R-factor --- as we could fit into our attics. However, modern building science has shifted the focus from R-factors to blocking air movement.
Unless the building was built of an especially modern material, like stress skins, structural insulated panels (SIPs) or insulated concrete forms (ICFs), it's best to focus on blocking the building's stack effect -- the natural flow of air from warm to cool within a leaky building envelope -- as shown in this diagram.
Beyond weatherizing around doors and windows, it's best to begin by sealing the attic and foundation and perimeter rim joists, where they intersect with the first floor's joists to block the stack effect.
Then, if the building still is especially leaky, consider insulating the walls. Besides addressing the stack effect, this approach is usually easier and requires less mess. If building a new structure, insulating the attic, rim boards and walls is a no-brainer.
If not blocked, the Stack Effect naturally moves air within a structure.