Welcome, Ken.
I’ve used Reflectix a number of times. I live in Alaska, work as an engineer, and need the air and water lines in my systems to operate year-round. I’ve also used it to move frozen foods (usually sockeye salmon and halibut but sometimes caribou) across state lines at 33,000 feet. I’ve got a bit of it in my own house (which I built) where I wanted more insulation in a tight space.
It’s highest use would be when you’ve got minimal volume, no compression load, and a high temperature differential (that’s when the IR reflection helps the most, well, works at all – when you have a delta T across an air gap. The extreme example is the vacuum gap in a Thermos bottle which is why the inner surfaces are silvered).
If I was going to crash somewhere in a bush plane, I’d rather have a roll of it along, then not.
But in lieu of groundcloth and foam pad? Cheap. Waterproof. But not nearly the R-value of closed-closed foam (CCF) or a self-inflating foam pad nor the toughness of tyvek nor the extreme lightness of polypro. If I was relying on it under my body, I’d throw in some CCF pads for under my hips and shoulders where my weight most compresses the insulation under me.
I think it would be more successful over your body but under your quilt, as you suggest, to allow for a thinner quilt 0 use a 40F quilt at 25-30F comfortably. It wouldn’t quite have that lovely, light-as-a-cloud feeling of a 850- or 900-FP down quilt, because it’s a bit stiff but it wouldn’t weigh much and I suspect it would add 10-15F of comfort. I wouldn’t plan on using it every night, but if I was cutting it close on my bag/quilt, it would be really nice on the coldest few nights.
But I’ve never tried as part of a sleep system. It’s the kind of thing that would great to try out in your backyard before trusting on it on the trail.
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