I was going to test the other bivvy, but it stared raining.
I was in there for only 15 minutes.

I would not even try to sleep if I didn't have any pine bows or other insultion between me and the ground.

Obviously the experience would have been significantly different with nothing between me and the ground. Conduction would have won.

I used the thermarest because I wanted to eliminate the variable of conduction of my body heat to the earth. I wanted to see exactly how well the bivvy would trap the heat coming off my body.