Danny, Some of that depends on how you define "outside noise". If you mean noise that doesn't come directly from your vehicle or its interaction with the environment, doors would be first since they let in most traffic noise. Doors make a good first project. They can be done without disturbing the rest of the vehicle. I know from personal experience that you can drive a car for days without trim panels. On warm days you can drive without glass Noise rejection treatments involve installing a barrier. If you hang this layer from the inner skin and have door mounted speakers, you'll also be isolating the front and rear wave coming from the cone, improving speaker performance. An interesting thing happens when you start by treating one area. In a noisy car, you are sitting in a swirl of sound coming from every direction. This makes it difficult to localize the sources. Treat just the doors and you are likely to be able hear engine, exhaust and tire/wheel noise better than you could before. It's not louder, just easier to localize and identify. That leaves noises generated by the vehicle - engine, exhaust and its interaction with the environment - tire/wheel and wind, maybe rain too. Engine, exhaust and tire/wheel noise are complicated. The firewall and floor are where they penetrate, but not exactly as we tend to think. Engine and exhaust noise do hit the passenger compartment in a direct line, but they also reflect off the pavement, meaning they even enter through areas from which you can't draw a straight line back to the source. Tire noise is especially interesting. Most of us think that most of it comes in through the wheel wells. That's led to all sorts of exterior treatments that are supposed to avoid the complication of getting to the interior surface of the floor. It seldom makes an audible difference. The important point is that tire noise starts at the contact patch with the pavement and radiates up in every direction from there. A lot of the sound that would otherwise pass through the wheel well is blocked by the tire and wheel. The reality is that tire noise enters through most of the floor. Interestingly and despite years of tradition pointing in exactly the opposite direction, you generally don't need much supplemental vibration damper on the floor of a vehicle, especially if you don't plan to remove and replace the factory application. The floor is almost always braced and reinforced in a variety of ways that make it much less susceptible to resonance than other areas like the doors, roof and quarter panels. Add the bolted down seats and the weight of a human being or more and you are most of the way there. A barrier layer is always going to yield the biggest improvement. Spare tire wells are another area that have been traditionally over treated to the extreme. The usual approach is to remove the spare, tap the sheet metal and marvel at the flimsy construction and extreme resonance. Many wheel wells WILL benefit from some supplemental treatment. The confusion results from the fact that the spare itself is part of the manufacturer's NVH treatment. With the spare in place, the well has completely different characteristics. The roof is interesting. Glass will always be the weak link in any noise attenuation program. This isn't as big a problem as we might fear because most of the noise we are concerned about strikes the vehicle below the glass line. What this means in terms of treatments is that a barrier isn't going to help much unless you are frequently exposed to noise sources directly overhead. I had one customer who spent a lot of his day driving around an airfield. For most of us it isn't a problem. Resonance can be a major problem for the roof, especially since it is such a large panel, right above your head. This resonance can be excited by a wide variety of sources. My rule is to tap on the roof from outside the vehicle. If it rings, treat it. If not, it's probably not worth the effort.