Interesting. One last thought: for sheltering-in-place in a car, I think the Thermalite2's would work well. The car acts as a shelter, forestalling the wind.
I bought a couple of Theralite2's for that purpose (one for my wife, one for me), and I keep my older, lighter Thermalite ("Thermalite1") in the bottom of my pack on day hikes.
Based on this thread, maybe I should get a smaller Heatsheet bivvy for my hikes. Hmmm.