Cheap insulation. Is there such a thing?
Two things about that reflective stuff - first is the adhesive backing. Most of them are petroleum based and can give off an odor, especially when it gets warm. Second, AFAIK, it's way more expensive per square foot than regular insulation. Not sure where you got yours and how much you paid for a roll but based on what I've seen it going for (even at the big box stores) it wouldn't be a cost-effective solution for a larger area.
I can speak from direct experience regarding the odor as I used some of it on the floor of my car. I had to drill a decent-sized hole through some of it. The heat from the drill caused the surrounding area to get warm and it definitely smelled of petroleum product. Not overwhelming but enough to notice, even from a small spot. If you covered your whole interior with it, you would notice it for sure.
