To be fair, when mounted flush the Reflectix will be somewhat of a conducting surface, but not a very good one. The air bubbles inside the Reflectix make it not a good conductive surface, and it will still be more effective as a reflective surface (radiant barrier) than it will be as a conductive surface (conductive barrier). If properly sealed (as with the silver reflective tape) it is also an effective moisture barrier all on its own.
The dead air space required by Reflectix is mainly for building construction applications. An application inside a van or truck, while similar, and the same principles apply, is still a little different. I mean, stapling Reflectix to 2x4 rafters (or equivalent) in a van just ain't gonna work, it's just not sturdy enough to handle the jostling of the roads. It's also not sturdy enough to handle being shouldered or butt-bumped while loading or securing a load. Compromises need to be made, otherwise it'll all come tumbling down (which I've seen in more than one installation).
The shiny silver reflective surface of Reflectix isn't really designed to reflect visible light, it's designed to reflect electromagnetic radiation (infrared light, or heat), and it'll do that whether it's mounted flush or with a dead air space behind it. The dead air space increases the R-factor of the installation, but doesn't increase the infrared reflective ability at all.
Reflectix all by itself, either glued right to the inside skin of a van or using spacers for an air barrier, isn't going to be very effective as an insulator in a van. But it will be an effective radiant barrier in either case. One way to tell is to glue Reflectix to bare metal inside the van, and leave another metal area free of Reflectix. On a nice hot sunny day, feel the bare metal and feel the Reflectix foil, and you'll notice a marked difference. Cut 2-inch strips of Reflectix to use as your spacers for the dead air space, and you'll feel even more of a different with Reflectix applied over the spacers. Insulate the bare metal with foam (board or canned) and then put Reflectix over that, and you'll get the most benefit.
But whether you use spacers to achieve the dead air space or use air-filled foam board or canned foam, the effect is still the same. So it's best to insulate over dead air spaces, or with foam, and then cover everything with Reflectix. There are places where it's not practical to insulate first with foam. In those spots gluing Reflectix right to the metal is certainly preferable to leaving the metal bare. I have several areas inside my van where Reflectix is glued right to the metal, and it's quite effective. Most of those are where there is already dead airspace inside, like the long strips running the length of the van where the wiring harness for cargo lighting and tail lights are contained, or the thick framing sections of the sliding door.
But for the large areas, especially the recessed ones where foam board or canned foam can be used, use the foam, then cover it all with Reflectix, and you've got an excellent radiant and conductive barrier insulation installation. Burt Reflectix alone, even with dead air spaces behind it, will not make an effective installation.
