Don't cheap out, quality product, proper installation = great results
Vans are really loud and one of the things we do to our builds is get them quiet enough so you can actually hear the radio, blue tooth, make it more comfortable to drive on long trips, and most importantly keep it cooler in summer and warmer in winter. We use only top quality products. Dynamat Extreme is the sound deadener we use. Unlike other brands, this actually has not asphalt product in it, has no smell, and is a superior to most other products in the market. Many popular products claim to be 100% free of asphalt, that is simply not true. We have worked with them all and we find this the best. In general if the product you are using is cheap...there is a reason. Sound deadener reduced vibration noise however it does not "stop" sound. To do this you have to use an acoustic product. We choose to use 3M Thinsulate version that is designed to reduce noise and is a version specific for automotive use. This is similar stuff you find in high end jackets and sleeping bags...but for automotive use. Its a high end product with superior results. Unlike many other materials it has no fibers to float around, actually reduces sound transmission, and of course as excellent thermal properties. It has resistance to mold and mildew as well and works well even when wet, breathes, and drys quickly. There is no comparison to wool products or certainly stuff from the home stores. Don't use stuff from there for your van. For flooring we used closed cell foam from 2nd skin or Dynamat, they make specific product just for this. You can also take it to next level and use a closed cell foam product and a mass loaded vinyl product for your floors if you want to take it next level...but it gets really expensive to do so in materials. We do this if customer requests, but find Dynamat Extreme and a closed cell foam work great with excellent result for the floor, but its a lot of material. There are also a few companies that make insulated drop in floors, but those are thousands of dollars and most of the time we are working with the stock floor so we don't go that route.
DO YOUR DOORS and ROOF
The largest mistake DIYer's do is they only do the walls because they are easy access, but in reality the most important area is the roof area - most of your heat loss and cold is from this area. You can cook an egg on your roof it gets so hot. The 2nd largest mistake and not doing the doors. Third mistake is not using a sound deadener with an insulation component, we even see a lot of builders and RV companies do this...its a cheap shortcut and you can tell the difference.
HOW HARD IS IT?
Most people are surprised when we tell them it take us on average about 20-22 man hours to do a passenger van (just the walls/roof). Its easy, but really time consuming. When you do this however you should run all of your electrical, do any support structures for future cabinets or L track, and any other work such as a fan, roof rack rails, or lighting. Its like taking down your drywall, you need to do all your work otherwise you have to take it down again. If you are paying us that gets very expensive. (just like redoing your drywall)
This a door...its way more of a difference on the larger panels!
You can see here we get all the areas possible...not just a few "strips" Lots of work...and a few hand cuts usually on the way. :) Be careful for the sharp edges when getting behind all the metal. In this door we used over 35 square feet of material...more than most use in an entire side wall of a van and this is a low roof rear door...nothing on top.
Insulation example, we also sound deaden wheel wells then cover them with carpet to give a finished look
More examples of the extensive work just for this process