How much warmth does a bivy add?

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=81507&skip_to_post=693810#693810

From my experience it isn't so much about adding warmth as it is protection. With a small tarp it'll protect from spray and the bottom is a ground cloth. It creates a layer of still air around the bag/quilt so it doesn't loose heat to moving air as easily.

I don't think it adds warmth per se. Bags are often rated for 'in-tent' conditions so the bivy makes up for the lack of coverage of tarp/cowboy camping.

I use a bivy for bug control and often for-go the insulation layer… but that's only when I'm out near the coast.

Oh, shoot. Bonehead moment. I'll go read my own thread….

This is so embarrassing.