Heat is transferred from a hotter object to a colder object 3 ways:
- Conduction: two objects touching each other, think of food in a heated pan on the stove
- Convection: air immediately adjacent to an object is heated or cooled to the temperature of the object, transferring a miniscule amount of heat energy; air currents occur (natural convection occurs as hotter air rises, cooler air sinks) which keep bringing fresh air at the prevailing room temperature. The quicker the air moves (the faster the 'turnover' time of the air layer next to the object), the quicker the heat transfer. This is how WIND CHILL can kill you when the absolute temp isn't that cold, but your body is constantly trying to warm the air next to your body, but it keeps being stripped away by the wind (and replaced by cold air again). Your body loses the battle of trying to heat up the world!
- Radiation: heat transfers (even through a vacuum...no air or physical contact) from the hot object to a cold object in its line of sight. Think about how sun feels on your body. That is radiation heating you up. Just like sitting in front of a fire.
Okay, with that out of the way: the roof gets REALLY HOT sitting in the sun. The air next to the roof on the inside heats up by convection, which in turn heats up the headliner, which in turn heats up the passenger compartment. The roof also directly heats the headliner via radiation. The bubble wrap reflective stuff really does work. The shiny stuff reflects MOST of the radiant heat gain from the roof, so the headliner can't "see" the roof and the reflective surface keeps the insulator itself cool. Also, there are little air bubbles that provide a bit of insulation, so the convective heat gain (from the heated air above the headliner) doesn't get to the headliner very well.
Trust me, it makes a BIG difference. Doors? Nah, it wouldn't make much difference. Heat gain through the vertical door is fairly low. You already have two layers of steel, door panel, etc. Now heat gain (radiation) through the GLASS is obviously an issue. That's why tinting helps so much. This isn't a good material for sound deadening, as it isn't going to absorb much sonic energy (basically converting vibrations in the air to vibrations in solid material which are converted to heat), nor is it dense enough to BLOCK sound. To do that, I put Damplifier Pro as a CLD (constrained layer damper = converts panel sound vibration to heat energy) on the door sheet metal, and then a layer of MLV over top (mass loaded vinyl which acts as a direct mechanical barrier to sound; think of listening to a noisy generator or lawnmower and then ducking down below a brick wall...the sound doesn't pass through the brick very well).
As to the heat GAIN in the cab: the question was about desert conditions, and I am assuming that is NOT referring to winter time. So the HOT place is OUTSIDE, and the cool place is INSIDE. Heat flows from HOT to COLD, so I am not sure what is meant by "reflective material also reflects heat back into the vehicle". Sorry, but that is not how heat transport works. Forgive the engineering geek stuff, but this is how this stuff works. Now if you put the reflective stuff OUTSIDE so it shined IN to the truck, yes, it would heat it up. But that isn't the plan.
Hopefully this makes sense and is useful.