Here's from another post I wrote.
I did this last summer and this spring. Used Prodex mostly , but came up a little short and finished up with reflectex(silver bubble wrap you see at lowes and Home Depot). My 23' used one full Prodex roll which was 48"x 170' or some length close, and a few rolls of 16" reflectex. They sell wider reflectex, but for what I needed to finish up it was a good size. Both were pretty easy to work with, the Prodex cut a little easier.
Key is getting the trapped air layers, you don't want it going right against the skin.
I did 2 layers of the insulation. The process involved cutting about 5,000, 1"x1" square pieces of 1/2" thick rigid insulation. Yep, 5,000, a little over a full sheet got cut up into little cubes. I set up a little station and would use a razor knife to cut a bunch of 1"x48" strips. Then just slice 1" pieces and feed another inch to the razor. Set up a little 'jig' on a small 2'x4' piece of plywood. Eventually cut through the plywood with the razor cutting in the same spot over and over.
Each section between horizontal and vertical ribs gets done individually do you have to measure and cut the Prodex or reflectex to fit each one.
First I would go to each section and take some of the 1" blocks, I would put a big dab of polyurethane sealant/adhesive(used black poly roofing sealant) on one side of the block and adhere some around the perimeter of the section and then some somewhat randomly in the interior portion, spaced to minimize conductive heat transfer, but enough so they would hold the insulation off the skin and create a air space. I'd let the poly set up over night then put a layer of insulation in. Then I hot glued(so I didn't have to wait for poly to set ) blocks to the face of the next layer of insulation, around its perimeter and spread out in the middle, then put that over the first layer. Each piece of of insulation was cut to fit snug at the edges of each section. You'll quickly realize that the ribs look straight and parallel but often aren't, do lots of measure and cut.
If you can imagine the layers from outside-in, it would be outer skin, foam block, insulation, foam block, insulation. But the foam block layers were really mostly air.
All the wiring got sandwiched between the layers of insulation so it wasn't against the outer or inner skins directly. Figured this will help keep it cooler and last longer and protect from chafing.
Took a lot of work, especially with my OCDism, and the way I did a few things, but I am very happy with it. No insulation can keep a closed up trailer sitting in the sun from getting hot inside. But if I go down to work on it and open up the windows and put the awnings out. It will easily stay very close to outdoor ambient. Haven't had a AC unit on the trailer for almost 2 years working on her so it definitely helps keep cool. No furnace either, When it's cold I turn on a small catalytic heater and it warms up quick(10 min), then I'll usually have to turn it off to keep from getting to hot, running it for 5-10 min an hour to keep it warm.
You can use foam strips instead of little cube blocks.
Here's the thread