I have slept out in the Thermarest Bivvy overnight, under cedar trees, also with a thermal pad (z-rest). I don't think its counter-productive because I was trying to simulate a real world bivouac, for which I do tend to carry my z-rest. If I didn't have any thermal layer from the ground I would have probably built up what protection I could, and shelter in the bivvy sitting up - not a very comfortable night. When I slept out in the bivvy under cedars it was fairly cold, mid-40s, with rain, but I was in normal outdoor clothing for me - a thermal layer, pants, long sleeve shirt, hat, gloves, fleece jacket, boots. I took off the gloves, too warm, and slept snug for the night. I didn't wake up in a pool of my own condensation, the outside of my fleece and pants were damp. The point is I slept through the night, comfortably, and woke up, then wrapped up the bivvy in its stuff sack, and went indoors (and slept some more as I recall). I don't recall my feet getting cold. I did this a year or two back, and I wish now I documented my tests better, but I think I reported on it somewhere earlier in this forum.

The Heatsheets Bivvy is one of a line of progressively improved products - small, lightweight, increasingly effective emergency shelter. Better than heatsheet's thermal blanket, which was better than the space blanket, which is better than the Tacoma Mountain Rescue emergency shelter I carried in packs for 25 years. It should help you get by if you are at least mildly prepared and seek appropriate shelter from the elements. Its not my bivvy of choice, I have a Montbell bivvy bag that is waterproof and much more durable. But I carry the Heatsheets Bivvy on every outing, on the odd chance that I need it or someone else does - to help recover from hypothermia or shelter them while injured. At $11-12 the Heatsheets bivvy is also much more likely to be slit down the side and wrapped around an immobile victim than my $100 Montbell.