Interesting to see this thread come alive again.

I ended up purchasing a few AMK Heatsheet bivvys. A bivvy is always warmer than a blanket when you need it to be. Based on the material, you can sweat, but it is easily fixed by adding a strip of duct tape towards the bottom and making a slit for ventilation. This acts similar to the much better and functional AMK Thermo-Lite bivvy which has velcro sides for heat adjustment. I've used the Thermo-Lite on multiple occasions and it is well worth the investment. I have every vote of confidence in the Heatsheet bivvy, knowing it's limitations, that it can perform as I expect. If I need it as a blanket for any reason, a knife will take care of that.

Happy Hiking!