I own two (2) Tac Bivvys and have used one (practice & familiarly) and they are an OUTSTANDING piece of Emergency Equipment.
About one week after receiving the Tact Bivvys (over 5 years ago) I opened one up to give it a try.
It was a "brisk" sort of day, about 55 degrees and the wind was blowing about 2-5 MPH, more of a breeze than a real wind. I opened one up and crawled inside of it. At the time I tipped the scales at about 260 of a 5'11" frame. The Bivvy was a little snug but not really confining. In less than 5 minutes I was feeling very warm and after about 15 minutes I was starting to sweat a little.
At about 55 degrees with a slight breeze I was starting to sweat?!
It's a little more rugged than an Emergency Mylar Blanket, I crawled inside the Bivvy with my regular clothes and boots. I also forgot to remove my Leatherman and Buck knife off of my belt. Nothing was cut, torn or abraded. I would not recommending climbing into this rig with your boots on or with anything attached to your belt. I only did it to test our the Bivvy.
I highly recommend that you wear clothing when using the Bivvy. First off wearing clothes when using the Bivvy will help you stay warmer (layering) and if you do perspire your clothing will insulate you so that if you move during sleep you won't be shocked by cold Mylar against warm skin. Since the Bivvy only reflects heat but does not hold heat it only makes sense to wear clothing when using it.
I know that the Bivvy is supposed to be…