To add a little "insulationology" to this, there are basically three methods of heat transfer in our universe (there's also a fourth, evaporative cooling, but that's not really relevant to anything except why people get colder when it's windy): conduction, convection, and radiation. Conduction occurs when atoms bump into other atoms and transfer their energy like pool balls running into each other on a billiards table. Convection occurs when a mass of material (either in gaseous or liquid form) moves and takes its heat energy with it.
Air (or any gas, really) transfers heat poorly via conduction because it is not very dense, but it transfers heat extremely well via convection (as does any gas or liquid). So for any kind of insulation to work effectively in a structure (like fiberglass, XPS foam, rock wool etc.) it has to trap lots and lots of tiny pockets of air in a physical matrix. Since insulation is composed of a large fraction of air or other gases, it conducts heat poorly, and since the tiny pockets of air are not free to move (much), it also convects heat poorly. The end result is a substance that heat moves through slowly, which is exactly what you want from insulation.
Heat transfer by radiation is different. Any substance above absolute zero temperature will occasionally emit photons of energy; the loss of this energy will cause the substance's temperature to gradually go down. This heat loss occurs without the material needing to come in contact with any other material and without the material needing to move anywhere (this is why it's so relevant in space, of course). The frequency of this photon emission (aka the rate at which heat is lost in this way) is a function of the material's absolute temperature taken to the fourth power.
This means that heat transfer via radiation increases rapidly as temperature increases, but it also means that heat transfer via radiation decreases rapidly as temperature decreases. It just so happens that at the temperatures humans typically live in (say, 100°F and below) the amount of heat transferred via radiation is nearly nothing when compared to the amount transferred via conduction and convection. When you get into the temperatures of a material heated above the ambient air temperature by direct sunlight (say, 140°F on the roof of a school bus or 165°F on the black asphalt roof of a house) you start to get a significant (but still pretty small) amount of heat transferred by radiation. When you start talking about the temperatures of a wood stove (maybe 400°F to 500°F) then the amount of heat transferred by radiation becomes more significant than that via conduction and convection (hence the need for a radiative heat shield with a wood stove, something not everybody in the skoolie world seems to actually install).
Long story short: bubble-and-foil reflectix-type insulation is pretty much worthless because a) the bubbles are too big to block heat transfer via convection and the overall material is too thin to have any real effect on conduction, and b) the foil is useless against radiative heat transfer because there's nearly none of that happening (except in the special case of sunlight coming through the windows, and aluminum foil would do the same job only a lot cheaper).
The insulation also leads to a special logical problem sometimes. In a house, reflective insulation can be used to somewhat block the radiative heat coming off the hot roof into the house. If the foil layer is installed in the attic with the foil facing upwards (and if the air in the attic is free to circulate to the outside), then it does provide a small cooling effect, usually given the equivalent R-rating ("equivalent" because R-rating is literally the inverse of thermal conductivity, something that does not really apply at all to radiative heat transfer) of 2.5. Since reflectix-type insulation is usually 1/4" thick, some people imagine that multiple layers of reflectix will provide a superb R-value of 10 per inch - definitely not the case.
