My van had the factory headliner, which took a bit of fussing with to remove, but then became easy to work with. I put a layer of Reflectix against the top, added a layer of closed cell upholstery foam (actually as much as i could stuff in in places, there's lot's of void in there) and then another layer of Reflectix on top of the headliner. The sides have all the factory glass, but where there is no glass i used the same procedure, Reflectix, foam, Reflectix. None of this was glued to the body, i felt that might be a trap for moisture against the metal and could potentially pull the paint from the skin and expose bare metal.
All of this has helped to a point. It's easier to heat and stays cool longer, and it's a lot quieter all around. The most noise now comes from the still unfinished floor and rear wheel housings.
What i do notice is that the large amounts of metal forming the body transfer the heat faster than the insulation can stop it. Unless you build another wall inside the body to separate the interior from the outside thermally, it's an uphill battle to keep up with extremes of hot and cold. Cold outside sucks the heat out, and hot outside heats the inside. Between that and the glass doing the same thing, it seems the only use the insulation is really good for was the sound deadening qualities.
But i'm really glad i did it all, and would do it again.
Jef