Well, I am no expert and I won't pretend to know all the science behind it, so I won't say anyone is wrong, but I will explain my thoughts so you all can review and critic my logic...

I looked into the "R" value of bubble foil yesterday and from what I could gather a single layer with one white side and one reflector side (like I use) has an R value of about 3. If you add dead air space the R value increases by a value of 1 for every inch of air space.

I'm guessing that the trapped air in the polyethylene layers traps heat to provide the first 3" of R value to bubble foil because just the reflector sheet (a space blanket) has no R value at all by itself. My "Baker's Tent" made from space blankets does trap the heat. That's easily proven. It's basically one big bubble made the same way as bubble foil.

Okay, so now we know that a space blanket will provide more R value as it's covering a bag with more loft. That's an important thing to understand in how it works.

But that's not the whole story as I figure it. I'd think that what we want for our purposes is a minimal amount of air between our skin and the reflector. What we want to do is retain our body heat in our body, not in a sleeping bag. That's the logic I'm working with.

With that in mind, it also makes sense to me that the more loft you have in your bag the less you'll feel the effects of a space blanket over it because that loft is absorbing the radiant heat you are emitting before it gets to the reflective layer.

If I am correct, then if you wore a thin layer of anything that has air space (fleece would work) with a space blanket over it, and then added R value (get inside your sleeping bag) you'd add the most warmth (to your body) with the least weight.

The issue there, we know, is condensation. That's a problem with the fabric, but as I've pointed out previously there are breathable reflective fabrics now. No one has tried them as far as I know. I think we have to test the potential for that.

Still, right now a space blanket laid loosely over your bag does breath quite a bit, and I've never experienced condensation doing that. I have experienced the increase in warmth, and very quickly too. This was with a 32º (optimistically rated) synthetic bag, that is not high end, in mid 20ºs nights.

What I will say for sure is that I know the stuff works (neoair is using it for that reason), we just haven't figured out how to make it work best for us yet. I'm playing with that a little and seeing some very good results.

And like you said, "there's no harm in experimenting"