Carleas wrote:I did mean it to be a puzzler, because for me, with a poor knowledge of thermodynamics, I can only use my intuition and reason (assuming I am capable of using those

. So pardon the rambling I'm about to do, and please point out where I'm wrong (not that you've ever held back).
Your intuition is the best solution because a precise answer would have to account for radiative, convective, and conductive heat transfers requiring more knowledge of your blanket and the floor in terms of air leakage and thermal conductivity.
When insulating an area, your biggest bang will come with eliminating air leaks between insulating pockets of air, so even a sheet of saran wrap over the entire floor would do a better job than a blanket folded in half and it's why people put plastic over their windows. Anytime you can divide one area from another area to prevent the volumes of air from intermingling, that's half the insulation battle. The other half is slowing the conduction of heat from one air pocket to the other via the material used for the division. One way to accomplish that is to slow the turbulence of the air in the pockets which is the purpose of the fiberglass stuffing. If air cannot move around, it cannot convect heat and it slows conduction through the barrier considerably which allows a mere paper backing to be a good-enough barrier dividing the 2 volumes. (Plus glass happens to not pass IR light easily, which is insulative as well.)
Heat, as you think of it (and not the proper definition) is infrared light which is only different from visible light in that it oscillates a bit slower. All light comes from the vibration (acceleration) of charged particles. So keeping your vibrating charged particles inside your room is of utmost importance. As molecules vibrate, they are emitting IR radiation (ie cooling). When the IR meets another charge, it induces a vibration and continues on its way. The charge then emits its own IR in all directions resulting from the induced vibration and etc, etc, etc from particle to particle. Now if you let the particle float out of the room, you've lost all that energy. Also, if you let that particle touch another particle, it will transfer all its energy immediately to the other particle. The best strategy is to keep all your particles still and let them vibrate (cool) slowly over time. So reduce air turbulence and seal all leaks.
You could put a tarp down and then a blanket over it, but it would be much more effective to put the blanket down first and the tarp over it because you will have created a pocket of air between the floor and the tarp wherein the stuffing inside the blanket would reduce air turbulence and essentially mimic typical pink insulation. You see how it works?