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I almost exclusively hammock camp. The idea is the foam sleeping pad inside the hammock won't compress as much as laying my full weight on it on the ground. Therefore retaining more of it's "R-value". An underquilt outside the hammock is that additional insulation that never compresses and the rainfly acts as a wind break. I have found these four pieces (tarp fly, hammock, foam pad, underquilt) all more lightweight and compact than a tent. I have enough real estate on my pack for a second sleeping bag, but rarely use it.

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It's not required, BUT it does allow you to use a lighter/smaller sleeping bag, which is very nice if you're backpacking. It's about the combo, not just one or the other. If you're car camping just bring an old Coleman bag or two to put on the bottom, I've done that down to -10 with snow on the ground at 8k feet and been comfortable.

If you give off a lot of water vapor like me, it can get really uncomfortable if one side is cold, but yet your feet are still sweating.

This, once you invest in a hammock and under quilt, you wonder why you slept on the ground so long...

When tent camping (scouts and SCA!) we always used the ratio of 2-3 times the insulation below to the amount above. I've never slept cold when following these rules. Plus you don't wake up feeling like you slept on roots and rocks.

I did a night at -10 on an air matress with a -40 rated bag, and an electric blanket below .  It was great until the battery ran out then it was time to wake up. Lol. Get an insulated layer for cold weather.

The insulated pads work by having more reflective material inside the pad. A common complaint for years was that sleeping on the high. R value pad was like sleeping on a chips bag because of the noise
I personally have a separate winter pad . It's heavier but worth it.
If money is an option you might want to consider buying a surplus military casualty blanket and laying it under your summer rated pad with the reflective side facing up. It doesn't weigh that much and accomplishes much the same role as the metallic innards of a high R value pad
A sleeping bag liner is a second option

Placing a mylar sheet between, and in compressive contact with, the ground and a sleeping pad provides no additional insulating value.

Placing the mylar sheet between a pad and sleeping bag also does nothing.  The only reason manufacturers can show a numerical difference in R values of diffuse-surface closed-cell foam pads and closed-cell foam pads with a reflective coating on the surface ... is just owing to a quirk of the testing setup.  The standardized test involves a heated flat plate applied to the pad with x pounds of force - the flat plate creates meaningful voids when compressed against a corrugated closed cell foam pad, and reducing radiation heat transfer at these air voids does make a measurable difference in a laboratory environment.  It's an air void.  Not contact conduction.  A real life application reduces the thermal radiation resistance to near zero.

Winter rated air-core mattresses work because there is a physical gap between the mylar surfaces.  Ideally, this gap would be a vacuum.  That's why they are called space blankets.  With a pad, you have a convective air gap, and convection is a very finicky thing.

Turn the ASTM F3340-18 test upside down, literally, and the air-core mattresses will have a lower R-Value compared to the standard test.  Put a box fan in the lab, and the air-core mattresses will have a lower R-Value compared to the standard test, whereas a closed-cell foam pad will show little to no change; depending on the design, self-inflating open-cell-foam air mattresses can be largely immune to outside air convection, like the closed-cell pads, or they can be degraded by this convection, just depends on the geometry of any weight-saving cutouts made in the foam.