When I purchased my brand new Sprinter cargo van, it had no windows and was essentially a bare metal can. This includes the cab are in which the two front seats are, so the driving experience is not enjoyable for even moderate trips due to the high road noise from other vehicles and thermal influence from the outside temperatures. On a hot sunny day, before adding insulation, the roof would literally feel hot to the head that was 1-2 feet away from it. The heat could be felt radiating in. So one of my first tasks on my camper van build out project schedule was to insulate the van, thoroughly and well.
I started with some sheets of Noico vibration reduction product--any similar product will work. And installed the recommend size to each of the van's bare metal interior panels. Recommend size was about 1/4 of the sheet metal section. There are many of these sections--dozens in the roof and one each walll. I used this sparingly as it is very heavy and it not for thermal insulation--only to reduce the vibrations from the metal panels, which are significant contributions to noise and some of these panels are very large such as the wall panel sections where windows can be installed.
I then added a silicon chalk to the inside of the lower plastic exterior trim clips from the inside to ensure that water and other debris doesn't enter thru those clips as has been reported. I then sprayed a rubberized sealant to the bottom of the wall panel cavities as an additional sealant to prevent any rust that could build up in those lower wall panels. They are already very well coated with a thick rust sealant by Mercedes.
I then added Thinsulate SM600 (used by many vehicle OEMs) for both thermal and acoustic comfort. This was first cut into appropriate sized strips and stuffed and pulled thru the many, many wall, door and ceiling pockets and ribs so that there were stuffed full. This is really important as they are direct connections from outside wall to inside wall to transfer heat, and the doors are filled with these pockets. So I stuffed Thinsulate into all of the door pocket sections including around the windows on rear doors and slider, and also stuffed the front door pockets too.
I think glued using spray adhesive the sheets of Thinsulate that I precut to fit each of the Sprinter's wall and ceiling panel sections. Turns out the Mercedes engineers must use French curves to create a new curve for every wall panel and section, and really like to make each section a different size. It sure would've saved hundreds of hours over the build out if they used right angles, flat sections and kept the wall and ceiling sections panelized so most were the same size.
Now that the Thinsulate was pulled thru wall and ceiling structural ribs, door pockets and window frames, and attached to the larger panels, I then added Loow-E reflective foam core insulation over the entire wall sections, including over the structural ribs to thermally break them from the interior wall panels. I also added rivnuts into existing wall and ceiling holes to mount my ceiling and interior wall elements. I repeated the same for the ceiling. All of this reflective foam insulation was in continuous sheets along the length of the van, and overlapped at a midpoint. Then sealed with foil tape so that the entire interior is "sealed" from the outside for any draft penetrations, as well as a moisture barrier between inside and outside. Incidentally, I do not use any propane in my van, so that certainly keeps moisture down, and even drain out refrigerator moisture to the outside.
For the floor, I cut 3/8" high density foam into strips with trapezoidal cuts to match the convex ribs of the floor and fill them so level with the top of the ribs. Then used full sheets of the same 3/8" high density foam over the entire width and length of the van to cover the top of ribs and add a second layer to the troughs. I wish I added one more sheet, as the floor can be a little cool when camping in the winter. Walls and ceiling not--they certainly insulate well. Factory subfloor was reinstalled, and covered with a Commercial vinyl flooring, continuous one piece from front to rear, and glued down. Factory floor transition strip was screwed down at the front with longer and better stainless rounded head screws, and a new beefier transition piece was fabricated from aluminum angle for the rear to accommodate the now taller/thicker floor.
The ceiling not fully covered in reflective foam with thinsulate as well, I attached fiberglass U channels to the rivnuts running the length of the van, and then acrylic ceiling panels (only two the entire length of van) attached to the fiberglass U channels so that they act as a thermal break to the ceiling. Meaning, the interior ceiling is never directly attached to the exterior ceiling. This was a lot of work and time, but does prevent any condensation from forming at all, a key focus, and I have camped and traveled thru areas with below 0F temperatures without any condensation or noticeable cold bridges other than windows, so it works well. Incidentally, my ceiling lighting is LED strips running continuously in several strips along the length of the van (front and rear circuits as well as "emergency/aisle circuit) above the frosted acrylic sheet so that the ceiling is a continuous smooth ceiling with no breaks or visible light fixtures.
I tried to connect the wall panels the same with a thermal break to the van walls, but could not. So what I did was used 60" wide panels of expanded PVC foam board 1/4" thick, which not only adds thermal insulation, but is self-extinguishing (I tested this) and approved for use in mobile applications including boats, cars and airplanes for flame spread, as there is none). The foam core panels also will not absorb moisture or odors--my goal was to not use any wood at all in my build and the wall panels are a massive section of wood that would absorb moisture, get moldy on the outside and cause an entire van build out to have to be demolished and rebuilt. The foam cores also bend very nicely with the many compound wall curves that Mercedes loves to use to my frustration, are easily cut and yet hold screws very well. They also are non-conductive, unlike wood, so any errant wire that could rub and conduct current won't make it to the exterior metal van skin. I should also add that I did not run any wires inside the wall panels to avoid trying to solve a problem later or have a wire issue that could be dangerous yet hidden in a wall. So the wall sections, all five of them--yes only five due to this wide panels, unlike the thirteen wall panel sections that most van builders use when using wood for their panels since it comes in 48" widths and yet Mercedes in their humor loves to use wall sections that range from 50 to 54" wide. So my 60" wide foam panels were cut into five full height sections, covered in 1/8" foam padding for additional thermal insulation and "give", and a marine grade, wash-down fabric was adhered to each wall section, wrapped around the edges and not only glued but also stapled to the back edges. These wall sections were then screwed to the wall ribs, which are now covered in reflective foam, so only thermal connection to the van's interior wall structure is thru these screws, which were kept to four per panel, which was enough to hold them in place. Care was taken to let the interior wall covering fabric and foam overlap slightly at each interior wall to wall section to force an overlapped seal. These are tight from ceiling to floor. Another note, is that my interior cabinets are connected directly thru the factory floor tie down points. Each of these are rated to over 1,000 pounds, there are twelve or thirteen, and I replaced all factory bolts with even stronger stainless bolts that are about an inch longer as the frame mounts could accommodate a longer threaded section as well as the added thickness of my floor insulation. These are so strong that my cabinets will not budge and any attempt just rocks the van and not the cabinets. Part of the benefit of metal framed cabinets-again, no wood. Secondly, these bolts have a thermal break washer at the connection to the cabinets, and limit the dozens to hundreds of drilling screw connections used by most van builders to the van's walls. So my solution significantly reduces thermal transfer points and I have not yet seen any bit of condensation at my area or bolted connection.
Now I have a fully sealed camper section. However, I also added three layers of various vehicle flooring insulation and left over high density foam, reflective foam and thinsulate to under the factory rubber floor, inside the seat bases, and up the fire wall. I also added thinsulate to the A pillars where I could without impeding the factory air bags, and stuffed the heck out of the headliner area before it was reinstalled with reflective foam tapped in and thinsulate added every where I could including the entire height of the B pillars. Key tip, there are foam plugs at the bottom of the B pillars that hide huge interior structural "holes" that connect directly to the interior B pillars and can be stuffed with about a sack of potatoes or a much of Thinsulate remaining strips, and I have found that before adding this insulation, there was much discernible road noise from this area. Same goes true for the driver's side as much can be stuffed around the fuel filler pipe. I also stuffed and tapped thinsulate and reflective foam to the front doors after removing the interior panels and replacing the pathetic factory speakers. Another tip: Mercedes likes really pathetic stereo speakers and embarrassing horn sounds for the Sprinter (same for the Unimog I drove). Also, each interior door panel has large pockets in which more insulation can be added, just be sure the insulation does not interfere with the window mechanism.
One more tip: the rear door floor panel is not insulated at all, I could see light from inside the van from outside thru large gaps in rear door floor filler between the rear doors and main floor where there are plastic slips over the bolts that hold it in place and the ones that can be removed to lower the spare tire from factory location. When camping, I noticed drafts, cold and sounds coming thru this area, so I stuffed high density foam up into this floor panel from below solving that issue.
All of this insulation significantly adds to your driving and camping comfort for reduced noise and more stable temperatures. When camping in the winter, I usually just run my heater for about 20-40 mins to heat the van from 50 degrees to 70 degrees and that's it. It's then comfy until the next morning. My old Sprinter van without all of this attention to good insulation was far from as comfortable or noise free.
Hope this helps. I will make a video with more details and photos.