Boy oh boy is this a complicated topic. There is no clear solid consensus on what the “best” way to insulate a van is, but there sure are a lot of opinions with very little in the way of empirical data. At best, you're making an educated guess and almost everyone insulates their van differently.
FarOutRide's guide is a good place to start.
Insulation can help retain heat in cold weather, and reduce transfer from hot body panels in direct sunlight in warmer weather. Insulation will not, by itself, make your van cooler than ambient in the summer or warmer than ambient in the winter. But it will help when aided with either heating or ventilation to keep the interior more livable.
It is better to insulate the entire van poorly than to only insulate part of the van well.
Windows are going to be the largest source of both heat loss or heat gain by far. Especially If you have a van with windows all around, it's pointless to do any insulation to the body at all if you're not doing something about the windows in the form of insulated or reflective window coverings. Again, exactly what type of window covering you have isn't as important as just having SOME sort of window covering.
Similarly, the sheet metal of the van makes an incredibly effective thermal bridge. You can go crazy with stuffing Thinsulite into every single nook and cranny, but even just a few strips of exposed body sheet metal to the interior will almost completely negate all of that work. To effectively insulate your van, everything needs to be covered. Even a thin covering of carpet spray-glued to exposed sheet metal will make a world of difference.
4-Way stretch carpet is ideal for this. Because it has some give to it, it's possible to mold it over and around all the sort of crazy compound curves that makes up the van body. Many videos on youtube showing this process, here https://www.youtube.com/watch?v=TON7xuJkQs8 or here: https://www.youtube.com/watch?v=ob9TmLn8D8o
Total R-Value is the total R-Value of a given construction, taking into account all the different R-values of the different sections that it might have. For example, a well-constructed residential wall in a house in a cold climate might have an R-Value of 201) (1/2“ plywood, then 1” of Poly-Sty, then a vapor barrier, 3.5“ of fiberglass batting with wood studs forming the wall, then 1/2” drywall), while a solid-core door might have an R-Value of 4 and triple-pane windows might have an R-Value of 2.5-32).
Single-pane windows (Such as used in cars) has an R-Value of 1 5).
Let's look at some examples of how this dramatic heat loss from windows affects the total insulation of your van using this total R-Value calculator.
Let's say we're insulating a van with windows all around, like a former passenger van/shuttle bus. In this case, about 35% of the van's surface area is going to be glass.
We'll start with a typical kinda basic insulation, like what you'd get from the factory or most people who are doing a full build out of their own and not going too crazy. An R-Value of 5 for the walls, and 1 for the windows.
Huh! So in spite of all that insulation work, you got an R-Value of barely over 2. You're very chilly at night, so let's say you DO go crazy with the insulation! You spend a thousand bucks on 600 thickness thinsulite, you stuff it in EVERY nook and cranny, you put mini-cell foam pads on all the sheet metal and then cover the whole inside of the van with some sheet poly-sty and then a solid sheet of plywood or wood paneling. You've gotten the R-Value up to 10 (Which is REALLY REALLY hard). Let's try it again:
Wait so you DOUBLED the insulation on the walls, and it barely made a dent in your total R-Value! Damn. Well, let's go REALLY nuts! Let's build it like we do a house, and do a full wood frame inside with an inch of poly-sty, wood studs spacing out 3.5“ of fiberglass batt, plus a vapor barrier and 1/2” of plywood. That can get us up to an R-Value of 20 for the walls (This would lose you a HUGE amount of space inside, but let's just do it as an experiment):
WUT?! You went bat-shit nuts and lost a ton of interior space, and your van is still leaking heat like a sieve because of the massive loss through those windows.
BUT. Let's assume we do a bit of insulation on the windows. Nothing crazy, just add some window covers DIYed out of scrap material. Some thick cardboard, wrapped in some $5 fleece blankets that you got from wal-mart and tightly fit into the window openings to covering them completely is a great start. This will get the window R-value up to around 3 if you leave a bit of an air gap. Let's see how that changes our calculations, even with our basic lazy van insulation:
WILL YOU LOOK AT THAT! Just by covering the windows, poorly, and insulating the rest of the van to a kinda-mediocre level, you've got NEARLY TWICE the total system insulation as if you did a completely nutso, bonkers, over-the-top insulation job but haven't covered the windows!
TL;DR - COVER YOUR DANG WINDOWS IF YOU'RE TRYING TO STAY WARM
Many companies make window coverings for vans, some are available with Reflective coating to help keep the sun out. Others are thickly insulated to help keep heat in. Prices and quality range from very minimal, to very expensive. Plenty of people DIY these as well.
Van Essentials has a pretty good comparison chart of costs (biased as it may be).
Some other sources:
Insulating the floor is done not only for thermal comfort, but for sound while driving. A bare metal cargo van receives a shocking amount of road noise, and the large slabs of metal act like a drum to amplify it.
For extremely simple builds, throwing some basic rubber mats over the floor (and cutting to fit, possibly attaching down with spray-glue) will make a world of difference over just a plain metal floor. Anti-Fatigue mats can be very cheap and are easy to cut to fit whatever space you happen to have.
Rigid-core foam board under plywood works well for thermal insulation, but can lead to more vibration and squeaks as the van moves while driving. Mini-cell foam is a popular sub-floor insulation that gives both reasonable thermal as well as acoustic insulation.
Even if you're sticking to mild climates and have a van with a ton of windows, there is something to be gained from doing some kind of insulation on at least the roof underneath the solar panels. Heat re-radiated from the underside of the panels may increase roof temperatures even though the roof is shaded by the panels.6)
If you don't have panels, the heat from the sun can easily make the interior sheet metal burning hot to the touch. Covering it with some insulation (and ventilating the van well) will help keep things more tolerable.
See this write-up of testing floor vs ceiling insulation.
See this test of different insulation types compared side-by-side in pseudo-laboratory conditions.