Yes, the insulation actually works.
One of the most frequent questions I get from people looking to build out a Sprinter Van is, "Does the insulation actually work?" and I would say "Yes*". Yes, it does, but with a footnote.
The insulation we put in the van does indeed work. I can sit in the van and notice that when I go outside it's much chillier or if it's been cool out and then suddenly the sun comes out I notice I am much more cool in the van than outside.
However, it is a van. Which is just a giant metal box with glass holes in it. Any insulation you put in the van is going to be a bit less effective than if you were to put it in, say, a house. That being said, I am glad we insulated the van since it also acts as a bit of a sound barrier (although not much) and keeps the van at a more moderate temperature.
Insulfoam 3/4 in. x 2 ft. x 4 ft. R-2.89 Insulating Sheathing - http://thd.co/2aQGW3r
Rmax Thermasheath-3 2 in. x 4 ft. x 8 ft. R-13.1 Polyisocyanurate Rigid Foam Insulation Board - http://thd.co/1UCOA2x
Step 1: Cut the 3/4 insulfoam to fit the ceiling of the van. If you've taken everything out of the van you will see ribs running along the roof that are about 2 feet apart. Fortunately, the insulfoam sheets are almost exactly that wide! So we just had to shorten them a bit on the ends using an Exacto knife.
Step 2: Spray the insulfoam boards with the 3M adhesive and stick them to the van walls. Note: the adhesive off gasses a lot and we got a bit dizzy using it. Make sure you have a fan and lots of air circulation!
Step 3: Cut the Thermasheath block foam insulation boards to fit, spray them with the 3M adhesive and stick them on the walls.
Step 4: Use the Great Stuff window and door/gaps and cracks foam sealant around the edges of all the block insulation. This is fun and super easy! It also helps keep the insulating blocks from popping off the walls. The spray adhesive doesn't actually do that much to keep the heavier boards on the walls.
This step is actually pretty easy, however, we missed a crucial step that may end up rusting the van from the inside out.
We did not, but SHOULD HAVE used some kind of rubber undercoating to protect the inside of the van from rust. I'm quoting directly from the Sprinter Van Conversion sourcebook (which is 100% amazing, go check it out) and he has this to say about materials that will help prevent rust.
So! Make sure to put some of that on first to prevent rusting on the inside! For now, I'm keeping her well ventilated and taking advantage of my two dehumidifiers. Best of luck to you!