The endless amount of bus conversion Youtube tutorials I consumed leading up to actually acquiring my bus had me convinced the only way forward was to fully rip out the walls, ceiling, and floor. That's exactly what I did. But hindsight is twenty-twenty, and despite what people will try to tell you, the truth is that: like all decisions you'll make along your journey, there is no absolute right or wrong answer. It all depends on your circumstances, the resources you have available, and your ultimate goals for your build.
I'm going to discuss three factors that will help you decide how much to strip, or not strip the interior of your conversion. Along with the additional considerations that might affect your decision.
Water is life, but water also will destroy any and everything in its path. Silicon has a lifespan of around 20 years, so the majority of vehicles approaching that age will have some level of leaking in the windows and ceiling. The environment where the bus has been stored can lengthen or shorten this life. Newer buses are less likely to have compromised seals, but it can be challenging to ensure a 100% water-tight seal especially if you are unable to see where the leaks are (this took several tries for me). My floor was rife with holes and rust where moisture had seeped through.
Counterpoint: While seeing the leaks in my skoolie helped me determine trouble areas and confirm with 100% certainty they were sealed, all but one of my leaks were actually sealed from the outside so I didn’t necessarily need to see them to seal them. Also, I can’t help but suspect all of that hammering and drilling away on rivets may have caused some seals to break that may have otherwise been ok.
While most buses do contain fiberglass insulation behind their walls, fiberglass insulation loses much of its R-value (the measure of insulation) if it gets wet. My insulation was wet and moldy and likely not providing much insulation value. Also, my plans to put in a wood plank ceiling meant making sure it would never be exposed to water.
Counterpoint: I have seen buses in which the original insulation is intact and in pristine condition, but without checking it's basically a gamble. If the floor plan permits the loss of space, insulation can be added on top of existing walls, rendering any existing insulation that might be damaged a non-factor.
Removing the existing materials allows for adding studs, insulation, and wall coverings without sacrificing interior inches. Beneath skoolie walls are a void of around 3 inches (varies by bus model) that provide great space for the guts (wiring, plumbing, studs, etc.) of your build. The amount of insulation I wanted to add would have subtracted at least six inches from my interior width. In a tiny space, every inch counts, and I especially wanted to keep maintain my ceiling height as much as possible.
Counterpoint: If the floorplan/insulation goals allow, subtracting some inches from the living space may not be a huge obstacle to accomplishing your design goals. Also, depending on your height and the interior height of your vehicle., ceiling height may not be an issue.